Tissues
Tissues in Levels of Organization
The human body is organized at several levels of scale that can each be examined.
LEARNING OBJECTIVES
Characterize where tissues fall in levels of organization
KEY TAKEAWAYS
Key Points
The human body has many levels of structural organization: atoms, cells, tissues, organs, and organ system.
The simplest level is the chemical level, which includes tiny building blocks such as atoms.
Cells are the smallest functional units of life.
Tissues are groups of similar cells that have a common function.
An organ is a structure that is composed of at least two or more tissue types and performs a specific set of functions for the body.
Many organs working together to accomplish a common purpose is called an organ system.
Tissues in the Human Body
The human body has many levels of structural organization. The simplest level is the chemical level, which includes tiny building blocks such as atoms. Cells are the smallest functional units of life. The simplest living creatures are single cell creatures, but in complex life forms, such as human beings, cells also exist in the tissue level.
Tissues are groups of similar cells that have a common function. The four basic tissue types are epithelial, muscle, connective, and nervous tissue. Each tissue type has a characteristic role in the body:
Epithelium covers the body surface and lines body cavities.
Muscle provides movement.
Connective tissue supports and protects body organs.
Nervous tissue provides a means of rapid internal communication by transmitting electrical impulses.
Organs: Made of Tissues
An organ is a structure that is composed of at least two or more tissue types and performs a specific set of functions for the body. The liver, stomach, brain, and blood are all different organs and perform different functions. Each organ is a specialized functional center responsible for a specific function of the body.
At the organ level, complex functions become possible because of the specialized activities of various tissues. Most organs contain more than one tissue type. For example, the stomach consists of smooth muscle tissue for churning movement while it is innervated, but it is also supplied by blood, which is a connective tissue.
The next level is the organ system level. Many organs working together to accomplish a common purpose create an organ system. For example, the heart and the blood vessels of the cardiovascular system circulate blood and transport oxygen and nutrients to all the body cells.
Levels of Organization: Molecules form cells. Cells form tissues, and tissues form organs. Organs that fulfill related functions are called organ systems. An organism is made up of interconnected organ systems.
Characteristics of Epithelial Tissue
The human body consists of four types of tissue: epithelial, connective, muscular, and nervous. Epithelial tissue covers the body, lines all cavities, and composes the glands.
LEARNING OBJECTIVES
Describe the primary functions and characteristics of epithelial tissue
KEY TAKEAWAYS
Key Points
Epithelial tissue is composed of cells laid together in sheets with the cells tightly connected to one another. Epithelial layers are avascular, but innervated.
Epithelial cells have two surfaces that differ in both structure and function.
Glands, such as exocrine and endocrine, are composed of epithelial tissue and classified based on how their secretions are released.
Key Terms
epithelium: A membranous tissue composed of one or more layers of cells that form the covering of most internal and external surfaces of the body and its organs.
avascular: Lacking blood vessels.
vascular: Containing blood vessels.
Functions of the Epithelium
Epithelia tissue forms boundaries between different environments, and nearly all substances must pass through the epithelium. In its role as an interface tissue, epithelium accomplishes many functions, including:
Protection for the underlying tissues from radiation, desiccation, toxins, and physical trauma.
Absorption of substances in the digestive tract lining with distinct modifications.
Regulation and excretion of chemicals between the underlying tissues and the body cavity.
The secretion of hormones into the blood vascular system. The secretion of sweat, mucus, enzymes, and other products that are delivered by ducts come from the glandular epithelium.
The detection of sensation.
Characteristics of Epithelial Layers
Epithelial tissue is composed of cells laid out in sheets with strong cell-to-cell attachments. These protein connections hold the cells together to form a tightly connected layer that is avascular but innervated in nature.
The epithelial cells are nourished by substances diffusing from blood vessels in the underlying connective tissue. One side of the epithelial cell is oriented towards the surface of the tissue, body cavity, or external environment and the other surface is joined to a basement membrane. The basement layer is non-cellular in nature and helps to cement the epithelial tissue to the underlying structures.
Types of Epithelial Tissue
Epithelial tissues are identified by both the number of layers and the shape of the cells in the upper layers. There are eight basic types of epithelium: six of them are identified based on both the number of cells and their shape; two of them are named by the type of cell (squamous) found in them. Epithelial tissue is classified based on the number of cells, the shape of those cells, and the types of those cells.
Epithelial Tissue Cells
Cells
Locations
Function
Simple squamous epithelium
Air sacs of the lungs and the lining of the heart, blood vessels and lymphatic vessels
Allows materials to pass through by diffusion and filtration, and secretes lubricating substances
Simple cuboidal epithelium
In ducts and secretory portions of small glands and in kidney tubules
Secretes and absorbs
Simple columnar epithelium
Ciliated tissues including the bronchi, uterine tubes, and uterus; smooth (nonciliated tissues) are in the digestive tract bladder
Absorbs; it also secretes mucous and enzymes.
