<Cells and Tissues><Your Goals
After completing this chapter,
you will have a working knowledge of
° Cells carry out all the chemical activities needed to sustain life.
° Tissues provide for a division of labor among body cells.
Cells (pp. 56=77)
° Name the four elements that make up the bulk of living matter and list several trace elements.
° Define cell, organelle, and inclusion.
° Identify on a cell model or diagram the three major cell regions (nucleus, cytoplasm, and plasma membrane).
° List the structures of the nucleus and explain the function of chromatin and nucleoli.
° Identify the organelles on a cell model or describe them, and discuss the major function of each.
° Define selective permeability, diffusion (including simple and facilitated diffusion and osmosis), active transport, passive transport, solute pumping, exocytosis, endocytosis, phagocytosis, bulk-phase endocytosis, hypertonic, hypotonic, and isotonic.
° Describe the structure of the plasma membrane, and explain how the various transport processes account for the directional movements of specific substances across the plasma membrane.
° Describe briefly the process of DNA replication and of mitosis. Explain the importance of mitotic cell division.
° In relation to protein synthesis, describe the roles of DNA and of the three varieties of RNA.
° Name some cell types and relate their overall shape and internal structure to their special functions.
Body Tissues (pp. 77=87)
° Name the four major tissue types and their chief subcategories. Explain how the four major tissue types differ structurally and functionally.
° Give the chief locations of the various tissue types in the body.
° Describe the process of tissue repair (wound healing).
(Objective Checklist, continued)
Developmental Aspects of Cells and Tissues (pp. 87=90)
neoplasm, and distinguish between
benign and malignant
° Explain the significance of the fact that some tissue types (muscle and nerve) are largely amitotic after the growth stages are over.><3><Part 1: Cells
In the late 1600s, Robert Hooke was looking through a primitive microscope at some plant tissue—cork. He saw some cubelike structures that reminded him of the long rows of monk’s rooms (or cells) at the monastery, so he named these structures cells. The living cells that had formed the cork were long since dead. However, the name stuck and is still used to describe the unit, or the building block, of all living things, plants and animals alike. The human body has trillions of these microscopic building blocks.
Overview of the Cellular
Perhaps the most striking thing about a cell is its organization. If we chemically analyze cells, we find that they are made up primarily of four elements—carbon, oxygen, hydrogen, and nitrogen—plus much smaller amounts of several other elements (such as iron, sodium, and potassium). Although the four major elements build most of the cell’s structure (which is largely protein), the lesser, or trace, elements are very important for certain cell functions. For example, calcium is needed for blood clotting (among other things), and iron is necessary to make hemoglobin, which carries oxygen in the blood. Iodine is required to make the thyroid hormone that controls metabolism. In their ionic form, many of the metals (such as calcium, sodium, and potassium) can carry an electrical charge; when they do they are called electrolytes (e-lek9tro-l¯ıts). Sodium and potassium ions are essential if nerve impulses are to be transmitted and muscles are to contract. (A more detailed account of body chemistry appears in Chapter 2.)
Strange as it may seem, especially when we feel our firm muscles, living cells are about 60 percent water, which is one of the reasons water is essential for life. In addition to containing large amounts of water, all the cells of the body are constantly bathed in a dilute saltwater solution (something like seawater) called interstitial fluid, which is derived from the blood. All exchanges between cells and blood are made through this fluid.
Cells vary tremendously in length—ranging from 2 micrometers (1/12,000th of an inch) in the smallest cells to over a meter (3 feet) or more in the nerve cells that cause you to wiggle your toes. Furthermore, a cell’s structure often reflects its function; this will become clear later in this chapter. Cells can have amazingly different shapes. For example, some are disk-shaped (red blood cells), some have many threadlike extensions (nerve cells), others are like toothpicks pointed at each end (smooth muscle cells), and others are cubelike (some types of epithelial cells).
Cells also vary dramatically in the functions, or roles, they play in the body. For example, white blood cells wander freely through the body tissues and protect the body by destroying bacteria and other foreign substances. Some cells make hormones or chemicals that regulate other body cells. Still others take part in gas exchanges in the lungs or cleanse the blood (kidney tubule cells).
Anatomy of a
Although no one cell type is exactly like all others, cells do have the same basic parts, and there are certain functions common to all cells. Here we will talk about the generalized cell, which demonstrates these many typical features.
In general, all cells have three main regions or parts—a nucleus (nu9kle-us), cytoplasm (si9to-plazm0), and a plasma membrane (Figure 3.1). The nucleus is usually located near the center of the cell. It is surrounded by the semifluid cytoplasm, which in turn is enclosed by the plasma membrane, which forms the outer cell boundary. (Figure 3.4 on p. 60 shows the more detailed structure of the generalized cell as revealed by the electron microscope.)
Anything that works, works best when it is controlled. For cells, “headquarters,” or the control center, is the gene-containing nucleus. The genetic material, or deoxyribonucleic acid (DNA), is much like a blueprint that contains all the instructions needed for building the whole body; so as one might expect, human DNA differs from that of a frog. More specifically, DNA has the instructions for building proteins. DNA is also absolutely necessary for cell reproduction. A cell that has lost or ejected its nucleus (for whatever reason) is programmed only to die.
While most often oval or spherical, the shape of the nucleus usually conforms to the shape of the cell. For example, if the cell is elongated, the nucleus is usually extended as well. The nucleus has three distinct regions or structures: the nuclear envelope, nucleoli, and chromatin.
The nucleus is bound by a double membrane barrier called the nuclear envelope or nuclear membrane (see Figure 3.1). Between the two membranes is a fluid-filled “moat,” or space. At various points, the two layers of the nuclear envelope approach each other and fuse, and nuclear pores penetrate through the fused regions. Like other cellular membranes, the nuclear envelope is selectively permeable, but passage of substances through it is much freer than elsewhere because of its relatively large pores. The nuclear membrane encloses a jellylike fluid called nucleoplasm (nu9kle-o-plazm0) in which the nucleoli and chromatin are suspended.
The nucleus contains one or more small, dark-staining, essentially round bodies called nucleoli (nu-kle9o-li). Nucleoli are sites where ribosomes are assembled. The ribosomes, which eventually migrate into the cytoplasm, serve as the actual sites of protein synthesis, as described shortly.
When a cell is not dividing, its DNA is combined with protein and forms a loose network of bumpy threads called chromatin (kro9mah-tin) that is scattered throughout the nucleus. When a cell is dividing to form two daughter cells, the chromatin threads coil and condense to form dense, rodlike bodies called chromosomes—much the way a stretched spring becomes shorter and thicker when allowed to relax. The functions of DNA and the mechanism of cell division are discussed in the “Cell Physiology” section later in this chapter.
The Plasma Membrane
The flexible plasma membrane is a fragile, transparent barrier that contains the cell contents and separates them from the surrounding environment. (The term cell membrane is often used instead, but since nearly all cellular organelles are composed of membranes, we will specifically refer to the cell’s surface or outer limiting membrane as the plasma membrane.) Although the plasma membrane is important in defining the limits of the cell, it is much more than a passive envelope, or “baggie.” As you will see, its unique structure allows it to play a dynamic role in many cellular activities.
The structure of the plasma membrane consists of two lipid (fat) layers arranged “tail to tail” in which protein molecules float (Figure 3.2). Although most of the lipid portion is phospholipids (some with attached sugar groups), a substantial amount of cholesterol is also found in plasma membranes. (The characteristics of these specialized lipids are described in Chapter 2, pp. 40–43.) The olive oil–like lipid bilayer forms the basic “fabric” of the membrane. The polar “heads” of the lollipop-shaped phospholipid molecules are hydrophilic (“water loving”) and interact with water and other polar molecules, while their nonpolar “tails” are hydrophobic (“water hating”). It is the hydrophobic makeup of the membrane interior that makes the plasma membrane relatively impermeable to most water-soluble molecules. The cholesterol has a stabilizing effect and helps keep the membrane fluid.
The proteins scattered in the lipid bilayer are responsible for most of the specialized functions of the membrane. Some proteins are enzymes. Many of the proteins mounted on the cell exterior are receptors, or binding sites, for hormones or other chemical messengers. Most proteins that span the membrane are involved in transport functions. For example, some cluster together to form tiny pores through which water and small water-soluble molecules or ions can move; others act as carriers that bind to a substance and move it through the membrane. Branching sugar groups are attached to most of the proteins abutting the extracellular space. Such “sugar-proteins” are called glycoproteins, and because of their presence, the cell surface is a fuzzy, sticky, sugar-rich area. (You can think of your cells as being sugar-coated.) Among other things, these glycoproteins determine your blood type, act as receptors that certain bacteria, viruses, or toxins can bind to, and play a role in cell-to-cell interactions. Definite changes in glycoproteins occur in cells that are being transformed into cancer cells. (Cancer is discussed in the box on pp. 88–89.)
Specializations of the Plasma Membrane
Specializations of the plasma membrane—such as microvilli and membrane junctions—are commonly displayed by the (epithelial) cells that form the linings of hollow body organs, such as the small intestine (Figure 3.3). Microvilli (mi0kro-vil9i; “little shaggy hairs”) are tiny fingerlike projections that greatly increase the cell’s surface area for absorption so that the process occurs more quickly.
The membrane junctions vary structurally depending on their roles.
• Tight junctions are impermeable junctions that bind cells together into leakproof sheets that prevent substances from passing through the extracellular space between cells. In tight junctions, adjacent plasma membranes fuse together tightly like a zipper. In the small intestine, for example, these junctions prevent digestive enzymes from seeping into the bloodstream.
• Desmosomes (des9mo-somz) are anchoring junctions that prevent cells subjected to mechanical stress (such as skin cells) from being pulled apart. Structurally, these junctions are buttonlike thickenings of adjacent plasma membranes, which are connected by fine protein filaments.
• Gap junctions, commonly seen in the heart and between embryonic cells, function mainly to allow communication. Chemical molecules, such as nutrients or ions, can pass directly from one cell to another. In gap junctions, the neighboring cells are connected by connexons, which are hollow cylinders composed of proteins that span the entire width of the abutting membranes.
The cytoplasm is the cellular material outside the nucleus and inside the plasma membrane. It is the site of most cellular activities, so it can be thought of as the “factory area” of the cell. Although early scientists believed that the cytoplasm was a structureless gel, the electron microscope has revealed that it has three major elements: the cytosol, organelles, and inclusions. The cytosol is semitransparent fluid that suspends the other elements. Dissolved in the cytosol, which is largely water, are nutrients and a variety of other solutes (dissolved substances).
The organelles (or0gah-nelz9), described in detail shortly, are the metabolic machinery of the cell. Each type of organelle is “engineered” to carry out a specific function for the cell as a whole. Some synthesize proteins, others package those proteins, and so on.
Inclusions are not functioning units, but instead are chemical substances that may or may not be present, depending on the specific cell type. Most inclusions are stored nutrients or cell products. They include the fat droplets common in fat cells, glycogen granules, pigments such as melanin seen in skin and hair cells, mucus and other secretory products, and various kinds of crystals.
