1.2 Simple Squamous Epithelium

This type of simple epithelium consists of a single layer of flat, scale-like cells. Note the example of this epithelium shown in image 1.2a, which is the wall of a capillary. From a side view the cells are so flat that the nucleus makes the cell bulge upward, and from a top-surface view the epithelium would resemble a tiled floor. The nucleus of each cell is centrally located and oval or spherical. Since a simple squamous epithelium has only one layer of cells, it is highly adapted to diffusion, osmosis, and filtration. Thus, it lines the air sacs of the lungs where oxygen is exchanged with carbon dioxide (image 1.2b). Note the alveolus of the lung (indicated by the central star in image 1.2b) surrounded by the very thin simple squamous epithelial wall. It is also present in the part of the kidney that filters the blood. In image 1.2c, the arrow is pointing to the thin simple squamous epithelium which makes up the wall of Bowman's capsule, a balloon-like structure that collects filtrate in each nephron in the kidney. It also lines the inner surfaces of the membranous labyrinth and tympanic membrane of the ear. Simple squamous epithelium, because it is so thin and fragile, is usually found in body parts that have little wear and tear.

1.3 Simple Cuboidal Epithelium

Image 1.3a shows the simple cuboidal epithelium which lines the collecting ducts (CD) of the kidney. Note that the cells are about as wide as they are tall (a definitive character of simple cuboidal epithelium), and they appear somewhat hexagonal in surface view. Short microvilli are present on the cell surface (which are hard to see here), and a centrally located cilium (Ci) projects outward from each cell into the tubule lumen. A number of the tubule cells have been internally exposed revealing spherical cell nucleus (Nu).

A light-microscope cross-sectional view of the collecting ducts of the kidney is shown in image 1.3b. Note that the cells are approximately as tall as they are wide (there is a good example in the middle of slide). Simple cuboidal epithelium is often used in the body to make tiny tubules in organs. That is so because it is stronger than simple squamous and can withstand more internal fluid pressure. As fluid-carrying tubules become larger, the simple cuboidal epithelium can thicken to become an even stronger stratified cuboidal epithelium.

1.4 Simple Columnar Epithelium

Simple columnar epithelium looks like simple cuboidal epithelium with the cells stretched into elongated vertical columns (as can readily be seen in image 1.4a). In image 1.4b, you can see a light microscopic view of the same type of columnar cells from the gall bladder. Note that the nuclei are large and oval-shaped, and are usually located near the base of the cell. Simple columnar epithelium may be ciliated or nonciliated, depending on its location and function. In the uterine tube (oviduct), for instance, ciliated cells help sweep the egg cell toward the uterus after it leaves the ovary. Simple columnar epithelia may also have microvilli (labeled as "Mv" in image 1.4a) on their free surfaces. Cells in the small intestine are covered with microvilli that increase the surface area greatly and aid the absorption of food from the intestine into the bloodstream. Simple columnar epithelium is commonly found in the stomach, intestines, digestive glands, and in the gallbladder it protects delicate linings and assists in the absorption and secretion of substances in and out of the bile.

In the kidneys, simple columnar epithelia form the kidney tubules and contains microvilli (as seen in image 1.4c). Here in the distal collecting duct it functions in water reabsorption from the tubular lumen (labled "a" in image 1.4c). These epithelia also line the smaller ducts of some glands and the secreting units of glands, such as the thyroid. This tissue usually performs the functions of secretion and absorption. Secretion is the production and release by cells of a fluid that may contain a variety of substances such as mucus, perspiration, or enzymes. Absorption is the intake of fluids or other substances by cells of the skin or mucous membranes.

1.5 Ciliated Columnar Epithelium

Another modification of columnar epithelium is found in cells that carry hair-like processes called cilia. Cilia are whip-like organs that give the columnar epithelium the ability to move objects or fluid substances across its surface. For example, in human oviducts the cilia push the fertilized egg towards the uterus. Cilia (Ci) (and microvilli, Mv) can be seen in image 1.5a and also in the epithelium histological slide of the trachea in image 1.5b (indicated by the arrow). In these portions of the upper respiratory tract, ciliated columnar cells are interspersed with goblet cells. Mucus secreted by the goblet cells forms a film over the respiratory surface and this film then traps foreign particles that are inhaled. The cilia beat in unison and move the mucus, with any trapped foreign particles, toward the throat, where it can be swallowed or eliminated. Air is therefore filtered by this process before entering the lungs. Ciliated columnar epithelium is also found in some paranasal sinuses, the central canal of the spinal cord, and the ventricles of the brain. Image 1.5c shows the ciliated columnar ependymal cells that line the ventricles of the brain and help move the cerebro-spinal fluid through the central nervous system.