Pseudostratified columnar epithelium
Ciliated tissue lines the trachea and much of the upper respiratory tract
Secrete mucous; ciliated tissue moves mucous
Stratified squamous epithelium
Lines the esophagus, mouth, and vagina
Protects against abrasion
Stratified cuboidal epithelium
Sweat glands, salivary glands, and mammary glands
Protective tissue
Stratified columnar epithelium
The male urethra and the ducts of some glands.
Secretes and protects
Transitional epithelium
Lines the bladder, urethra and ureters
Allows the urinary organs to expand and stretch
Types of Epithelial Tissue
Epithelial tissue is classified by cell shape and the number of cell layers.
LEARNING OBJECTIVES
Classify epithelial tissue by cell shape and layers
KEY TAKEAWAYS
Key Points
There are three principal cell shapes associated with epithelial cells: squamous epithelium, cuboidal epithelium, and columnar epithelium.
There are three ways of describing the layering of epithelium: simple, stratified, and pseudostratified.
Pseudostratified epithelium possesses fine hair-like extensions called cilia and unicellular glands called goblet cells that secrete mucus. This epithelium is described as ciliated pseudostratified epithelium.
Stratified epithelium differs from simple epithelium in that it is multilayered. It is therefore found where body linings have to withstand mechanical or chemical insult.
In keratinized epithelia, the most apical layers (exterior) of cells are dead and and contain a tough, resistant protein called keratin. An example of this is found in mammalian skin that makes the epithelium waterproof.
Transitional epithelia are found in tissues such as the urinary bladder where there is a change in the shape of the cell due to stretching.
Key Terms
simple columnar: A columnar epithelium that is uni-layered.
pseudostratified epithelium: A type of epithelium that, though comprising only a single layer of cells, has its cell nuclei positioned in a manner suggestive of stratified epithelia.
squamous: Flattened and scale-like.
cuboidal: Resembling a cube.
Keratinized: To produce or become like keratin.
columnar: Having the shape of a column.
Most epithelial tissue is described with two names. The first name describes the number of cell layers present and the second describes the shape of the cells. For example, simple squamous epithelial tissue describes a single layer of cells that are flat and scale-like in shape.
Epithelial Tissue: There are three principal classifications associated with epithelial cells. Squamous epithelium has cells that are wider than they are tall. Cuboidal epithelium has cells whose height and width are approximately the same. Columnar epithelium has cells taller than they are wide.
Simple Epithelia
Simple epithelium consists of a single layer of cells. They are typically where absorption, secretion and filtration occur. The thinness of the epithelial barrier facilitates these processes.
Simple epithelial tissues are generally classified by the shape of their cells. The four major classes of simple epithelium are: 1) simple squamous; 2) simple cuboidal; 3) simple columnar; and 4) pseudostratified.
Simple Squamous
Simple squamous epithelium cells are flat in shape and arranged in a single layer. This single layer is thin enough to form a membrane that compounds can move through via passive diffusion. This epithelial type is found in the walls of capillaries, linings of the pericardium, and the linings of the alveoli of the lungs.
Simple Cuboidal
Simple cuboidal epithelium consists of a single layer cells that are as tall as they are wide. The important functions of the simple cuboidal epithelium are secretion and absorption. This epithelial type is found in the small collecting ducts of the kidneys, pancreas, and salivary glands.
Simple Columnar
Simple columnar epithelium is a single row of tall, closely packed cells, aligned in a row. These cells are found in areas with high secretory function (such as the wall of the stomach), or absorptive areas (as in small intestine ). They possess cellular extensions (e.g., microvilli in the small intestine, or the cilia found almost exclusively in the female reproductive tract).
Pseudostratified
These are simple columnar epithelial cells whose nuclei appear at different heights, giving the misleading (hence pseudo) impression that the epithelium is stratified when the cells are viewed in cross section.
Pseudostratified epithelium can also possess fine hair-like extensions of their apical (luminal) membrane called cilia. In this case, the epithelium is described as ciliated pseudostratified epithelium. Ciliated epithelium is found in the airways (nose, bronchi), but is also found in the uterus and fallopian tubes of females, where the cilia propel the ovum to the uterus.
Stratified Epithelium
Stratified epithelium differs from simple epithelium by being multilayered. It is therefore found where body linings have to withstand mechanical or chemical insults.
Stratified epithelia are more durable and protection is one their major functions. Since stratified epithelium consists of two or more layers, the basal cells divide and push towards the apex, and in the process flatten the apical cells.
Stratified epithelia can be columnar, cuboidal, or squamous type. However, it can also have the following specializations:
Keratinized Epithelia
In keratinized epithelia, the most apical layers (exterior) of cells are dead and lose their nucleus and cytoplasm. They contain a tough, resistant protein called keratin. This specialization makes the epithelium waterproof, and it is abundant in mammalian skin. The lining of the esophagus is an example of a non-keratinized or moist stratified epithelium.