The cytoplasmic organelles, literally “little organs,” are specialized cellular compartments (see Figure 3.4), each performing its own job to maintain the life of the cell. Many organelles are bounded by a membrane similar to the plasma membrane. The membrane boundaries of such organelles allow them to maintain an internal environment quite different from that of the surrounding cytosol. This compartmentalization is crucial to their ability to perform their specialized functions for the cell. Let us consider what goes on in each of the workshops of our cellular factory.
MitochondriaMitochondria (mi0to-kon9dre-ah) are usually depicted as tiny threadlike (mitos 5 thread) or sausage-shaped organelles (Figure 3.4), but in living cells they squirm, lengthen, and change shape almost continuously. Their wall consists of a double membrane, equal to two plasma membranes, placed side by side. The outer membrane is smooth, but the inner membrane has shelflike protrusions called cristae (kris9te). Enzymes dissolved in the fluid within the mitochondria, as well as enzymes that form part of the cristae membranes, carry out the reactions in which oxygen is used to break down foods. As the foods are broken down, energy is released. Much of this energy escapes as heat, but some is captured and used to form ATP molecules. ATP provides the energy for all cellular work, and every living cell requires a constant supply of ATP for its many activities. Because the mitochondria supply most of this ATP, they are referred to as the “powerhouses” of the cell.
Metabolically “busy” cells, like liver and muscle cells, use huge amounts of ATP and have hundreds of mitochondria. By contrast, cells that are relatively inactive (an unfertilized egg, for instance) have just a few.
RibosomesRibosomes (ri9bo-s¯omz) are tiny, round, dark bodies made of proteins and one variety of RNA called ribosomal RNA. Ribosomes are the actual sites of protein synthesis in the cell. Some ribosomes float free in the cytoplasm, and others attach to membranes. When ribosomes are attached to membranes, the whole ribosome-membrane combination is called the rough endoplasmic reticulum.
Endoplasmic ReticulumThe endoplasmic reticulum (en0do-plas9mik r˘e-tik9u-lum; “network within the cell”) (ER) is a system of fluid-filled cisterns (tubules, or canals) that coil and twist through the cytoplasm. It accounts for about half of a cell’s membranes. It serves as a minicirculatory system for the cell because it provides a network of channels for carrying substances (primarily proteins) from one part of the cell to another. There are two forms of ER; a particular cell may have both forms or only one, depending on its specific functions.
The rough ER is so called because it is studded with ribosomes. Because essentially all of the building materials of cellular membranes are formed either in it or on it, the rough ER can be thought of as the cell’s membrane factory. Within its tubules, the proteins made on the ribosomes fold into their functional three-dimensional shapes and then are dispatched to other areas of the cell. In general the amount of rough ER a cell has is a good clue to the amount of protein that cell makes. Rough ER is especially abundant in cells that export protein products—for example, pancreas cells, which produce digestive enzymes to be delivered to the small intestine. The enzymes that catalyze the synthesis of membrane lipids reside on the external face of the rough ER, where the needed building blocks are readily available.
Although the smooth ER is a continuation of the rough variety, it plays no role in protein synthesis. Instead it functions in cholesterol synthesis and breakdown, fat metabolism, and detoxification of drugs. Hence it is not surprising that the liver cells are chock-full of smooth ER. So too are body cells that produce steroid-based hormones—for instance, cells of the male testes that manufacture testosterone.
Golgi ApparatusThe Golgi (gol9je) apparatus appears as a stack of flattened membranous sacs, associated with swarms of tiny vesicles. It is generally found close to the nucleus and is the principal “traffic director” for cellular proteins. Its major function is to modify and package proteins (sent to it by the rough ER via transport vesicles) in specific ways, depending on their final destination (Figure 3.5).
As proteins “tagged” for export accumulate in the Golgi apparatus, the sacs swell. Then their swollen ends, filled with protein, pinch off and form secretory vesicles (ves9˘ı-kuls), which travel to the plasma membrane. When the vesicles reach the plasma membrane, they fuse with it, the membrane ruptures, and the contents of the sac are ejected to the outside of the cell. Mucus is packaged this way, as are digestive enzymes made by pancreas cells.
In addition to its packaging-for-release functions, the Golgi apparatus pinches off sacs containing proteins and phospholipids destined to become part of the plasma membrane and packages hydrolytic enzymes into membranous sacs called lysosomes that remain in the cell.
LysosomesLysosomes (li9so-s¯omz; literally “breakdown bodies”), which appear in different sizes, are membrane “bags” containing powerful digestive enzymes. Because lysosomal enzymes are capable of digesting worn-out or nonusable cell structures and most foreign substances that enter the cell, lysosomes function as the cell’s demolition sites. Lysosomes are especially abundant in white blood cells that engulf bacteria and other potentially harmful substances because they digest and rid the body of such “foreigners.” As described above, the enzymes they contain are formed by ribosomes and “packaged” by the Golgi apparatus (see Figure 3.5).
Homeostatic ImbalanceThe lysosomal membrane is ordinarily quite stable, but it becomes fragile when the cell is injured or deprived of oxygen and when excessive amounts of vitamin A are present. Lysosomal rupture results in self-digestion of the cell. s
PeroxisomesPeroxisomes (per-ok9sih-s¯omz) are membranous sacs containing powerful oxidase (ok9s˘ı-d¯az) enzymes that use molecular oxygen (O2) to detoxify a number of harmful or poisonous substances, including alcohol and formaldehyde. However, their most important function is to “disarm” dangerous free radicals. Free radicals are highly reactive chemicals with unpaired electrons that can scramble the structure of proteins and nucleic acids. Although free radicals are normal by-products of cellular metabolism, if allowed to accumulate, they can have devastating effects on cells. Peroxisomes convert free radicals to hydrogen peroxide (H2O2), a function indicated in their naming (peroxisomes 5 peroxide bodies). The enzyme catalase (kat9ah-l¯as) then converts excess hydrogen peroxide to water. Peroxisomes are especially numerous in liver and kidney cells, which are very active in detoxification.
Although peroxisomes look like small lysosomes (Figure 3.4), they do not arise by budding from the Golgi apparatus. Instead, they appear to replicate themselves by simply pinching in half.
CytoskeletonAn elaborate network of protein structures extends throughout the cytoplasm (see Figures 3.2 and 3.4). This network, or cytoskeleton, acts as a cell’s “bones and muscles” by furnishing an internal framework that determines cell shape, supports other organelles, and provides the machinery needed for intracellular transport and various types of cellular movements. From its largest to its smallest elements, the cytoskeleton is made up of microtubules, intermediate filaments, and microfilaments (Figure 3.6). Although there is some overlap in roles, generally speaking the strong, stable ropelike intermediate filaments help form desmosomes (see Figure 3.3) and provide internal guy wires to resist pulling forces on the cell. Microfilaments (such as actin and myosin) are most involved in cell motility and in producing changes in cell shape. You could say that cells move when they get their act(in) together. The tubelike microtubules determine the overall shape of a cell and the distribution of organelles. They are very important during cell division, as described later.
CentriolesThe paired centrioles (sen9tre-¯olz) lie close to the nucleus. They are rod-shaped bodies that lie at right angles to each other; internally they are made up of fine microtubules. During cell division, the centrioles direct the formation of the mitotic spindle.
In addition to the cell structures described above, some cells have projections called cilia (sil9e-ah; “eyelashes”), whiplike cellular extensions that move substances along the cell surface. For example, the ciliated cells of the respiratory system lining move mucus up and away from the lungs. Where cilia appear, there are usually many of them projecting from the exposed cell surface. When a cell is about to make cilia, its centrioles multiply and then line up beneath the plasma membrane at the free cell surface. Microtubules then begin to “sprout” from the centrioles and put pressure on the membrane, forming the projections. When the projections formed by the centrioles are substantially longer, they are called flagella (flah-jel9ah). The only example of a flagellated cell in the human body is the sperm, which has a single propulsive flagellum called its tail (Figure 3.7). Notice that cilia propel other substances across a cell’s surface, whereas a flagellum propels the cell itself.
So far in this chapter, we have focused on an “average” human cell. However, the trillions of cells in the human body are made up of 200 different cell types that vary greatly in size, shape, and function. They include sphere-shaped fat cells, disk-shaped red blood cells, branching nerve cells, and cube-shaped cells of kidney tubules. Figure 3.7 illustrates how the shapes of cells and the relative numbers of the various organelles they contain relate to specialized cell functions. Let’s take a look at some of these cell “specialists.”
1. Cells that connect body parts:
Fibroblast. The elongated shape of this cell lies along the cable-like fibers that it secretes. It also has an abundant rough ER and a large Golgi apparatus, to make and secrete the protein building blocks of these fibers.
Erythrocyte (red blood cell). This cell carries oxygen in the bloodstream. Its concave disk shape provides extra surface area for the uptake of oxygen and streamlines the cell so it flows easily through the bloodstream. So much oxygen-carrying pigment is packed in erythrocytes that all other organelles have been shed to make room.
2. Cells that cover and line body organs:
Epithelial cell. The hexagonal shape of this cell is exactly like a “cell” in a honeycomb of a beehive. This shape allows epithelial cells to pack together in sheets. An epithelial cell has abundant intermediate filaments that resist tearing when the epithelium is rubbed or pulled.
3. Cells that move organs and body parts:
Skeletal muscle and smooth muscle cells. These cells are elongated and filled with abundant contractile filaments, so they can shorten forcefully and move the bones or change the size of internal organs.
4. Cell that stores nutrients:
Fat cell. The huge spherical shape of a fat cell is produced by a large lipid droplet in its cytoplasm.
5. Cell that fights disease:
Macrophage (a phagocytic cell). This cell extends long pseudopods (“false feet”) to crawl through tissue to reach infection sites. The many lysosomes within the cell digest the infectious microorganisms it takes up.
6. Cell that gathers information and controls body functions:
Nerve cell (neuron). This cell has long processes for receiving messages and transmitting them to other structures in the body. The processes are covered with an extensive plasma membrane and a plentiful rough ER is present to synthesize membrane components.
7. Cells of reproduction:
Oocyte (female). The largest cell in the body, this egg cell contains many copies of all organelles, for distribution to the daughter cells that arise when the fertilized egg divides to become an embryo.
Sperm (male). This cell is long and streamlined, built for swimming to the egg for fertilization. Its flagellum acts as a motile whip to propel the sperm.
As mentioned earlier, each of the cell’s internal parts is designed to perform a specific function for the cell. Most cells have the ability to metabolize (use nutrients to build new cell material, break down substances, and make ATP), digest foods, dispose of wastes, reproduce, grow, move, and respond to a stimulus (irritability). Most of these functions are considered in detail in later chapters. For example, metabolism is covered in Chapter 14, and the ability to react to a stimulus is covered in Chapter 7. Here, we will consider only the functions of membrane transport (the means by which substances get through plasma membranes), protein synthesis, and cell reproduction (cell division).
The fluid environment on both sides of the plasma membrane is an example of a solution. Although the various types of mixtures were mentioned in Chapter 2 (p. 36), it is important that you really understand solutions before we dive into an explanation of membrane transport. In the most basic sense, a solution is a homogeneous mixture of two or more components. Examples include the air we breathe (a mixture of gases), seawater (a mixture of water and salts), and rubbing alcohol (a mixture of water and alcohol). The substance present in the largest amount in a solution is called the solvent (or dissolving medium). Water is the body’s chief solvent. Components or substances present in smaller amounts are called solutes.