1.6 Pseudostratified Columnar Epithelium

Pseudostratified columnar epithelium consists of a single layer of cells that appears to be stratified or multilayered in sections when viewed with the light microscope, as can be easily be observed in image 1.6a. Some histologists do not consider this to be a type of epithelium distinct from columnar, but simply believe that if a simple columnar epithelium is compressed horizontally, the cell nuclei will move to different vertical levels within the epithelium. This type of epithelium is often found with simple ciliated columnar. For example, pseudostratified also makes up the lining of the trachea (image 1.6b). It also lines the large bronchi, the nasal mucosa, the epididymis and vas deferens of the male reproductive tract. Although all cells of pseudostratified epithelium are in contact with the basement membrane, many become narrow toward their apical end, thus appearing not to reach the free surface.

In Image 1.6c, note the cilia (indicated by the arrow) on the pseudostratified columnar epithelium which lines some of the kidney tubules. In the center uppermost area of image 1.6d, also observe the "pseudostratified" -- the false appearance of a multilayered epithelium--nature of the inner lining. The red nuclei present a many-layered appearance that is characteristic of this epithelium.

1.7 Stratified Squamous Epithelium

1.8 Stratified Cuboidal Epithelium

Stratified cuboidal epithelium (imge 1.8a), which is a multicellular layer, is found only in a few places in the body, and its cells are rarely shaped like true cubes. This type of epithelium is found in the ducts of sweat glands, in sebaceous (oil) glands, and in the developing epithelium in the ovaries and testes. In the tubules of the testes this epithelium makes spermatozoa which start from large cuboidal cells known as spermatogonia (image 1.8b). The seminiferous tubule wall is different from most other types of stratified cuboidal epithelia in which the superficial cells are larger than the basal cells and a distinct basement membrane is clearly evident.

1.9 Stratified Columnar Epithelium

Stratified columnar epithelium is characterized by a regular arrangement of columnar cells at the superficial layer, with underlying smaller, cuboidal-type cells in contact with the basement membrane (image 1.9a). This type of epithelium can be ciliated, as in the larynx and the nasal surface of the soft palate. It is also found in the pharynx, urethra, and in the larger excretory ducts of the salivary and mammary glands. Image 1.9b is the excretory duct of a salivary gland which is lined internally with a stratified columnar epithelium. Note the similarity of the epithelium along the bottom of the slide with the diagram on the computer screen. Although the cells are small in this section, the dark blue nuclei are tightly assembled in cuboidal cells at the base of the epithelium, and they are seen as more scattered in the columnar cells above. Also note the large, light-colored goblet cells which secrete mucus into the duct.

1.10 Transitional Epithelium

Transitional epithelium is very similar to a nonkeratinized stratified squamous epithelium. The major distinction is that cells in the outer layer in transitional epithelium tend to be large and round, rather than small and flat. When stretched, they are drawn out into squamous-like cells. Because of this stretch property, transitional epithelium lines hollow structures that are subjected to expansion from within, such as the urinary bladder, ureters, and parts of the urethra.

The cells of transitional epithelium lining the bladder vary in shape, depending on how distended they are by the fluid the organ contains. This flexibility is a useful feature in the urinary bladder, where it enlarges or shrinks depending on how much total urine it is holding. Transitional epithelium is clearly stratified and lines the entire urinary tract, including the ureters, urinary bladder, urethra, pelvis, and calyxes of the kidneys. When the surface cells are not being stretched, they appear round. Note the large rounded "pear-shaped" cells at the top of the bladder epithelium in both image 1.10a and image 1.10b. When the bladder is full, the surface cells are stretched out and assume a flat appearance. But when the bladder is empty, its inner membranes are often pleated like an accordion. Image 1.10c demonstrates pleated epithelium of the urinary bladder when it is relaxed. As the bladder fills, the pleats flatten out and the epithelium can stretch, thus allowing the organ to stretch without breaking.

1.11 Glandular Epithelium (Alveolar)

The function of glandular epithelium is secretion, accomplished by the glandular cells that lie under the surface. All glands in the body are classified as exocrine (glandular epithelium) or endocrine (hormone-secreting), according to whether they secrete substances onto a free surface or into the blood. Exocrine glands often secrete their products into ducts before they pass onto the free surface. The secretions of exocrine glands can include mucus, oils, perspiration, and digestive juices. Some examples of exocrine glands are: sweat glands, which eliminate perspiration to cool the skin; salivary glands, which secrete digestive enzymes; and goblet cells, which produce mucus. Conversely, endocrine glands are ductless and they secrete their products into the blood. The secretions of endocrine glands are always hormones, chemical messengers that regulate various physiological activities.