Transitional Epithelia
Transitional epithelia are found in tissues that stretch and it can appear to be stratified cuboidal when the tissue is not stretched, or stratified squamous when the organ is distended and the tissue stretches. It is sometimes called the urothelium since it is almost exclusively found in the bladder, ureters, and urethra.
Characteristics of Connective Tissue
Connective tissue is incredibly diverse and contributes to energy storage, the protection of organs, and the body’s structural integrity.
LEARNING OBJECTIVES
Describe the main characteristics and functions of connective tissue
KEY TAKEAWAYS
Key Points
Connective tissue is the most abundant and widely distributed of the primary tissues.
Connective tissue has three main components: cells, fibers, and ground substance. Together the ground substance and fibers make up the extracellular matrix.
Connective tissue is classified into two subtypes: soft and specialized connective tissue.
Major functions of connective tissue include: 1) binding and supporting, 2) protecting, 3) insulating, 4) storing reserve fuel, and 5) transporting substances within the body.
Connective tissues can have various levels of vascularity. Cartilage is avascular, while dense connective tissue is poorly vascularized. Others, such as bone, are richly supplied with blood vessels.
Key Terms
extracellular matrix: Cells of the connective tissue are suspended in a non-cellular matrix that provides structural and biochemical support to the surrounding cells.
fibroblast: A type of cell found in connective tissue that synthesizes the extracellular matrix and collagen.
connective tissue: A type of tissue found in animals whose main function is to bind, support, and anchor the body.
Connective tissue (CT) is a one of the four main classes of tissues. Although it is the most abundant and widely distributed of the primary tissues, the amount of connective tissue in a particular organ varies. Like to the timber framing of a house, the connective tissue provides structure and support throughout the body.
Structure of Connective Tissue
Connective tissue has three main components:
Ground substance
Fibers
Cells
Together the ground substance and fibers make up the extracellular matrix. The composition of these three elements vary tremendously from one organ to the other. This offers great diversity in the types of connective tissue.
Structural elements of connective tissue: Connective tissues consist of three parts: cells suspended in a ground substance or matrix; and most have fibers running through it.
Ground substance is a clear, colorless, viscous fluid that fills the space between the cells and fibers. It is composed of proteoglycans and cell adhesion proteins that allow the connective tissue to act as glue for the cells to attach to the matrix. The ground substance functions as a molecular sieve for substances to travel between blood capillaries and cells.
Connective tissue fibers provide support. Three types of fibers are found in connective tissue:
Collagen
Elastic fibers
Reticular fibers
Collagen Fibers
Collagen: Collagen fibers are the strongest and most abundant of all the connective tissue fibers.
Collagen fibers are fibrous proteins and are secreted into the extracellular space and they provide high tensile strength to the matrix.
Elastic Fibers
Elastic fibers are long, thin fibers that form branching network in the extracellular matrix. They help the connective tissue to stretch and recoil.
Reticular Fibers
Reticular fibers are short, fine collagenous fibers that can branch extensively to form a delicate network.
Function of Connective Tissue
The major functions of connective tissue include:
Binding and supporting.
Protecting.
Insulating.
Storing reserve fuel.
Transporting substances within the body.
Types of Connective Tissue
Connective tissues encompass a diverse array of tissue types that are involved in binding and supporting body structure and tissues.
LEARNING OBJECTIVES
Describe the diverse types of connective tissue
KEY TAKEAWAYS
Key Points
The lymphatic system is a part of the circulatory system, comprising a network of conduits called lymphatic vessels that carry a clear fluid called lymph unidirectionally towards the heart.
Blood is considered a specialized form of connective tissue. In vertebrates, it is composed of blood cells suspended in a liquid called blood plasma.
The primary tissue of bone, osseous tissue, is a relatively hard and lightweight composite material, formed mostly of calcium phosphate in the chemical arrangement termed calcium hydroxylapatite.
Adipose tissue or body fat is loose connective tissue composed of adipocytes.
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes, and the intervertebral discs.
In humans, adipose tissue is located beneath the skin (subcutaneous fat), around internal organs (visceral fat), in bone marrow (yellow bone marrow), and in breast tissue.
Key Terms
cartilage: A type of dense, non-vascular connective tissue, usually found at the end of joints, the rib cage, the ear, the nose, in the throat, and between intervertebral disks.
adipose tissue: Connective tissue that stores fat and cushions and insulates the body.
blood: A vital liquid flowing in the bodies of many types of animals that usually conveys nutrients and oxygen. In vertebrates, it is colored red by hemoglobin, is conveyed by arteries and veins, is pumped by the heart, and is usually generated in bone marrow.
Connective tissue is divided into four main categories:
Connective proper
Cartilage
Bone
Blood
Connective tissue proper has two subclasses: loose and dense. Loose connective tissue is divided into 1) areolar, 2) adipose, 3)
reticular. Dense connective tissue is divided into 1) dense regular, 2) dense irregular, 3) elastic.