Intracellular fluid (collectively, the nucleoplasm and the cytosol) is a solution containing small amounts of gases (oxygen and carbon dioxide), nutrients, and salts, dissolved in water. So too is interstitial fluid, the fluid that continuously bathes the exterior of our cells. Interstitial fluid can be thought of as a rich, nutritious, and rather unusual “soup.” It contains thousands of ingredients, including nutrients (amino acids, sugars, fatty acids, vitamins), regulatory substances such as hormones and neurotransmitters, salts, and waste products. To remain healthy, each cell must extract from this soup the exact amounts of the substances it needs at specific times and reject the rest.
The plasma membrane is a selectively permeable barrier. Selective permeability means that a barrier allows some substances to pass through it while excluding others. Thus, it allows nutrients to enter the cell but keeps many undesirable substances out. At the same time, valuable cell proteins and other substances are kept within the cell, and wastes are allowed to pass out of it.
Homeostatic ImbalanceThis property of selective permeability is typical only of healthy, unharmed cells. When a cell dies or is badly damaged, its plasma membrane can no longer be selective and becomes permeable to nearly everything. This phenomenon is evident when someone has been severely burned. Precious fluids, proteins, and ions “weep” from the dead and damaged cells of the burned areas. s
Movement of substances through the plasma membrane happens in basically two ways—passively or actively. In passive transport processes, substances are transported across the membrane without any energy input from the cell. In active transport processes, the cell provides the metabolic energy (ATP) that drives the transport process.
Diffusion (d˘ı-fu9zhun) is an important means of passive membrane transport for every cell of the body. The other passive transport process, filtration, generally occurs only across capillary walls. Let us examine how these two types of passive transport differ.
DiffusionDiffusion is the process by which molecules (and ions) tend to scatter themselves throughout the available space. All molecules possess kinetic energy (energy of motion), as described in Chapter 2. As the molecules move about randomly at high speeds, they collide and change direction with each collision. Since the overall effect of this erratic movement is that molecules move away from a region where they are more concentrated (more numerous) to a region where they are less concentrated (fewer of them), we say that molecules move down their concentration gradient. Because the driving force (source of energy) is the kinetic energy of the molecules themselves, the speed of diffusion is affected by the size of the molecules (the smaller the faster) and temperature (the warmer the faster).
An example should help you understand diffusion. Picture yourself pouring a cup of coffee, and then adding a cube of sugar (but not stirring the cup). After adding the sugar, the phone rings and you are called in to work. You never do get to drink the coffee. Upon returning that evening, you find that the coffee tastes sweet even though it was never stirred. This is because the sugar molecules moved around all day and eventually, as a result of their activity, became equally distributed throughout the coffee. A laboratory example that might be familiar to some students is illustrated in Figure 3.8. The plasma membrane is a physical barrier to diffusion. Molecules will move passively through the plasma membrane by diffusion if (1) they are small enough to pass through its pores, or (2) they can dissolve in the fatty portion of the membrane (see Figure 3.9).
The unassisted diffusion of solutes through the plasma membrane (or any selectively permeable membrane) is called simple diffusion (Figure 3.9a). Solutes transported this way are either lipid-soluble (fats, fat-soluble vitamins, oxygen, carbon dioxide) or small enough to pass through the membrane pores (some small ions such as chloride ions, for example).
Diffusion of water through a selectively permeable membrane such as the plasma membrane is specifically called osmosis (oz-mo9sis). Because water is highly polar, it is repelled by the (nonpolar) lipid core of the plasma membrane, but it can and does pass easily through the pores created by the proteins in the membrane (Figure 3.9a). Osmosis into and out of cells is occurring all the time as water moves down its concentration gradient.
Still another example of diffusion is facilitated diffusion (see Figure 3.9b). Facilitated diffusion provides a means for certain needed substances, notably glucose, that are both lipid-insoluble and too large to pass through the membrane pores, to enter the cell. Although facilitated diffusion follows the “laws” of diffusion—that is, the substances move down their concentration gradient—a protein “carrier” is needed as a transport vehicle. Hence, some of the proteins in the plasma membrane act as carriers to move glucose passively across the membrane and make it available for cell use.
Substances that pass into and out of cells by diffusion save the cell a great deal of energy. When you consider how vitally important water, glucose, and oxygen are to cells, it becomes apparent just how necessary these passive transport processes really are. Glucose and oxygen continually move into the cells (where they are in lower concentration because the cells keep using them up), and carbon dioxide (a waste product) continually moves out of the cells into the blood (where it is in lower concentration).
FiltrationFiltration is the process by which water and solutes are forced through a membrane (or capillary wall) by fluid, or hydrostatic, pressure. In the body, hydrostatic pressure is usually exerted by the blood. Like diffusion, filtration is a passive process, and a gradient is involved. In filtration, however, the gradient is a pressure gradient that actually pushes solute-containing fluid (filtrate) from the higher-pressure area to the lower-pressure area. Filtration is necessary for the kidneys to do their job properly. In the kidneys, water and small solutes filter out of the capillaries into the kidney tubules because the blood pressure in the capillaries is greater than the fluid pressure in the tubules. Part of the filtrate formed in this way eventually becomes urine. Filtration is not very selective. For the most part, only blood cells and protein molecules too large to pass through the membrane pores are held back.
Active Transport Processes
Whenever a cell uses some of its ATP supply to move substances across the membrane, the process is referred to as active. Substances moved actively are usually unable to pass in the desired direction by diffusion. They may be too large to pass through the pores, they may not be able to dissolve in the fat core, or they may have to move “uphill” against their concentration gradients. The two most important examples of active transport mechanisms, solute pumping and bulk transport, are described next.
Solute PumpingSolute pumping (more simply called active transport by some) is similar to facilitated diffusion in that both processes require protein carriers that combine reversibly with the substances to be transported across the membrane. However, facilitated diffusion is driven by the kinetic energy of the diffusing molecules, whereas solute pumping uses ATP to energize its protein carriers, which are called solute pumps. Amino acids, some sugars, and most ions are transported by solute pumps, and in most cases these substances move against concentration (or electrical) gradients. This is opposite to the direction in which substances would naturally flow by diffusion, which explains the need for energy in the form of ATP. Amino acids are needed to build cellular proteins but are too large to pass through the pores and are not lipid-soluble. The sodium-potassium pump that simultaneously carries sodium ions out of and potassium ions into the cell is absolutely necessary for normal transmission of impulses by nerve cells. Sodium ions (Na+) are moved out of cells by solute pumps (Figure 3.10). There are more sodium ions outside the cells than inside, so they tend to remain in the cell unless the cell uses ATP to force, or “pump,” them out. Likewise, there are relatively more potassium ions inside cells than in the interstitial (extracellular) fluid, and potassium ions that leak out of cells must be actively pumped back inside. Since each of the pumps in the plasma membrane transports only specific substances, solute pumping provides a way for the cell to be very selective in cases where substances cannot pass by diffusion. (No pump—no transport.)
Bulk TransportSome substances that cannot get through the plasma membrane in any other way are transported with the help of ATP out of or into cells by bulk transport. The two types of bulk transport are exocytosis and endocytosis.
Exocytosis (ek0so-si-to9sis; “out of the cell”) moves substances out of cells (Figure 3.11). It is the means by which cells actively secrete hormones, mucus, and other cell products or eject certain cellular wastes. The product to be released is first “packaged” (typically by the efforts of the Golgi apparatus) into a small membrane vesicle or sac. The sac migrates to the plasma membrane and fuses with it. The fused area then ruptures, spilling the sac contents out of the cell (also see Figure 3.5).
Endocytosis (en0do-si-to9sis; “into the cell”) includes those ATP-requiring processes that take up, or engulf, extracellular substances by enclosing them in a small membranous vesicle (Figure 3.12). Once the vesicle, or sac, is formed, it detaches from the plasma membrane and moves into the cytoplasm, where it fuses with a lysosome and its contents are digested (by lysosomal enzymes). If the engulfed substances are relatively large particles such as bacteria or dead body cells, which are separated from the external environment by flowing cytoplasmic extensions called pseudopods, the endocytosis process is more specifically called phagocytosis (fag0o-si-to9sis), a term that means “cell eating.” Certain white blood cells and other “professional” phagocytes of the body act as scavenger cells that police and protect the body by ingesting bacteria and other foreign debris as well as dead body cells.
If we say that cells can eat, we can also say that they can drink. This is bulk-phase endocytosis, formerly called pinocytosis (pi0no-si-to9sis; “cell drinking”). In this process the plasma membrane invaginates to form a tiny pit and then its edges fuse around the droplet of extracellular fluid containing dissolved proteins or fats (Figure 3.12b). Unlike phagocytosis, it is a routine activity of most cells. It is especially important in cells that function in absorption (for example, cells forming the lining of the small intestine and kidney tubule cells).
The cell life cycle is the series of changes a cell goes through from the time it is formed until it divides. The cycle has two major periods: interphase, in which the cell grows and carries on its usual metabolic activities, and cell division, during which it reproduces itself. Although the term interphase leads one to believe that it is merely a resting time between the phases of cell division, this is not the case. During interphase, which is by far the longer phase of the cell cycle, the cell is very active and is resting only from division. A more accurate name for interphase would be metabolic phase.
Preparations: DNA Replication
The function of cell division is to produce more cells for growth and repair processes. Because it is essential that all body cells have the same genetic material, an important event always precedes cell division: The genetic material (the DNA molecules that form part of the chromatin) is duplicated exactly. This occurs toward the end of the cell’s interphase period.
You will recall from Chapter 2 that DNA is a very complex molecule (review Figure 2.17). It is composed of building blocks called nucleotides, each consisting of deoxyribose sugar, a phosphate group, and a nitrogen-containing base. Essentially DNA is a double helix, a ladderlike molecule that is coiled into a spiral staircase shape. The upright parts of the DNA “ladder” are alternating phosphate and sugar units, and the rungs of the ladder are made of pairs of nitrogen-containing bases.
The precise trigger for DNA synthesis is unknown, but once it starts, it continues until all the DNA has been replicated. The process begins as the DNA helix uncoils and gradually separates into its two nucleotide chains (see Figure 3.13). Each nucleotide strand then serves as a template, or set of instructions, for building a new nucleotide strand.
Remember that nucleotides join in a complementary way: adenine (A) always bonds to thy-mine (T), and guanine (G) always bonds to cytosine (C) (see Chapter 2). Hence, the order of the nucleotides on the template strand also determines the order on the new strand. For example, a TACTGC sequence on a template strand would bond to new nucleotides with the order ATGACG. The end result is that two DNA molecules are formed that are identical to the original DNA helix, and each consists of one old and one newly assembled nucleotide strand.
Events of Cell Division
In all cells other than bacteria and some cells of the reproductive system, cell division consists of two events. Mitosis (mi-to9sis), or division of the nucleus, occurs first. The second event is division of the cytoplasm, cytokinesis (si0to-k˘ı-ne9sis), which begins when mitosis is nearly completed.