Exocrine glands are classified into two types: unicellular and multicellular. Unicellular glands are single-celled. A good example of a unicellular gland is a goblet cell. Goblet cells are found in the epithelial lining of the digestive, respiratory, urinary, and reproductive systems. They produce mucus which lubricates the inner surfaces of these membranes; you will observe these glands in the next topic, tubular glandular epithelium.

Multicellular glands occur in two basic types. If the secretory portions of the gland are tubular, it is referred to as tubular. If they are flask-like, then it is called acinar. If the gland contains both tubular and flask-like secretory portions, it is called a tubulo-acinar gland. Further, if the duct of the gland does not branch, then it is referred to as a simple gland; if the duct does branch and subbranch, it is called a compound gland.  Image 1.11a depicts acinar glands (AU) located in the pancreas. Image 1.11b depicts a mucus-secreting acinus of a salivary gland.

1.12 Glandular Epithelium (Tubular)

The second major type of glandular epithelium is tubular. Here the secretory cells are organized into simple or compound tube-like elements. Good examples of simple tubular glands (TG) are the "colonic pits" of the large intestine (image 1.12a). The secretory cells produce mucus which then flows out of the tubular gland through a pore (the lumen, labled "Lu" in image 1.12a) into the colonic cavity. A light microscope slide of these same colonic pits can be seen in image 1.12b. Note the numerous and large "goblet cells" which appear clear white on the screen. Image 1.12c is another view of colonic pits, this time stained with a mucus (green) stain. Again, note the obvious unicellular goblet cells which produce the mucus.

1.13 Loose (Areolar) Connective Tissue

Loose or areolar connective tissue consists of numerous fibers and cells embedded in a semifluid intercellular substance and is one of the common tissues found in the human body. Various cell types and fibers are identified in image 1.13a.  In image 1.13b, a fiber is labeled "a", and a fibroblast cell, is labeled "b". The term "loose" refers to the loosely woven arrangement of fibers and cells in the intercellular substance.

The intercellular substance consists principally of a viscous material called hyaluronic acid which facilitates the passage of nutrients from the blood vessels to adjacent cells. The three types of fibers embedded between the cells of loose connective tissue are collagenous, elastic, and reticular fibers. Collagenous (white) fibers are very tough and resistant to a pulling force, but are somewhat flexible because they are usually wavy. Collagen fibers--a large fiber-- is indicated by the arrow in image 1.13c. These fibers often occur in bundles and are composed of many minute fibers called fibrils, an arrangement that affords a great deal of strength. Chemically, collagenous fibers consist of the protein called collagen. Elastic (yellow) fibers, by contrast, are smaller than collagenous fibers and freely branch and rejoin one another. Elastic fibers (indicated by the arrow in image 1.13d) consist of a protein called elastin. These fibers also provide some strength and have great elasticity, up to 50 percent of their length. Reticular fibers also consist of collagen, plus some glycoprotein. They are very thin fibers that branch extensively but are not as strong as collagenous fibers.

The cells in loose connective tissue are diverse. Many are fibroblasts which are often seen as large, flat cells with tapered processes. If the tissue is injured, the fibroblasts form fibers and the viscous ground substance. When mature, fibroblasts are referred to as fibrocytes. The basic distinction between the two is that fibroblasts (or "blasts" of any form of cell) are involved in the formation of immature tissue or repair of mature tissue, and fibrocytes (or "cytes" of any form of cell) are inactive cells that no longer produce fibers or matrix. Other cells found in loose connective tissue are called macrophages. They are irregular shaped with short branching projections and are capable of engulfing bacteria and cellular debris by the process of phagocytosis. A third kind of cell in loose connective tissue is the plasma cell. These cells are small and ovoid in shape. Plasma cells develop from white blood cells called "B" Iymphocytes. They produce protective antibodies. Another common cell in loose connective tissue is the mast cell which develops from the blood-born basophil. It forms heparin, which is an anticoagulant that prevents blood from clotting in the vessels. Mast cells are also believed to produce histamine and serotonin, chemicals that dilate small blood vessels.

1.14 Fibrocyte

The shape of fibroblasts and fibrocytes is highly variable. In some instances (e.g., loose areolar connective tissue in the adventitia of the trachea as can be seen in image 1.14a), the fibroblasts (labeled Fi in image 1.14a) may be fusiform. For a size comparison, note the presence of the nearby erythrocyte (labled Er in image 1.14a). In image 1.14b, this tapered shape is also evident in the fibroblast (labled "b" in image 1.14b) imbedded in collagen fibers (labled "a" in image 1.14a). Image 1.14c is a light microscope slide of fibroblasts (labled "b" in image 1.14c). Also note another type of connective tissue cell, the mast cell (labeled "a" in image 1.14c) which appears full of dark stained granules.