Areolar Connective Tissue
These tissues are widely distributed and serve as a universal packing material between other tissues. The functions of areolar connective tissue include the support and binding of other tissues.
It also helps in defending against infection. When a body region is inflamed, the areolar tissue in the area soaks up the excess fluid as a sponge and the affected area swells and becomes puffy, a condition called edema.
Adipose Tissue or Body Fat
Adipose tissue: Yellow adipose tissue in paraffin section with lipids washed out.
This is loose connective tissue composed of adipocytes. It is technically composed of roughly only 80% fat. Its main role is to store energy in the form of lipids, although it also cushions and insulates the body.
The two types of adipose tissue are white adipose tissue (WAT) and brown adipose tissue (BAT). Adipose tissue is found in specific locations, referred to as adipose depots.
Reticular Connective Tissue
This tissue resembles areolar connective tissue, but the only fibers in its matrix are the reticular fibers, which form a delicate network. The reticular tissue is limited to certain sites in the body, such as internal frameworks that can support lymph nodes, spleen, and bone marrow.
Dense Regular Connective Tissue
This consists of closely packed bundles of collagen fibers running in the same direction. These collagen fibers are slightly wavy and can stretch a little bit.
With the tensile strength of collagen, this tissue forms tendons, aponeurosis and ligaments. This tissue forms the fascia, which is a fibrous membrane that wraps around the muscles, blood vessels, and nerves.
Dense Irregular Tissue
This has the same structural elements as dense regular tissue, but the bundles of collagen fibers are much thicker and arranged irregularly. This tissue is found in areas where tension is exerted from many different directions. It is part of the skin dermis area and in the joint capsules of the limbs.
Elastic Connective Tissue
The main fibers that form this tissue are elastic in nature. These fibers allow the tissues to recoil after stretching. This is especially seen in the arterial blood vessels and walls of the bronchial tubes.
Cartilage
This is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes, and the intervertebral discs.
Cartilage is composed of specialized cells called chondroblasts and, unlike other connective tissues, cartilage does not contain blood vessels. Cartilage is classified in three types: 1) elastic cartilage, 2) hyaline cartilage, and 3) fibrocartilage, which differ in the relative amounts of these three main components.
Elastic Cartilage
This is similar to hyaline cartilage but is more elastic in nature. Its function is to maintain the shape of the structure while allowing flexibility. It is found in the external ear (known as an auricle) and in the epiglottis.
Hyaline Cartilage
This is is the most abundant of all cartilage in the body. Its matrix appears transparent or glassy when viewed under a microscope. It provides strong support while providing pads for shock absorption. It is a major part of the embryonic skeleton, the costal cartilages of the ribs, and the cartilage of the nose, trachea, and larynx.
Fibrocartilage
This is a blend of hyaline cartilage and dense regular connective tissue. Because it is compressible and resists tension well, fibrocartilage is found where strong support and the ability to withstand heavy pressure are required. It is found in the intervertebral discs of the bony vertebrae and knee meniscus.
Bone tissue is also called the osseous tissue. The osseous tissue is relatively hard and lightweight in nature. It is mostly formed of calcium phosphate in the chemical arrangement termed calcium hydroxyapatite, which gives bones their rigidity. It has relatively high compressive strength, but poor tensile strength, and very low shear stress strength.
The hard outer layer of bones is composed of compact bone tissue, so-called due to its minimal gaps and spaces. Its porosity is 5–30%. This tissue gives bones their smooth, white, and solid appearance, and accounts for 80% of the total bone mass of an adult skeleton.
Filling the interior of the bone is the trabecular bone tissue (an open cell porous network also called cancellous or spongy bone), which is composed of a network of rod and plate-like elements that make the overall organ lighter and allow room for blood vessels and marrow.
Blood
This is considered a specialized form of connective tissue. Blood is a bodily fluid in animals that delivers necessary substances, such as nutrients and oxygen, to the cells and transports metabolic waste products away from those same cells.
It is an atypical connective tissue since it does not bind, connect, or network with any body cells. It is made up of blood cells and is surrounded by a nonliving fluid called plasma.
Characteristics of Nervous Tissue
Nervous tissue is the main component of the nervous system, which includes the brain, spinal cord, and nerves.
LEARNING OBJECTIVES
Describe the characteristics of nervous tissue
KEY TAKEAWAYS
Key Points
Nervous tissue is one of four major classes of tissues and makes up the central nervous system and the peripheral nervous system.
Integration and communication are the two major functions of nervous tissue.
Nervous tissue contains two categories of cells — neurons and neuroglia.
Neurons are highly specialized nerve cells that generate and conduct nerve impulses.
Neuroglia are supporting cells that provide physical sport, remove debris, and provide electrical insulation.
Key Terms
myelin: A substance produced by cells of the neuroglia that increases the speed of impulses along the axon of the neuronal fiber.
nervous tissue: The principal constituent of the central and peripheral nervous system, comprised neurons and neuroglia cells.
brain: The control center of the central nervous system, located in the skull.