MitosisMitosis results in the formation of two daughter nuclei with exactly the same genes as the mother nucleus. As explained above, DNA replication precedes mitosis, so that for a short time the cell nucleus contains a “double dose” of genes. When the nucleus divides, each daughter cell ends up with exactly the same genetic information as the original mother cell and the original fertilized egg from which it came.
The stages of mitosis, diagrammed in Figure 3.14, include the following events:
• Prophase (pro9faz). As cell division begins, the chromatin threads coil and shorten so that visible barlike bodies called chromosomes (chromo 5 colored; soma 5 body) appear. Because DNA replication has already occurred, each chromosome is actually made up of two strands, each called a chromatid (kro9mah-tid), held together by a small buttonlike body called a centromere (sen9tro-mer). The centrioles separate from each other and begin to move toward opposite sides of the cell, directing the assembly of the mitotic spindle (composed of thin microtubules) between them as they move. The spindle provides a “scaffolding” for the attachment and movement of the chromosomes during the later mitotic stages. By the end of prophase, the nuclear envelope and the nucleoli have broken down and disappeared, and the chromosomes have become attached randomly to the spindle fibers by their centromeres.
• Metaphase (met9ah-faz). In this short stage, the chromosomes cluster and become aligned at the center of the spindle midway between the centrioles so that a straight line of chromosomes is seen.
• Anaphase (an9ah-faz). During anaphase, the centromeres that have held the chromatids together split, and the chromatids (now called chromosomes again) begin to move slowly apart, toward opposite ends of the cell. The chromosomes seem to be pulled by their half-centromeres, with their “arms” dangling behind them. Anaphase is over when chromosome movement ends.
• Telophase (tel9o-faz). Telophase is essentially prophase in reverse. The chromosomes at opposite ends of the cell uncoil to become threadlike chromatin again. The spindle breaks down and disappears, a nuclear envelope forms around each chromatin mass, and nucleoli appear in each of the daughter nuclei.
Mitosis is basically the same in all animal cells. Depending on the type of tissue, it takes from 5 minutes to several hours to complete, but typically it lasts about 2 hours. Centriole replication is deferred until late interphase of the next cell cycle, when DNA replication begins before the onset of mitosis.
CytokinesisCytokinesis, or the division of the cytoplasm, usually begins during late anaphase and completes during telophase. Due to the activity of a contractile ring made of microfilaments, a cleavage furrow appears over the midline of the spindle, and it eventually squeezes or pinches the original cytoplasmic mass into two parts.
Thus, at the end of cell division, two daughter cells exist. Each is smaller and has less cytoplasm than the mother cell, but it is genetically identical to it. The daughter cells grow and carry out normal cell activities until it is their turn to divide.
Although mitosis and division of the cytoplasm usually go hand in hand, in some cases the cytoplasm is not divided. This condition leads to the formation of binucleate (two nuclei) or multinucleate cells. This is fairly common in the liver.
As mentioned earlier, mitosis provides the “new” cells for body growth in youth and is necessary to repair body tissue all through life. Mitosis “gone wild” is the basis for tumors and cancers.
Genes: The Blueprint for Protein Structure
In addition to replicating itself for cell division, DNA serves as the master blueprint for protein syntheses. A gene is a DNA segment that carries the information for building one protein or polypeptide chain.
Proteins are key substances for all aspects of cell life. As described in Chapter 2, fibrous (structural) proteins are the major building materials for cells. Other proteins, the globular functional proteins, do things other than build structures. For example, all enzymes, biological catalysts that regulate chemical reactions in the cells, are functional proteins. The importance of enzymes cannot be overstated. Every chemical reaction that goes on in the body requires an enzyme. It follows that DNA regulates cell activities largely by specifying the structure of enzymes, which in turn control or direct the chemical reactions in which carbohydrates, fats, other proteins, and even DNA itself are made and broken down.
How does DNA bring about its miracles? It appears that DNA’s information is “encoded” in the sequence of bases along each side of the ladderlike DNA molecules. Each sequence of three bases (a triplet) calls for a particular amino acid (See Figure 3.15). (Amino acids are the building blocks of proteins that are joined during protein synthesis.) For example, a DNA base sequence of AAA specifies an amino acid called phenylalanine, while CCT calls for glycine. Just as different arrangements of notes on sheet music are played as different melodies, variations in the arrangements of A, C, T, and G in each gene allow cells to make all the different kinds of proteins needed. It has been estimated that a single gene has between 300 and 3000 base pairs in sequence.
The Role of RNA
By itself, DNA is rather like a strip of magnetic recording tape; its information is not useful until it is decoded. Furthermore, ribosomes—the man-ufacturing sites for proteins—are in the cytoplasm, but in interphase cells DNA never leaves the nucleus. Thus, DNA requires not only a decoder but also a messenger to achieve its task of specifying the structure of proteins to be built at the ribosomes. These messenger and decoder functions are carried out by the second type of nucleic acid, called ribonucleic (ri0bo-nu-kle9ik) acid, or RNA.
As you learned in Chapter 2, RNA differs from DNA in being single-stranded and in having ribose sugar instead of deoxyribose and a uracil (U) base instead of thymine (T). There are three varieties of RNA, and as described shortly, each has a special role to play in protein synthesis. Transfer RNA (tRNA) molecules are small cloverleaf-shaped molecules. Ribosomal RNA (rRNA) helps form the ribosomes, where proteins are built. Messenger RNA (mRNA) molecules are long, single nucleotide strands that resemble half of a DNA molecule and carry the “message” containing instructions for protein synthesis from the DNA gene in the nucleus to the ribosomes in the cytoplasm.
Essentially, protein synthesis involves two major phases: transcription, when complementary mRNA is made at the DNA gene, and translation, when the information carried in mRNA molecules is “decoded” and used to assemble proteins. These steps are summarized simply in Figure 3.15, and described in more detail next.
The word transcription often refers to one of the jobs done by a secretary—converting notes from one form (shorthand notes or a dictaphone recording) into another form (a typewritten letter, for example). In other words, the same information is transformed from one form or format to another. In cells, transcription involves the transfer of information from DNA’s base sequence into the complementary base sequence of mRNA (Figure 3.15, step 1). Only DNA and mRNA are involved in transcription. Whereas each three-base sequence specifying a particular amino acid on the DNA gene is called a triplet, the corresponding three-base sequences on mRNA are called codons. The form is different, but the same information is being conveyed. Thus, if the (partial) sequence of DNA triplets is AAT-CGT-TCG, the related codons on mRNA would be UUA-GCA-AGC.
A translator takes words in one language and restates them in another language. In the translation phase of protein synthesis, the language of nucleic acids (base sequence) is “translated” into the language of proteins (amino acid sequence). Translation occurs in the cytoplasm and involves all three varieties of RNA. As illustrated in Figure 3.15, steps 2–5, translation consists of the following events. Once the mRNA attaches to the ribosome (see step 2), tRNA comes into the picture. Its job is to ferry, or “transfer,” amino acids to the ribosome, where they are bound together by enzymes in the exact sequence specified by the gene (and its mRNA). There are some 20 common types of tRNAs, each capable of carrying one of the 20 or so common types of amino acid to the protein synthesis sites. But that is not the only job of the tiny tRNAs. They also have to recognize the mRNA codons “calling for” the amino acid they are toting. They can do this because they have a special three-base sequence called an anticodon on their “head” that can bind to the complementary codons (step 3).
Once the first tRNA has maneuvered itself into the correct position at the beginning of the mRNA message, the ribosome moves the mRNA strand along, bringing the next codon into position to be “read” by another tRNA. As amino acids are brought to their proper positions along the length of mRNA, they are joined together by enzymes (step 4). As an amino acid is bonded to the chain, its tRNA is released and moves away from the ribosome to pick up another amino acid (step 5). When the last codon (the termination or “stop” codon)is read, the protein is released.
Part 2: Body Tissues
The human body, complex as it is, starts out as a single cell, the fertilized egg, which divides almost endlessly. The millions of cells that result become specialized for particular functions. Some become muscle cells, others the transparent lens of the eye, still others skin cells, and so on. Thus, there is a division of labor in the body, with certain groups of highly specialized cells performing functions that benefit the organism as a whole and contribute to homeostasis.
Cell specialization carries with it certain hazards. When a small group of cells is indispensable, its loss can disable or even destroy the body. For example, the action of the heart depends on a very specialized cell group in the heart muscle that controls its contractions. If those particular cells are damaged or stop functioning, the heart will no longer work efficiently, and the whole body will suffer or die from lack of oxygen.
Groups of cells that are similar in structure and function are called tissues. The four primary tissue types—epithelium, connective tissue, nervous tissue, and muscle—interweave to form the “fabric” of the body. If we had to assign a single term to each primary tissue type that would best describe its overall role, the terms would most likely be covering (epithelium), support (connective), movement (muscle), and control (nervous). However, these terms reflect only a tiny fraction of the functions that each of these tissues performs.
As explained in Chapter 1, tissues are organized into organs such as the heart, kidneys, and lungs. Most organs contain several tissue types, and the arrangement of the tissues determines each organ’s structure and what it is able to do. Thus, a study of tissues should be helpful in your later study of the body’s organs and how they work.
For now, we want to become familiar with the major similarities and differences in the primary tissues. Because epithelium and some types of connective tissue will not be considered again, they are emphasized more in this section than are muscle, nervous tissues, and bone (a connective tissue), which are covered in more depth in later chapters.
Epithelial tissue, or epithelium (ep0˘ı-the9le-um; epithe 5 laid on, covering) is the lining, covering, and glandular tissue of the body. Glandular epithelium forms various glands in the body. Covering and lining epithelium covers all free body surfaces and contains versatile cells. One type forms the outer layer of the skin. Others dip into the body to line its cavities. Since epithelium forms the boundaries that separate us from the outside world, nearly all substances given off or received by the body must pass through epithelium.
Epithelial functions include protection, absorption, filtration, and secretion. For example, the epithelium of the skin protects against bacterial and chemical damage, and that lining the respiratory tract has cilia, which sweep dust and other debris away from the lungs. Epithelium specialized to absorb substances lines some digestive system organs such as the stomach and small intestine, which absorb food into the body. In the kidneys, epithelium both absorbs and filters. Secretion is a specialty of the glands, which produce such substances as perspiration, oil, digestive enzymes, and mucus.
Special Characteristics of Epithelium
Epithelium generally has the characteristics listed below:
• Epithelial cells fit closely together to form continuous sheets. Neighboring cells are bound together at many points by cell junctions, including desmosomes and tight junctions.
• The membranes always have one free (unattached) surface or edge. This so-called apical surface is exposed to the body’s exterior or to the cavity of an internal organ. The exposed surfaces of some epithelia are slick and smooth, but others exhibit cell surface modifications, such as microvilli or cilia.
• The lower surface of an epithelium rests on a basement membrane, a structureless material secreted by the cells.
• Epithelial tissues have no blood supply of their own (that is, they are avascular) and depend on diffusion from the capillaries in the underlying connective tissue for food and oxygen.
• If well nourished, epithelial cells regenerate themselves easily.
Classification of Epithelium
Each epithelium is given two names. The first indicates the relative number of cell layers it has (Figure 3.16a). The classifications by cell arrangement (layers) are simple epithelium (one layer of cells) and stratified epithelium (more than one cell layer). The second describes the shape of its cells (Figure 3.16b). On this basis there are squamous (skwa9mus) cells, flattened like fish scales (squam 5 scale), cuboidal (ku-boi9dal) cells, which are cube-shaped like dice, and columnar cells, shaped like columns. The terms describing the shape and arrangement are then combined to describe the epithelium fully. Stratified epithelia are named for the cells at the free surface of the epithelial membrane, not those resting on the basement membrane.
The simple epithelia are most concerned with absorption, secretion, and filtration. Because simple epithelia are usually very thin, protection is not one of their specialties.
Simple Squamous EpitheliumSimple squamous epithelium is a single layer of thin squamous cells resting on a basement membrane. The cells fit closely together, much like floor tiles. This type of epithelium usually forms membranes where filtration or exchange of substances by rapid diffusion occurs. It is in the air sacs of the lungs, where oxygen and carbon dioxide are exchanged (see Figure 3.17a), and it forms the walls of capillaries, where nutrients and gases pass between the tissue cells and the blood in the capillaries. Simple squamous epithelium also forms the serous membranes, or serosae (se-ro9se), the slick membranes that line the ventral body cavity and cover the organs in that cavity. The serous membranes are described in more detail in Chapter 4.
Simple Cuboidal EpitheliumSimple cuboidal epithelium, which is one layer of cuboidal cells resting on a basement membrane, is common in glands and their ducts (for example, the salivary glands and pancreas). It also forms the walls of the kidney tubules and covers the surface of the ovaries (see Figure 3.17b).
Simple Columnar EpitheliumSimple columnar epithelium is made up of a single layer of tall cells that fit closely together. Goblet cells, which produce a lubricating mucus, are often seen in this type of epithelium. Simple columnar epithelium lines the entire length of the digestive tract from the stomach to the anus (see Figure 3.17c). Epithelial membranes that line body cavities open to the body exterior are called mucosae (mu-ko9se) or mucous membranes.
Pseudostratified Columnar EpitheliumAll of the cells of pseudostratified (soo0do-str˘a9t˘ı-f¯ıd) columnar epithelium rest on a basement membrane. However, some of its cells are shorter than others, and their nuclei appear at different heights above the basement membrane. As a result, this epithelium gives the false (pseudo) impression that it is stratified; hence its name. Like simple columnar epithelium, this variety mainly functions in absorption and secretion. A ciliated variety (more precisely called pseudostratified ciliated columnar epithelium) lines most of the respiratory tract (see Figure 3.17d). The mucus produced by the goblet cells in this epithelium traps dust and other debris, and the cilia propel the mucus upward and away from the lungs.
Stratified epithelia consist of two or more cell layers. Being considerably more durable than the simple epithelia, these epithelia function primarily to protect.
Stratified Squamous EpitheliumStratified squamous epithelium is the most common stratified epithelium in the body. It usually consists of several layers of cells. The cells at the free edge are squamous cells, whereas those close to the basement membrane are cuboidal or columnar. Stratified squamous epithelium is found in sites that receive a good deal of abuse or friction, such as the esophagus, the mouth, and the outer portion of the skin (Figure 3.17e).
Stratified Cuboidal and Stratified Columnar Epithelia Stratified cuboidal epithelium usually has just two cell layers with (at least) the surface cells being cuboidal in shape. The surface cells of stratified columnar epithelium are columnar cells, but its basal cells vary in size and shape. Both of these epithelia are fairly rare in the body, being found mainly in the ducts of large glands. (Because of the extremely limited distribution of these two epithelia, they are not illustrated in Figure 3.17. They are described here only to provide a complete listing of the epithelial tissues.)
Transitional EpitheliumTransitional epithelium is a highly modified, stratified squamous epithelium that forms the lining of only a few organs—the urinary bladder, the ureters, and part of the urethra. All these organs are part of the urinary system and are subject to considerable stretching (Figure 3.17f). Cells of the basal layer are cuboidal or columnar; those at the free surface vary in appearance. When the organ is not stretched, the membrane is many-layered, and the superficial cells are rounded and domelike. When the organ is distended with urine, the epithelium thins, and the surface cells flatten and become squamouslike. This ability of transitional cells to slide past one another and change their shape allows the ureter wall to stretch as a greater volume of urine flows through the tubelike organ. In the bladder, it allows more urine to be stored.
A gland consists of one or more cells that make and secrete a particular product. This product, called a secretion, typically contains protein molecules in an aqueous (water-based) fluid. Secretion is an active process in which the glandular cells obtain needed materials from the blood and use them to make their secretion, which they then discharge.
Two major types of glands develop from epi-thelial sheets. Endocrine (en9do-krin) glands lose their connection to the surface (duct); thus they are often called ductless glands. Their secretions (all hormones) diffuse directly into the blood vessels that weave through the glands. Examples of endocrine glands include the thyroid, adrenals, and pituitary.
Exocrine (ek9so-krin) glands retain their ducts, and their secretions empty through the ducts to the epithelial surface. Exocrine glands, which include the sweat and oil glands, liver, and pancreas, are both internal and external. They are discussed with the organ systems to which their products are related.
Connective tissue, as its name suggests, connects body parts. It is found everywhere in the body. It is the most abundant and widely distributed of the tissue types.
The characteristics of connective tissue include the following:
• Variations in blood supply. Most connective tissues are well vascularized (that is, they have a good blood supply), but there are exceptions. Tendons and ligaments have a poor blood supply, and cartilages are avascular. Consequently, all these structures heal very slowly when injured. (This is why some people say that, given a choice, they would rather have a broken bone than a torn ligament.)
• Extracellular matrix. Connective tissues are made up of many different types of cells plus varying amounts of a nonliving substance found outside the cells, called the extracellular matrix.
The extracellular matrix deserves a bit more explanation because it is what makes connective tissue so different from the other tissue types. The matrix, which is produced by the connective tissue cells and then secreted to their exterior, has two main elements, a structureless ground substance and fibers. The ground substance of the matrix is composed largely of water plus some adhesion proteins and large, charged polysaccharide molecules. The cell adhesion proteins serve as a glue that allows the connective tissue cells to attach themselves to the matrix fibers embedded in the ground substance. The charged polysaccharide molecules trap water as they intertwine. As the relative abundance of these polysaccharides increases, they cause the matrix to vary from fluid to gel-like to firm to rock-hard in its consistency. Various types and amounts of fibers deposited in the matrix and forming part of it include collagen (white) fibers, elastic (yellow) fibers, and reticular (fine collagen) fibers depending on the connective tissue type.
Because of its extracellular matrix, connective tissue is able to form a soft packing tissue around other organs, to bear weight, and to withstand stretching and other abuses, such as abrasion, that no other tissue could endure. But there is variation. At one extreme, fat tissue is composed mostly of cells, and the matrix is soft. At the opposite extreme, bone and cartilage have very few cells and large amounts of hard matrix, which makes them extremely strong.
Connective tissues perform many functions, but they are primarily involved in protecting, supporting, and binding together other body tissues. A less well known function of the matrix (ground substance) is its ability to absorb large amounts of water and thus serve as a water reservoir in the body. Find the various types of connective tissues in Figure 3.18 as you read their descriptions that follow.
Types of Connective Tissue
As noted above, all connective tissues consist of living cells surrounded by a matrix. Their major differences reflect fiber type and the number of fibers in the matrix. From most rigid to softest, the major connective tissue classes are bone, cartilage, dense connective tissue, loose connective tissue, and blood. Each of these is described briefly next.
Bone, sometimes called osseous (os9e-us) tissue, is composed of bone cells sitting in cavities called lacunae (lah-ku9ne) and surrounded by layers of a very hard matrix that contains calcium salts in addition to large numbers of collagen fibers. Because of its rocklike hardness, bone has an exceptional ability to protect and support other body organs (for example, the skull protects the brain).
Cartilage is less hard and more flexible than bone. It is found in only a few places in the body. Most widespread is hyaline (hi9ah-lin) cartilage, which has abundant collagen fibers hidden by a rubbery matrix with a glassy (hyalin 5 glass), blue-white appearance. It forms the supporting structures of the larynx, or voice box, attaches the ribs to the breastbone, and covers the ends of bones where they form joints. The skeleton of a fetus is made of hyaline cartilage, but by the time the baby is born most of that cartilage has been replaced by bone.
Although hyaline cartilage is the most abundant type of cartilage in the body, there are others. Highly compressible fibrocartilage forms the cushionlike disks between the vertebrae of the spinal column. Elastic cartilage is found where a structure with elasticity is desired. For example, it supports the external ear. (Elastic cartilage is not illustrated in Figure 3.18.)
Dense Connective Tissue
Dense connective tissue, also called dense fibrous tissue, has collagen fibers as its main matrix element. Crowded between the collagen fibers are rows of fibroblasts (fiber-forming cells) that manufacture the fibers. Dense connective tissue forms strong, ropelike structures such as tendons and ligaments. Tendons attach skeletal muscles to bones; ligaments connect bones to bones at joints. Ligaments are more stretchy and contain more elastic fibers than tendons. Dense connective tissue also makes up the lower layers of the skin (dermis), where it is arranged in sheets.
Loose Connective Tissue
Relatively speaking, the loose connective tissues are softer and have more cells and fewer fibers than any other connective tissue type except blood.
Areolar TissueAreolar (ah-re9o-lar) tissue, the most widely distributed connective tissue variety in the body, is a soft, pliable, “cobwebby” tissue that cushions and protects the body organs it wraps. It functions as a universal packing tissue and connective tissue “glue” because it helps to hold the internal organs together and in their proper positions. A soft layer of areolar connective tissue called the lamina propria (lah9m˘ı-nah pro9pre-ah) underlies all mucous membranes. Its fluid matrix contains all types of fibers, which form a loose network. In fact, when viewed through a microscope, most of the matrix appears to be empty space, which explains the name of this tissue type (areola 5 small open space). Because of its loose and fluid nature, areolar connective tissue provides a reservoir of water and salts for the surrounding tissues, and essentially all body cells obtain their nutrients from and release their wastes into this “tissue fluid.” When a body region is inflamed, the areolar tissue in the area soaks up the excess fluid like a sponge, and the area swells and becomes puffy, a condition called edema. Many types of phagocytes wander through this tissue scavenging for bacteria, dead cells, and other “debris,” which they destroy.
Adipose TissueAdipose (ad9˘ı-p¯os) tissue is commonly called fat. Basically, it is an areolar tissue in which fat cells predominate. A glistening droplet of stored oil occupies most of a fat cell’s volume and compresses the nucleus, displacing it to one side. Since the oil-containing region looks empty, and the thin rim of cytoplasm containing the bulging nucleus looks like a ring with a seal, fat cells are sometimes called signet ring cells.
Adipose tissue forms the subcutaneous tissue beneath the skin, where it insulates the body and protects it from extremes of both heat and cold. Adipose tissue also protects some organs individually. For example, the kidneys are surrounded by a capsule of fat, and adipose tissue cushions the eyeballs in their sockets. There are also fat “depots” in the body, such as the hips and breasts, where fat is stored and available for fuel if needed.