Nervous Tissue
Nervous tissue is one of four major classes of tissues. It is specialized tissue found in the central nervous system and the peripheral nervous system. It consists of neurons and supporting cells called neuroglia.
The nervous system is responsible for the control of the body and the communication among its parts. Nervous tissue contains two categories of cells—neurons and neuroglia.
Neurons
Neurons are highly specialized nerve cells that generate and conduct nerve impulses. A typical neuron consists of dendrites, the cell body, and an axon.
Dendrites
Dendrites are responsible for responding to stimuli; they receive incoming signals towards the cell body. The axons are responsible for transmitting impulses over long distances from cell body. The cell body is like a factory for the neuron. It produces all the proteins and contains specialized organelles such as nucleus, granules and Nissl bodies.
Neuron: This image illustrates the parts of a neuron. The dendrites receive incoming signals while axons propagate signals away from the neuron cell body. The myelin sheath surrounds and insulates the axon.
Dendrite
The axon is surrounded by a whitish, fatty layer called the myelin sheath. Outside the myelin sheath there is a cellular layer called the neurilemma.
Schwann Cells
In the peripheral nervous system, Schwann cells are neuroglia cells that support neuronal function by increasing the speed of impulse propagation. The Schwann cells are underlain by the medullary sheath. The medullary sheath is interrupted at intervals by the nodes of Ranvier.
Illustration of the Schwann Cells and the Myelin Sheath: Transmission electron micrograph of a myelinated axon. The myelin layer (concentric) surrounds the axon of a neuron, showing Schwann cells.
Types of Nervous Tissue
The nervous system consists of nervous tissue, which is composed of two principal types of cells called neuron and neuroglia.
LEARNING OBJECTIVES
Describe the main cells that comprise nervous tissue
KEY TAKEAWAYS
Key Points
Nervous tissue is composed of neurons and supporting cells called neuroglia, or ” glial cells.”
There are six types of neuroglia. Four are found in the central nervous system, while two are found in the peripheral nervous system.
The four types of neuroglia found in the central nervous system are astrocytes, microglial cells, ependymal cells, and oligodendrocytes.
The two types of neuroglia found in the peripheral nervous system are satellite cells and Schwann cells.
Neurons are the other the other type of cell that comprise nervous tissue. Neurons have cell bodies, dendrites, and axons.
Key Terms
neuron: The main cell type in nervous tissue.
neuroglia: Supporting cells in nervous tissue.
Nervous tissue, one of the four main tissue types, is composed of neurons and supporting cells called neuroglia. Neuroglia are also called “glial cells.”
Neuroglia
There are six types of neuroglia—four in the central nervous system and two in the PNS. These glial cells are involved in many specialized functions apart from support of the neurons. Neuroglia in the CNS include astrocytes, microglial cells, ependymal cells and oligodendrocytes. In the PNS, satellite cells and Schwann cells are the two kinds of neuroglia.
Astrocytes
Astrocytes are shaped like a star and are the most abundant glial cell in the CNS. They have many radiating processes which help in clinging to the neurons and capillaries. They support and brace the neurons and anchor them to the nutrient supply lines. They also help in the guiding the migration of young neurons. Astrocytes control the chemical environment around the neurons.
Microglial Cells
Microglial cells are small and ovoid un shape with thorny processes. They are found in the CNS. When invading microorganism or dead neurons are present, the microglial cells can transform into a phagocytic macrophage and help in cleaning the neuronal debris.
Ependymal Cells
Ependymal cells are ciliated and line the central cavities of the brain and spinal cord where they form a fairly permeable barrier between the cerebrospinal fluid that fills these cavities and the tissue cells of the CNS.
Oligodendrocytes
Oligodendrocytes line up along the nerves and produce an insulating cover called myelin sheath. They are found in the CNS.
Satellite Cells
Satellite cells surround neuron cell bodies in the peripheral nervous system (PNS). They are analogous to the astrocytes in the CNS.
Schwann Cells
Schwann cells surround all nerve fibers in the peripheral nervous system and form myelin sheaths around the nerve fibers. They are found in the PNS. Their function is similar to oligodendrocytes.
Neurons
Neurons consist of cell body and one or more slender processes. The neuronal cell body consists of a nucleus and rough endoplasmic reticulum or Nissl Bodies. The cell body is the major biosynthetic center of a neuron and contains the usual organelles for the synthesis of proteins and other chemicals. Arm like processes extend from the cell body to all neurons.
The two types of neuron processes are called dendrites and axons. Dendrites are motor neurons that are short and have a large surface area for receiving signals from other neurons. Dendrites convey incoming messages towards the cell body and are therefore called the receptive input region.