Reticular Connective TissueReticular connective tissue consists of a delicate network of interwoven reticular fibers associated with reticular cells, which resemble fibroblasts. Reticular tissue is limited to certain sites: It forms the stroma (literally, bed or mattress), or internal supporting framework, which can support many free blood cells (largely lymphocytes) in lymphoid organs such as lymph nodes, the spleen, and bone marrow.
Blood, or vascular tissue, is considered a connective tissue because it consists of blood cells, surrounded by a nonliving, fluid matrix called blood plasma. The “fibers” of blood are soluble protein molecules that become visible only during blood clotting. Still, we must recognize that blood is quite atypical as connective tissues go. Blood is the transport vehicle for the cardiovascular system, carrying nutrients, wastes, respiratory gases, and many other substances throughout the body. Blood is considered in detail in Chapter 10.
Muscle tissues are highly specialized to contract, or shorten, to produce movement. Because muscle cells are elongated to provide a long axis for contraction, they are called muscle fibers.
Types of Muscle Tissue
The three types of muscle tissue are illustrated in Figure 3.19. Notice their similarities and differences as you read through the descriptions that follow.
Skeletal muscle is packaged by connective tissue sheets into organs called skeletal muscles, which are attached to the skeleton. These muscles, which can be controlled voluntarily (or consciously), form the flesh of the body, the so-called muscular system (see Chapter 6). When the skeletal muscles contract, they pull on bones or skin. The result of their action is gross body movements or changes in our facial expressions. The cells of skeletal muscle are long, cylindrical, and multinucleate; they have obvious striations (stripes).
Cardiac muscle, covered in more detail in Chapter 11, is found only in the heart. As it contracts, the heart acts as a pump and propels blood through the blood vessels. Like skeletal muscle, cardiac muscle has striations, but cardiac cells are uninucleate, branching cells that fit tightly together (like clasped fingers) at junctions called intercalated disks. These intercalated disks contain gap junctions that allow ions to pass freely from cell to cell, resulting in rapid conduction of the exciting electrical impulse across the heart. Cardiac muscle is under involuntary control, which means that we cannot consciously control the activity of the heart. (There are, however, rare individuals who claim they have such an ability.)
Smooth, or visceral, muscle is so called because no striations are visible. The individual cells have a single nucleus and are spindle-shaped (pointed at each end). Smooth muscle is found in the walls of hollow organs such as the stomach, bladder, uterus, and blood vessels. When smooth muscle contracts, the cavity of an organ alternately becomes smaller (constricts) or enlarges (dilates) so that substances are propelled through the organ along a specific pathway. Smooth muscle contracts much more slowly than the other two muscle types. Peristalsis (per0˘ı-stal9sis), a wavelike motion that keeps food moving through the small intestine, is typical of its activity.
When we think of nervous tissue, we think of cells called neurons. All neurons receive and conduct electrochemical impulses from one part of the body to another; thus irritability and conductivity are their two major functional characteristics. The structure of neurons is unique (see Figure 3.20). The cytoplasm is drawn out into long extensions (as much as 3 feet or more in the leg), which allows a single neuron to conduct an impulse over long distances in the body. Neurons, along with a special group of supporting cells that insulate, support, and protect the delicate neurons, make up the structures of the nervous system—the brain, spinal cord, and nerves.
Tissue Repair (Wound Healing)
The body has many techniques for protecting itself from uninvited guests or injury. Intact physical barriers such as the skin and mucous membranes, cilia, and the strong acid produced by stomach glands are just three examples of body defenses exerted at the local tissue level. When tissue injury does occur, it stimulates the body’s inflammatory and immune responses, and the healing process begins almost immediately. Inflammation is a generalized (nonspecific) body response that attempts to prevent further injury. The immune response, on the other hand, is extremely specific and mounts a vigorous attack against recognized invaders (bacteria, viruses, toxins). These protective responses are considered in detail in Chapter 12. Here we will concentrate on the process of tissue repair itself.
Tissue repair, or wound healing, occurs in two major ways: by regeneration and by fibrosis. Regeneration is the replacement of destroyed tissue by the same kind of cells, whereas fibrosis involves repair by dense (fibrous) connective tissue, that is, by the formation of scar tissue. Which occurs depends on (1) the type of tissue damaged and (2) the severity of the injury. Generally speaking, clean cuts (incisions) heal much more successfully than ragged tears of the tissue.
Tissue injury sets a series of events into motion.
• The capillaries become very permeable. This allows fluid rich in clotting proteins and other substances to seep into the injured area from the bloodstream. Then leaked clotting proteins construct a clot, which stops the loss of blood, holds the edges of the wound together, and “walls off” the injured area, preventing bacteria or other harmful substances from spreading to surrounding tissues. Where the clot is exposed to air, it quickly dries and hardens, forming a scab.
• Granulation tissue forms. Granulation tissue is a delicate pink tissue composed largely of new capillaries that grow into the damaged area from undamaged blood vessels nearby. These capillaries are fragile and bleed freely, as when a scab is picked away from a skin wound. Granulation tissue also contains phagocytes that eventually dispose of the blood clot and connective tissue cells (fibro-blasts) that synthesize collagen fibers (scar tissue) to permanently bridge the gap.
• The surface epithelium regenerates. As the surface epithelium begins to regenerate, it makes its way across the granulation tissue just beneath the scab, which soon detaches. The final result is a fully regenerated surface epithelium that covers an underlying area of fibrosis (the scar). The scar is either invisible or visible as a thin white line, depending on the severity of the wound.
The ability of the different tissue types to regenerate varies widely. Epithelial tissues such as the skin epidermis and mucous membranes regenerate beautifully. So, too, do most of the fibrous connective tissues and bone. Skeletal muscle regenerates poorly, if at all, and cardiac muscle and nervous tissue within the brain and spinal cord are replaced largely by scar tissue.
Homeostatic ImbalanceScar tissue is strong, but it lacks the flexibility of most normal tissues. Perhaps even more important is its inability to perform the normal functions of the tissue it replaces. Thus, if scar tissue forms in the wall of the bladder, heart, or another muscular organ, it may severely hamper the functioning of that organ. s
We all begin life as a single cell, which divides thousands of times to form our multicellular embryonic body. Very early in embryonic development, the cells begin to specialize to form the primary tissues, and by birth, most organs are well formed and functioning. The body continues to grow and enlarge by forming new tissue throughout childhood and adolescence.
Cell division is extremely important during the body’s growth period. Most cells (except neurons) undergo mitosis until the end of puberty, when adult body size is reached and overall body growth ends. After this time, only certain cells routinely divide—for example, cells exposed to abrasion that continually wear away, such as skin and intestinal cells. Liver cells stop dividing; however, they still have this ability should some of them die or become damaged and need to be replaced. Still other cell groups (for example, heart muscle and ner-vous tissue) completely lose their ability to divide when they are fully mature; that is, they become amitotic (am0˘ı-tot9ik). Amitotic tissues are severely handicapped by injury because the lost cells cannot be replaced by the same type of cells. This is why the heart of an individual who has had several severe heart attacks becomes weaker and weaker. Damaged cardiac muscle cannot regenerate and is replaced by scar tissue that cannot contract, so the heart becomes less and less capable of acting as an efficient blood pump.
The aging process begins once maturity has been reached. (Some believe it begins at birth.) No one has been able to explain just what causes aging, but there have been many suggestions. Some believe it is a result of little “chemical insults,” which occur continually through life—for example, the presence of toxic chemicals (such as alcohol, certain drugs, or carbon monoxide) in the blood, or the temporary absence of needed substances such as glucose or oxygen. Perhaps the effect of these chemical insults is cumulative and finally succeeds in upsetting the delicate chemical balance of the body cells. Others think that external physical factors such as radiation (X rays or ultraviolet waves) contribute to the aging process. Several believe that the aging “clock” is genetically programmed, or built into our genes. We all know of cases like the “radiant woman of 50 who looks about 35” or the “barely-out-of-adolescence man of 24 who looks 40.” It appears that such traits can run in families.
There is no question that certain events are part of the aging process. For example, epithelial membranes become thinner and are more easily damaged, and the skin loses its elasticity and begins to sag. The exocrine glands of the body (epithelial tissue) become much less active as we age, which is why we begin to “dry out” as less oil, mucus, and sweat are produced. Some endocrine glands produce decreasing amounts of hormones, and the body processes that they control (such as metabolism and reproduction) become less efficient or stop altogether.
Connective tissue structures also show changes with age. Bones become porous and weaken, and the repair of tissue injuries slows. Muscles and nervous tissues begin to atrophy. Although a poor diet may contribute to some of these changes, there is little doubt that decreased efficiency of the circulatory system, which reduces nutrient and oxygen delivery to body tissues, is a major factor.
Besides the tissue changes associated with aging, which accelerate in the later years of life, other modifications of cells and tissues may occur at any time. For example, when cells fail to honor normal controls on cell division and multiply wildly, an abnormal mass of proliferating cells, known as a neoplasm (ne9o-plazm0; “new growth”), results. Neoplasms may be benign or malignant (cancerous). See “A Closer Look,” pp. 88–89. However, not all increases in cell number involve neoplasms. Certain body tissues (or organs) may enlarge because there is some local irritant or condition that stimulates the cells. This is called hyperplasia (hi0per-pla9ze-ah). For example, a woman’s breasts enlarge during pregnancy in response to increased hormones; this is a normal but temporary situation that doesn’t have to be treated. On the other hand, atrophy (at9ro-fe), or decrease in size, can occur in an organ or body area that loses its normal stimulation. For example, muscles that are not used or that have lost their nerve supply begin to atrophy and waste away rapidly.
Media study tools that could provide you with additional help in reviewing specific key topics of Chapter 3 are referenced below.
5 Interactive Physiology; 5 A&P Place website.
Part 1: Cells (pp. 56=77)
1. Overview of the Cellular Basis of Life
a. A cell is composed primarily of four elements—carbon, hydrogen, oxygen, and nitrogen—plus many trace elements. Living matter is over 60 percent water. The major building material of the cell is protein.
b. Cells vary in size from microscopic to over a meter in length. Shape often reflects function. For example, muscle cells have a long axis to allow shortening.
2. Anatomy of a Generalized Cell
a. Cells have three major regions—nucleus, cytoplasm, and plasma membrane.
(1) The nucleus, or control center, directs cell activity and is necessary for reproduction. The nucleus contains genetic material (DNA), which carries instructions for synthesis of proteins.
(2) The plasma membrane limits and encloses the cytoplasm and acts as a selective barrier to the movement of substances into and out of the cell. It is composed of a lipid bilayer containing proteins. The water-impermeable lipid portion forms the basic membrane structure. The proteins (many of which are glycoproteins) act as enzymes or carriers in membrane transport, form membrane channels or pores, provide receptor sites for hormones and other chemicals, or play a role in cellular recognition and interactions during development and immune reactions.
Specializations of the plasma membrane include microvilli (which increase the absorptive area) and cell junctions (desmosomes, tight junctions, and gap junctions).
Exercise: Chapter 3, Structure of the Plasma Membrane.
(3) The cytoplasm is where most cellular activities occur. Its fluid substance, the cytosol, contains inclusions, stored or inactive materials in the cytoplasm (fat globules, water vacuoles, crystals, and the like) and specialized bodies called organelles, each with a specific function. For example, mitochondria are sites of ATP synthesis, ribosomes are sites of protein synthesis, and the Golgi apparatus packages substances for export from the cell. Lysosomes carry out intracellular digestion, and peroxisomes disarm dangerous chemicals in the cells. Cytoskeletal elements function in cellular support and motion. The centrioles play a role in cell division and form the bases of cilia and flagella.
Exercise: Chapter 3, Parts of the Cell: Structure.
3. Cell Physiology
a. All cells exhibit irritability, digest foods, excrete wastes, and are able to reproduce, grow, move, and metabolize.
b. Transport of substances through the cell membrane:
Exercise: Chapter 3, Membrane Transport.
(1) Passive transport processes include diffusion and filtration.
(a) Diffusion is the movement of a substance from an area of its higher concentration to an area of its lower concentration. It occurs because of kinetic energy of the molecules themselves. The diffusion of dissolved solutes through the plasma membrane is simple diffusion. The diffusion of water through the plasma membrane is osmosis. Diffusion that requires a protein carrier is facilitated diffusion.
Exercise: Chapter 3, Passive Transport.
(b) Filtration is the movement of substances through a membrane from an area of high hydrostatic pressure to an area of lower fluid pressure. In the body, the driving force of filtration is blood pressure.
(2) Active transport processes use energy (ATP) provided by the cell.
(a) In solute pumping, substances are moved across the membrane against an electrical or a concentration gradient by proteins called solute pumps. This accounts for the transport of amino acids, some sugars, and most ions.
(b) The two types of ATP-activated bulk transport are exocytosis and endo-cytosis. Exocytosis moves secretions and other substances out of cells; a membrane-bound vesicle fuses with the plasma membrane, ruptures, and ejects its contents to the cell exterior. Endocytosis, in which particles are taken up by enclosure in a plasma membrane sac, includes phagocytosis (uptake of solid particles) and bulk-phase endocytosis (uptake of fluids).
c. Osmotic pressure, which reflects the solute concentration of a solution, determines whether cells gain or lose water.
(1) Hypertonic solutions contain more solutes (and less water) than do cells. In these solutions, cells lose water by osmosis and crenate.
(2) Hypotonic solutions contain fewer solutes (and more water) than do the cells. In these solutions, cells swell and may rupture (lyse) as water rushes in by osmosis.
(3) Isotonic solutions, which have the same solute-to-solvent ratio as cells, cause no changes in cell size or shape.
d. Cell division has two phases, mitosis (nuclear division) and cytokinesis (division of the cytoplasm).
(1) Mitosis begins after DNA has been replicated; it consists of four stages—prophase, metaphase, anaphase, and telophase. The result is two daughter nuclei, each identical to the mother nucleus.
(2) Cytokinesis usually begins during anaphase and progressively pinches the cytoplasm in half.
(3) Mitotic cell division provides an increased number of cells for growth and repair.
e. Protein synthesis involves both DNA (the genes) and RNA.
(1) A gene is a segment of DNA that carries the instructions for building one protein. The information is in the sequence of bases in the nucleotide strands. Each three-base sequence (triplet) specifies one amino acid in the protein.
(2) Messenger RNA carries the instructions for protein synthesis from the DNA gene to the ribosomes. Transfer RNA transports amino acids to the ribosomes. Ribosomal RNA forms part of the ribosomal structure and helps coordinate the protein building process.
Part 2: Body Tissues (pp. 77=87)
1. Epithelium is the covering, lining, and glandular tissue. Its functions include protection, absorption, and secretion. Epithelia are named according to arrangement (simple, stratified) and cell shape (squamous, cuboidal, columnar).
2. Connective tissue is the supportive, protective, and binding tissue. It is characterized by the presence of a nonliving, extracellular matrix (ground substance plus fibers) produced and secreted by the cells; it varies in amount and consistency. Fat, ligaments and tendons, bones, and cartilage are all connective tissues or connective tissue structures.
Exercise: Chapter 3, Identifying Connective Tissue.
3. Nervous tissue is composed of supporting cells and irritable cells called neurons, which are highly specialized to receive and transmit nerve impulses and supporting cells. Neurons are important in control of body processes. Nervous tissue is located in nervous system structures—brain, spinal cord, and nerves.
4. Muscle tissue is specialized to contract, or shorten, which causes movement. There are three types—skeletal (attached to the skeleton), cardiac (forms the heart), and smooth (in the walls of hollow organs).
5. Tissue repair (wound healing) may involve regeneration, fibrosis, or both. In regeneration, the injured tissue is replaced by the same type of cells. In fibrosis, the wound is repaired with scar tissue. Epithelia and connective tissues regenerate well. Mature cardiac muscle and nervous tissue are repaired by fibrosis.
Developmental Aspects of Cells
1. Growth through cell division continues through puberty. Cell populations exposed to friction (such as epithelium) replace lost cells throughout life. Connective tissue remains mitotic and forms repair (scar) tissue. For the most part, muscle tissue becomes amitotic by the end of puberty, and nervous tissue becomes amitotic shortly after birth. Amitotic tissues are severely handicapped by injury.
2. The cause of aging is unknown, but chemical and physical insults, as well as genetic programming, are suggested.
3. Neoplasms, both benign and cancerous, represent abnormal cell masses in which normal controls on cell division are not working. Hyperplasia (increase in size) of tissue or organ may occur when tissue is strongly stimulated or irritated. Atrophy (decrease in size) of a tissue or organ occurs when the organ is no longer stimulated normally.
1. Which of the following would you expect to find in or on cells whose main function is absorption?
a. Microvilli c. Gap junctions
b. Cilia d. Secretory vesicles
2. Adult cell types you might expect to have gap junctions include:
a. skeletal muscle
c. heart muscle
d. smooth muscle
3. Which of the following are possible functions of the glycoproteins in the plasma membrane?
a. Determination of blood groups
b. Binding sites for toxins or bacteria
c. Aiding the binding of sperm to egg
d. Increasing the efficiency of absorption
4. A cell with abundant peroxisomes would most likely be involved in:
b. storage of glycogen
c. ATP manufacture
e. detoxification activities
5. A cell stimulated to increase its steroid production will have abundant:
a. ribosomes d. Golgi apparatus
b. rough ER e. secretory vesicles
c. smooth ER
6. For diffusion to occur, there must be:
a. a selectively permeable membrane
b. equal amounts of solute
c. a concentration difference
d. some sort of carrier system
e. all of these
7. In which of the following tissue types might you expect to find goblet cells?
a. Simple cuboidal
b. Simple columnar
c. Simple squamous
d. Stratified squamous
8. An epithelium “built” to withstand friction is:
a. simple squamous
b. stratified squamous
c. simple cuboidal
d. simple columnar
9. What kind of connective tissue acts as a sponge, soaking up fluid when edema occurs?
a. Areolar connective
b. Adipose connective
c. Dense irregular connective
d. Reticular connective
10. What type of connective tissue prevents muscles from pulling away from bones during contraction?
a. Dense connective
c. Elastic connective
d. Hyaline cartilage
11. Which of the following terms describe cardiac muscle?
b. Intercalated discs
Short Answer Essay
1. Name the four elements making up the bulk of living matter.
2. Define cell and organelle.
3. Although cells have differences that reflect their special functions in the body, what functional abilities do all cells exhibit?
4. Describe the general function of the nucleus. Describe the special function of DNA found in the nucleus. What nuclear structures contain DNA? Help to form ribosomes?
5. Describe the general structure and function of the plasma membrane.
6. Describe the general composition and function of the cytosol and inclusions of the cytoplasm.
7. Name the cellular organelles and explain the function of each.
8. What is the difference between active and passive transport processes?
9. Define diffusion, osmosis, simple diffusion, filtration, solute pumping, exocytosis, endocytosis, phagocytosis, and bulk-phase endocytosis.
10. What two structural characteristics of cell membranes determine if substances can pass through them passively? What determines whether or not a substance can be actively transported through the membrane?
11. Explain the effect of the following solutions on living cells: hypertonic, hypotonic, and isotonic.
12. Briefly describe the process of DNA replication.
13. Define mitosis. Why is mitosis important?
14. What is the role of the spindle in mitosis?
15. Why can an organ be permanently damaged if its cells are amitotic?
16. Describe the relative roles of DNA and RNA in protein synthesis.
17. Define tissue. List the four major types of tissue. Which of the four major tissue types is most widely distributed in the body?
18. Describe the general characteristics of epithelial tissue. List the most important functions of epithelial tissues and give examples of each.
19. How are epithelial tissues classified?
20. Where is ciliated epithelium found, and what role does it play?
21. How do the endocrine and exocrine glands differ in structure and function?
22. What are the general structural characteristics of connective tissues? What are the functions of connective tissues? How are their functions reflected in their structures?
23. Name a connective tissue with (a) a soft fluid matrix, and (b) a stony hard matrix.
24. What is the function of muscle tissue?
25. Name the three types of muscle tissue and tell where each would be found in the body. What is meant by “Smooth muscles are involuntary in action”? Which muscle type is voluntary in action?
26. What two functional characteristics are highly developed in neurons?
27. Define neoplasm, hyperplasia, and atrophy.
At the Clinic
1. Two examples of chemotherapeutic drugs (used to treat cancer) and their cellular actions are given below. Explain why each drug could be fatal to a cell.
• Vincristine: Damages the mitotic spindle.
• Adriamycin: Binds to DNA and blocks messenger RNA synthesis.
2. Hydrocortisone is an anti-inflammatory drug that stabilizes lysosomal membranes. Explain how this effect reduces cell damage and inflammation.
3. John has severely injured his knee during football practice. He is told that he has a torn knee cartilage and to expect that recovery and repair will take a long time. Why?