The axon arises from the cone shaped portion of the cell body called the axon hillock. Functionally, the axon is the conducting region of the neuron and is responsible for generating and transmitting impulses typically away from the cell body. A single axon routes the nerve impulse from the cell body to another neuron or an effector organ. The axon can have many terminal branches, so each time the nerve fires, it can stimulate more than one cell.
Characteristics of Muscle Tissue
The three types of muscle tissue are skeletal, smooth, and cardiac.
LEARNING OBJECTIVES
Describe the types of muscle tissue
KEY TAKEAWAYS
Key Points
Muscle tissue can be divided functionally, is it under voluntary or involuntary control; and morphologically, striated or non-striated.
By applying these classifications three muscle types can be described; skeletal, cardiac and smooth.
Skeletal muscle is voluntary and striated, cardiac muscle is involuntary and straited and smooth muscle is involuntary and non-striated.
Key Terms
skeletal muscle: The voluntary muscle of vertebrates, which is striated and anchored by tendons to bone, is used to effect skeletal movement such as locomotion.
involuntary: A muscle movement not under conscious control e.g. the beating of the heart.
striated: The striped appearance of certain muscle types in which myofibrils are aligned to produce a constant directional tension.
voluntary: A muscle movement under conscious control e.g. deciding to move the forearm.
smooth muscle: Involuntary muscle that is found within the intestines, throat, uterus, and blood vessel walls.
cardiac muscle: The striated and involuntary muscle of the vertebrate heart.
Muscle Tissue
Muscle is a soft tissue that is highly specialized for the production of tension which results in the generation of force. Muscle cells, or myocytes, contain myofibrils comprised of actin and myosin myofilaments which slide past each other producing tension that changes the shape of the myocyte. Numerous myocytes make up muscle tissue and the controlled production of tension in these cells can generate significant force.
Types of Muscle Tissue
Muscle tissue can be classified functionally, voluntary or involuntary and morphologically striated or non-striated. Voluntary refers to whether the muscle is under conscious control, striation refers to the presence of visible banding within myocytes which occurs due to organization of myofibrils to produce a constant direction of tension.
By applying the above classifications it is possible to describe three forms of muscle tissue which perform the wide range of functions described.
Skeletal Muscle
Skeletal muscle mainly attaches to the skeletal system via tendons to maintain posture and control movement for example contraction of the biceps muscle, attached to the scapula and radius, will raise the forearm. Some skeletal muscle can attach directly to other muscles or the skin, as seen in the face where numerous muscles control facial expression.
Skeletal muscle is under voluntary control, although this can be subconscious for example when maintaining posture or balance. Morphologically skeletal myocytes are elongated and tubular and appear striated with multiple peripheral nuclei.
Cardiac Muscle Tissue
Cardiac muscle tissue is found only in the heart where cardiac contractions pump blood throughout the body and maintain blood pressure.
As with skeletal muscle cardiac muscle is striated, however it is not consciously controlled and so is involuntary. Cardiac muscle can be further differentiated from skeletal muscle by the presence of intercalated discs which control the synchronized contraction of cardiac tissues. Cardiac myocytes are shorter than skeletal equivalents and contain only one or two centrally located nuclei.
Smooth Muscle Tissue
Smooth muscle tissue is found associated with numerous other organs and tissue systems such as the digestive system or respiratory system. It plays an important role in the regulation of flow in such tissues for example aiding the movement of food through the digestive system via peristalsis.
Smooth muscle is non-striated, although it contains the same myofilaments they are just organized differently, and involuntary. Smooth muscle myocytes are spindle shaped with a single centrally located nucleus.
Muscle Types: Cardiac and skeletal muscle are both striated in appearance, while smooth muscle is not. Both cardiac and smooth muscle are involuntary while skeletal muscle is voluntary.
Epithelial Membranes
The mucous membranes are linings of mostly endodermal origin, covered in epithelium, which are involved in absorption and secretion.
LEARNING OBJECTIVES
Describe the function of mucous membranes
KEY TAKEAWAYS
Key Points
The mucous membranes are linings of ectodermal origin. It consists of an epithelium layer and an underlying lamina propria of loose connective tissue.
The mucus membranes are involved in absorption and secretion.
Most mucosal membranes contain stratified squamous or simple columnar epithelial tissue types.
Submucosal exocrine glands secrete mucus to facilitate the movement of particles along the body’s various tubes, such as the throat and the intestines.
Key Terms
mucous membrane: Linings of cavities that are exposed to the external environment and to internal organs.
The mucous membranes are linings of ectodermal origin. It consists of an epithelium layer and an underlying lamina propria of loose connective tissue. These mucus membranes are involved in absorption and secretion. They line cavities that are exposed to the external environment and internal organs. These membranes exist in the hollow organs of the digestive, respiratory, and urogenital tracts.
The term “mucous membrane” refers to where they are found in the body; not every mucous membrane secretes mucus. Secreted mucous traps the pathogens in the body, preventing any further progression of microbes.
Most mucous membranes contain stratified squamous or simple columnar epithelial tissue. The epithelial tissue sheet lies directly over the layer of loose connective tissue called lamina propria. In some mucosa, the lamina propria rests on a deeper, third layer of smooth muscle.
The submucosa is the tissue that connects the mucosa to the muscle outside the tube. Submucosal glands consist of exocrine glands that secrete mucus. These glands excrete mucus to facilitate the movement of particles along the body’s various tubes, such as the throat and intestines. The submucosal glands are a companion to unicellular goblet cells, which also produce mucus, and are found lining the same tubes.
General organization of the gastrointestinial tract: Illustration of mucosa in relation to other lining components.
Synovial Membranes
A synovial membrane is the soft tissue found between the articular capsule (joint capsule) and the joint cavity of synovial joints.
LEARNING OBJECTIVES
Describe the role of synovial membranes in joint function
KEY TAKEAWAYS
Key Points
Connective tissue membranes do not contain an epithelial cell layer. Synovial membranes line the inner surface of the capsule of a synovial joint and contain synovial fluid within; which functions to lubricate joint movement.
Synovial membranes often have two layers: a fibrous outer layer, or subintima, and an inner layer, or intima.
The intimal cells are termed synoviocytes and are of two types, fibroblastic (type B synoviocytes) and macrophagic (type A synoviocytes).
The meninges is the system of membranes that envelopes the central nervous system.
The meninges is comprised of three layers, the dura mater, arachnoid mater and pia mater.
Key Terms
meninges: The system of membranes that envelopes the central nervous system.
synovial membrane: The connective tissue which lines the inner surface of the capsule of a synovial joint.
Connective tissue membranes: A membrane which does not contain an epithelial cell layer.
synoviocyte: The cell that forms the initima layer of a snynovial membrane.
Membranes are thin sheets of tissue found within the body which can line cover tissues or line cavities. Connective tissue membranes do not contain an epithelial cell layer and there are two forms found in the body; synovial and meninges membranes.
Synovial Membrane
The synovial membrane (or synovium ) is the connective tissue which lines the inner surface of the capsule of a synovial joint and secretes synovial fluid which serves a lubricating function, allowing joint surfaces to smoothly move across each other.
The morphology of synovial membranes may vary, but it often consists of two layers. The outer layer, or subintima, is a thicker and fibrous protecting the single cell initma layer which is composed of synoviocytes.
Synoviocytes
The intimal cells are termed synoviocytes and are of two types: fibroblastic (type B) and macrophagic (type A). It is the lack of epithelial cells within the initma which defines the synovial membrane as connective rather than epithelial.
The type B synoviocytes manufacture a long-chain sugar polymer called hyaluronan, which makes the synovial fluid together with a molecule called lubricin, which lubricates the joint surfaces. The water component of synovial fluid is effectively trapped in the joint space by the hyaluronan, due to its large, highly negatively charged moeties.
The type A synoviocytes are responsible for the removal of undesirable substances from the synovial fluid.
Structure of Synovium
Synovial Membrane: A synovial joint showing the location of the synovial membrane.
The surface of synovium may be flat or may be covered with finger-like projections (villi), to allow the soft tissue to change shape as the joint surfaces move on one another.
Just beneath the intima, most synovium has a dense net of small blood vessels that provide nutrients, not only for synovium, but also for the avascular cartilage. In any one position, much of the cartilage is close enough to get nutrition directly from the synovium.
Meninges
The meninges is the system of membranes that envelopes the central nervous system. In mammals, the meninges consist of three layers: the dura mater, the arachnoid mater, and the pia mater. The primary function of the meninges and of the cerebrospinal fluid is to protect the central nervous system.
Dura Mater
The dura mater is a thick, durable membrane which lies closest to the skull. It consists of two layers, the periosteal layer which lies closest to the calvaria, and the inner meningeal layer which lies closer to the brain. It is composed of dense fibrous tissue, and its inner surface is covered by flattened cells like those present on the surfaces of the pia mater and arachnoid.
Arachnoid Mater
The middle layer of the meninges is the arachnoid mater, so named because of its spider web-like appearance. It provides a cushioning effect for the central nervous system. The arachnoid mater is a thin, transparent membrane composed of fibrous tissue and, like the pia mater, is covered by flat cells also thought to be impermeable to fluid.
Pia Mater
The Meninges: This figure displays the meninges with respect to the skull and surface of the brain.
The pia mater is the innermost layer of the meninges. It firmly adheres to the surface of the brain and spinal cord, following the brain’s minor contours. As such it is a very thin, delicate membrane composed of fibrous tissue covered on its outer surface by a sheet of flat cells thought to be impermeable to fluid.
The subarachnoid space is the space that normally exists between the arachnoid and the pia mater, which is filled with cerebrospinal fluid. Normally, the dura mater is attached to the skull or to the bones of the vertebral canal in the spinal cord. The arachnoid is attached to the dura mater, while the pia mater is attached to the central nervous system tissue.