4. Three patients in an intensive care unit are examined by the resident doctor. One patient has brain damage from a stroke, another had a heart attack that severely damaged his heart muscle, and the third has a severely damaged liver (a gland) from a crushing injury in a car accident. All three patients have stabilized and will survive, but only one will have full functional recovery through regeneration. Which one and why?><#>< Chapter 3: Cells and Tissues#><Figure 3.1 Anatomy of the generalized animal cell. (a) The three main regions of the generalized cell. (b) Structure of the nucleus.><# Essentials of Human Anatomy and Physiology><Figure 3.2 Structure of the plasma membrane.>< Chapter 3: Cells and Tissues#><Figure 3.3 Cell junctions. An epithelial cell is shown joined to adjacent cells by the three common types of cell junctions: tight junctions, desmosomes, and gap junctions. Also illustrated are microvilli (seen projecting from the free cell surface).><# Essentials of Human Anatomy and Physiology><Figure 3.4 Structure of the generalized cell. No cell is exactly like this one, but this generalized cell drawing illustrates features common to many human cells.>< Chapter 3: Cells and Tissues#><# Essentials of Human Anatomy and Physiology><Figure 3.5 Role of the Golgi apparatus in packaging the products of the rough ER. Protein-containing transport vesicles pinch off the rough ER and migrate to fuse with the Golgi apparatus. As it passes through the Golgi apparatus, the protein product is sorted (and slightly modified). The product is then packaged within vesicles, which leave the Golgi apparatus and head for various destinations (#1–3), as shown.>< Chapter 3: Cells and Tissues#><Figure 3.6 The cytoskeleton. (a) In this light micrograph of the cytoskeleton of a nerve cell (14003), the microtubules appear green; the microfilaments are blue. Intermediate filaments form most of the rest of the network. (b–d) Diagrammatic views of the three types of cytoskeletal elements.><# Essentials of Human Anatomy and Physiology>< Chapter 3: Cells and Tissues#><Figure 3.7 Cell diversity. The shape of human cells and the relative abundances of their organelles relate to their function in the body.><# Essentials of Human Anatomy and Physiology>< Chapter 3: Cells and Tissues#><Figure 3.8 Diffusion. Particles in solution move continuously and collide con-stantly with other particles. As a result, particles tend to move away from areas where they are most highly concentrated and to become evenly distributed, as illustrated by the diffusion of dye molecules in a beaker of water.><# Essentials of Human Anatomy and Physiology><Figure 3.9 Diffusion through the plasma membrane. (a) Simple diffusion. As depicted on the left, fat-soluble molecules diffuse directly through the lipid bilayer of the plasma membrane, in which they can dissolve. On the right, small lipid-insoluble substances (water molecules or small ions) are shown diffusing through channels constructed by membrane proteins. (b) Facilitated diffusion moves large, lipid-insoluble molecules (e.g., glucose) across the membrane. The substance to be transported binds to a transmembrane carrier protein.><A Closer Look><IV Therapy and Cellular “Tonics”><W><hy is it essential that medical personnel give only the proper intravenous (IV), or into-the-vein, solutions to patients?
Consider that there is a steady traffic of small molecules across the plasma membrane. Although diffusion of solutes across the membrane is rather slow, osmosis, which moves water across the membrane, occurs very quickly. Anyone administering an IV must use the correct solution to protect the patient’s cells from life-threatening dehydration or rupture due to excessive water entry.
The tendency of a solution to hold water or “pull” water into it is called osmotic pressure. Osmotic pressure is directly related to the concentration of solutes in the solution. The higher the solute concentration, the greater the osmotic pressure and the greater the tendency of water to move into the solution. Many molecules, particularly proteins and some ions, are prevented from diffusing through the plasma membrane. Consequently, any change in their concentration on one side of the membrane forces water to move from one side of the membrane to the other, causing cells to lose or gain water. The ability of a solution to change the size and shape of cells by altering the amount of water they contain is called tonicity (ton-is9i-te; ton 5 strength).
Isotonic (i0so-ton9ik; “same tonicity”) solutions (such as Ringer’s lactate, 5 percent dextrose, and 0.9 percent saline) have the same solute and water concentrations as cells do. Isotonic solutions cause no visible changes in cells, and when such solutions are infused into the bloodstream, red blood cells retain their normal size and disklike shape (photo a). As you might guess, interstitial fluid and most intravenous solutions are isotonic solutions.
If red blood cells are exposed to a hypertonic (hi0per-ton9ik) solution—a solution that contains more solutes, or dissolved substances, than there are inside the cells—the cells will begin to shrink, or crenate (kre9n–at). This is because water is in higher concentration inside the cell than outside, so it follows its concentration gradient and leaves the cell (photo b). Hypertonic solutions are sometimes given to patients who have edema (swollen feet and hands because of fluid retention). Such solutions draw water out of the tissue spaces into the bloodstream so that excess fluid can be eliminated by the kidneys.
When a solution contains fewer
solutes (and therefore more water) than the cell does, it is said to be
hypotonic (hi0po-ton9ik) to the cell. Cells placed in hypotonic solutions plump
up rapidly as water rushes into them (photo c). Distilled water represents the
most extreme example of a hypotonic fluid. Since it contains no solutes at all,
water will enter cells until they finally burst or lyse. Hypotonic solutions are sometimes infused intravenously
(slowly and with care) to rehydrate the tissues of extremely dehydrated
patients. In less extreme cases, drinking hypotonic fluids usually does the
trick. (Many fluids that humans tend to drink regularly, such as tea, colas,
apple juice, and sports drinks, are hypotonic.)><The effect of IV solutions of varying
tonicity on living red blood cells.><#><# Essentials of Human Anatomy and Physiology><Figure 3.10 Operation of the
sodium-potassium pump, a solute pump. ATP provides the energy for a “pump”
protein to move three sodium ions out of the cell and two potassium ions into
the cell. Both ions are moved against their concentration gradients.>< Chapter 3: Cells and Tissues#><Figure 3.11 Exocytosis.
(a) A secretory vesicle migrates to the plasma membrane and the two
membranes fuse. The fused site opens and releases the contents to the outside
of the cell. (b) Electron micrograph
of a vesicle in exocytosis (100,0003).><Figure 3.12 Phagocytosis
and bulk-phase endocytosis—two types of endocytosis.
A medical transcriptionist is a medical language specialist who interprets and transcribes notes dictated by physicians and other health-care professionals. These reports, which cover all aspects of a patient’s assessment, diagnosis, treatment, and outcome, become part of the person’s confidential medical record. Medical transcriptionists work in hospitals, clinics, doctors’ offices, transcription services, insurance companies, and home health-care agencies. Some freelance as independent contractors or subcontractors.
What does it take to be a transcriptionist? “Certainly, you need a good English background,” says Pamela Shull, an experienced transcriptionist and administrative assistant at Santa Clara Valley Medical Center in San Jose, California. “Strong grammar, spelling, and punctuation skills are crucial. Physicians often dictate these records on the go, perhaps as they’re walking down a hospital corridor or driving back to the office, and a good transcriptionist must be able to edit the dictated material for grammar and clarity.”
Knowledge of anatomy and physiology, however, is even more important. Notes Shull, “If you have a basic understanding of anatomy and medical terminology, you will be much more accurate at interpreting and transcribing what you hear. A hospital transcriptionist must deal with terms from a wide variety of medical specialties—one dictation might be from a gynecologist, the next from an orthopedic surgeon, and the next from a pediatrician. You never know what you’ll encounter next.”
All health professionals who treat a patient, now and in the future, must rely on these typed documents, so accurate transcription is vital: “I see the transcriptionist as a partner with physicians. We work with them to create excellent medical records, so patients will always be assured of receiving the best and most appropriate care possible.”
This is why anatomy and physiology, medical terminology, and disease processes make up most of the curriculum in medical transcription training programs. Classes for medical transcription are offered through community colleges, proprietary schools, and home-study programs. Training programs vary in length from several months to two years, but the American Association for Medical Transcription (AAMT) recommends a two-year program that includes at least 240 hours of on-the-job training in a health-care facility. Please note that accreditation procedures vary from state to state. The AAMT also offers a voluntary certification exam to become a certified medical transcriptionist.
Shull enjoys the variety of medical transcription work. “It’s fascinating because you get to follow each patient’s story, from the initial problem to diagnosis to treatment,” she says. “You feel like you get to know these people. It’s like watching a gripping television drama—only this is real life!”
For more information, contact the American Association for Medical Transcription:
100 Sycamore Avenue
Modesto, CA 95354
Telephone: 800-982-2182 or
Fax number: 209-527-9633
additional information on this career, click the Focus on Careers link at www.
But, what causes transformation—the changes that convert a normal cell to a cancerous one? It is well known that radiation, mechanical trauma, certain viral infections, and many chemicals (tobacco tars, saccharine) can act as carcinogens (cancer-causers). What all of these factors have in common is that they all cause mutations—changes in DNA that alter the expression of certain genes. However, most carci-nogens are eliminated by peroxisomal or lysosomal enzymes or the immune system. Furthermore, one mutation doesn’t do it—apparently it takes a sequence of several genetic changes to change a normal cell to a full-fledged cancer cell (see figure a).
Clues to the role of genes were provided by the discovery of oncogenes (cancer-causing [onco 5 tumor] genes), and then proto-oncogenes. Proto-oncogenes code for proteins that are needed for normal cell division and growth. However, many have fragile sites that break when they are exposed to carcinogens, and this event converts them to oncogenes. An example of a problem that might result from this conversion is “switching on” of dormant genes that allow cells to become invasive (an ability of embryonic cells—and cancer cells—but not normal adult cells). However, oncogenes have been discovered in only 15 to 20 percent of human cancers, so the more recent discovery of tumor suppressor genes, which work to suppress or prevent cancer, was not too surprising. The tumor suppressor genes (such as p53) aid DNA repair, put the “brakes” on cell division, help to inactivate carcinogens, or enhance the ability of the immune system to destroy cancer cells. When the tumor suppressor genes are damaged or changed in some way, the oncogenes are free to “do their thing.” One of the best-understood of human cancers, colon cancer, illustrates this principle (see figure b). The first sign of colon cancer is an unusual increase in the division rate of apparently normal cells of the colon lining. Then, a polyp (benign tumor) appears in the colon wall, and finally the malignant neoplasm makes its appearance at the site. In most cases, these changes parallel cellular changes at the DNA level and include activation of an oncogene and inactivation of two tumor suppressor genes. Whatever the precise genetic factor at work, the seeds of cancer do appear to be in our own genes. Thus, as you can see, cancer is an intimate enemy indeed.
Almost half of all Americans develop cancer in their lifetime, and a fifth of us will die of it. Cancer can arise from almost any cell type, but the most common cancers originate in the skin, lung, colon, breast, male prostate gland, and urinary bladder.
Screening procedures, such as self-examination of one’s breasts or testicles for lumps and checking fecal samples for blood, aid in early detection of cancers. However, most cancers are diagnosed only after they have begun to cause symptoms (pain, bloody discharge, lump, etc.), and the diagnostic method most used is the biopsy. In a biopsy, a sample of the primary tumor is removed surgically (or by scraping) and examined microscopically for structural changes typical of malignant cells.
The treatment of choice for either type of neoplasm is surgical removal. If surgery is not possible—as in cases where the cancer has spread widely or is inoperable—radiation and drugs (chemotherapy) are used. Anticancer drugs have unpleasant side effects because most target all rapidly dividing cells, including normal ones. The side effects include nausea, vomiting, and loss of hair. X rays also have side effects because, in passing through the body, they kill healthy cells that lie in the path to the cancer cells.
Current cancer treatments—“cut, burn, and poison”—are recognized as crude and painful. Promising new methods focus on delivering anticancer drugs precisely to the cancer (via monoclonal antibodies that respond to one type of protein on a cancer cell) and on increasing the immune system’s ability to fend off cancer. The most recent research focuses on choking off tumors by inhibiting their ability to attract a rich blood supply.><T><#><#><# Essentials of Human Anatomy and Physiology><WEB><WEB><WEB><WEB>< Chapter 3: Cells and Tissues#><WEB><# Essentials of Human Anatomy and Physiology>< Chapter 3: Cells and Tissues#><# Essentials of Human Anatomy and Physiology>