ResourcesNotes: Chapter 5: Tissues | here are some study things | as well as hereConcept Map on Tissues - organize the tissues into this handy graphic organizerTissue Chart - simple cheat sheet for learning the tissuesConnective Tissue Coloring - color the matrixHistology - viewing images of tissues | Histology WebsiteTissues Review GuideQuizzesPractice Quiz over Tissues
Tissue -
a group or mass of similar cells working together to perform certain common functions
There are 4 major types of tissue
Epithelial
Connective
Muscle
Nervous
1. Epithelial Tissue
General Characteristics:
- Found throughout the body, covers all body surfaces both inside and out.
- Main glandular tissue.
- Attached to underlying connective tissue by noncellular nonliving basement membrane.
- Usually has no vascular tissue - blood supply
- Cells reproduce rapidly (rapid healing).
- Cells tightly packed together
Functions: Protection, secretion, absorption, excretion, sensory perception
Six Specific types of epithelial tissue -
categorized based on the shape of the cells and the layers of cells.
A. SIMPLE SQUAMOUS -
single layer (simple) of very thin, flattened cells (squamous). Function: diffusion and filtration. Found in air sacs of lungs, walls of capillaries.
B. SIMPLE CUBOIDAL -
single layer, cube-shaped cells. Function: Secretion and absorption. Found: Lining of kidney tubules, ducts of glands, covering surface of ovaries
C. SIMPLE COLUMNAR -
single layer, elongated cells with their nuclei in about the same position in each cell (usually near the basement membrane). Protection, secretion, absorption.
Found in the lining of digestive tract and uterous
- contains scatter goblet cells functioning in the secretion of mucus
- some columnar cells (involved in absorption) have tiny finger-like processes from their free surface called microvilli (increases surface area)
D. STRATIFIED SQUAMOUS -
muli-layered, squamous cells. Thicker tisse.
Functions in protection. Found lining body cavities like the mouth and outer layer of skin
E. PSEUDOSTRATIFIED COLUMNAR -
appear "stratified" but really a single layer with nuclei at various levels giving the appearance of layered cells. Usually ciliated (tiny, hair-like projections for sweeping materials along a surface). Contains goblet cells.
- Function: secretion and cilia-aided movement
- Location: lining air passages like the trachea and tubes of the reproductive system
TRANSITIONAL EPITHELIUM -
thick, layered cuboidal cells. "Stretchable" tissue, also forms barrier to block diffusion. Found: lining of urinary bladder.
2. Connective Tissue
General Characteristics: -Most abundant tissue in your body, found throughout
-Binds structures together
-Provides support, protection, framework, fills space, stores fat, produces blood cells, fights infection, and helps repair tissue.
-Composed of more scattered cells with abundant intercellular material ' matrix
-Made up of a ground substance (fluid, semi-solid) and fibers
-Most has a good blood supply
-Cells can reproduce
Three common types of cells:
1. mast cells (prevents blood clots)
2. macrophages (phagocytic) and
3. fibroblasts (most abundant, produce fibers)
Main types of fibers:
-collagenous fibers - thick, made of protein collagen, major structural protein in the body, appear in long parallel bundles. Strong, flexible, but not very elastic, also known as white fibers. (bones, ligaments, tendons)
- elastic fibers - microfibrils in protein elastin, yellow fibers. Not as strong, but very elastic (respiratory and vocal cords)
CATEGORIES OF CONNECTIVE TISSUE
A. LOOSE C.T. or AREOLAR TISSUE -
binds skin to underlying organs and organs to organs, space between muscles, throughout body
B. ADIPOSE TISSUE -
aka FAT, beneath skin, around kidneys and eyeballs, abdominal membranes. Function: Protective cushion, insulation to preserve body heat, stores energy, cells are called adipocytes
C. FIBROUS C.T. -
dense tissue, closely packed, thick collagenous fibers and fine network of elastic fibers. Few cells, poor blood supply, thus slow healing.
Tendons - connect muscles to bones
Ligaments - connect bones to bones
CARTILAGE (all cartilage cells are called chondrocytes)
D. HYALINE CARTILAGE -
very fine white (collagenous) fibers. Most common cartilage. Covers ends of bones and joints, noise, respiratory passages.
E. ELASTIC CARTILAGE -
more flexible and elastic, external ear and larynx
F. FIBROCARTILAGE -
very tough, large numerous collagenous fibers. Intervertebral disks, menisci
G. BONE TISSUE -
Osseus tissue. Rigid due to mineral salts.
Layers - lamellae, haversian canals, osteocytes
H. BLOOD TISSUE -
circulates throughout the body
3. Muscle Tissue
A. Skeletal - skeletal muscles - voluntary (striated)
B. Smooth - in hollow organs, stomach - involuntary
C. Cardiac - wall of the heart
4. Nerve Tissue -
Found in brain, spinal cord, nerves
A. Neurons - transmit signals
B. Neuroglia - protection, support