1.15 Macrophage

Phagocytic cells, such as macrophages and leukocytes, populate connective tissue and are prominent at sites of bacterial infections. Upon stimulus, macrophages are capable of ingesting foreign particles, microorganisms, and expired cells within the body. The fixed macrophages appear flattened and possess thin, tendril-like cytoplasmic extensions, whereas the wandering type (left of image 1.15a) appear rounder and exhibits a ruffled surface extension, or pseudopod (labeled Ps in image 1.15a), during movement. The wandering macrophage seen here is from the alveolar surface of the lung.

The light micrograph shown in image 1.15b reveals a resting macrophage (labeled "a" in image 1.15b) next to a fibrocyte (labeled "b" in image 1.15b) in loose connective tissue. Note the granular primary lysosomes in the cytoplasm of the macrophage. Once they are activated, macrophages increase greatly in size, develop cytoplasmic extensions called filopodia, and begin to ingest external objects. Image 1.15c depicts "activated" macrophages. These macrophages have a large number of membrane-bound inclusions called digestive vacuoles (or secondary lysosomes) which result from the fusion of phagocytized material with primary lysosomes.

1.16 Mast Cell

Mast cells are typically situated in the vicinity of blood vessels, respiratory airways, and the peritoneal lining of the body cavity. They are capable of both the synthesis and release of heparin, histamine, the slow-reacting substance of anaphylaxis, and the eosinophilic chemotatic factor of anaphylaxis,  which are stored within intracellular granules. Because of the action of these substances, the mast cells are thought to play the role in the regulation of vascular permeability and airway diameter. Viewed from the mast cell surface in image 1.16a, the rounded form of underlying granules can often be distinguished. This can also be seen, but perhaps less clearly, on the light microscope slide in image 1.16b where the mast cell (labeled "a" in image 1.16b) shows a cytoplasm full of dark blue-stained granules.

During their degranulation process, when the mast cell interior is exposed (image 1.16c), the distribution of intracellular granules becomes apparent. The process of histamine release is thought to consist of two steps. In the first step, fusion of the plasma membrane with the granule membrane allows communication of the extracellular fluid with the granules, at which time a number of granules may be expelled. Image 1.16d depicts a mast cell actively degranulating under the light microscope. In the second step, the granule disrupts, thereby allowing histamine to diffuse freely into the surrounding medium. Image 1.16c granules that are in the process of being extruded from the mast cell surface are illustrated. The stimulus for granule release under physiological conditions often involves the binding of a specific antigen to the surface of sensitized mast cells (those with immunoglobulin E on their surface).

1.17 Irregular Dense Connective Tissue

Dense connective tissues are distinguished by an abundance of collagen fibers, which provide the capacity to resist exceptional degrees of tension. Dense irregular connective tissue (image 1.17a) is essentially a dense areolar tissue that contains all the same elements as loose connective tissue but has fewer cells and more numerous collagenous fibers. The fibers are closely interwoven, forming a compact tissue with fewer spaces. The highly irregular arrangement of fibers can be seen in image 1.17b. Because the tensions that need to be resisted by these tissues come from all directions, the fibers are oriented randomly. An example of dense irregular connective tissue is the dermis layer of the skin. Image 1.17c shows an azan (blue) stain of dense connective tissue found in the dermis of the skin. Although this slide does not present a wealth of visual information--one can detect the randomness of the blue fibers and a few cells in the lower right--it shows some of the artistry to be found in the exploration of anatomy.

1.18 Regular Dense Connective Tissue

Regular dense connective tissue (image 1.18a) is characterized by a predominance of collagenous fibers (labled CF in image 1.18a) that are tightly packed in parallel bundles. In this tissue, the tensions to be resisted come from a single direction, parallel to the orientation of the fibers. Because of the prevalence of collagenous fibers, this tissue is sometimes referred to as white fibrous connective tissue. The only cells present are fibroblasts (labeled Fb in image 1.18a), which are located between the fiber bundles. This is also readily seen in image 1.18b where three fibroblasts are seen in the center of the screen (darker purple). The abundance of fibers gives this tissue great strength. It forms the tendons of muscles, the ligaments of joints (which also contain some elastic fibers), and various fibrous membranes, such as fascia and aponeuroses. Fascia surrounds the organs and the muscles. Aponeuroses are broad sheets that function as thin tendons, attaching muscles to other structures.

1.19 Adipose Cells

Fat cells are commonly found in loose connective tissue. In some areas of the body, however, the numbers of these cells become great enough to warrant classifying it as a separate type of connective tissue, termed adipose tissue. Adipose tissue (image 1.19a) consists of large aggregations of fat-storing cells, referred to as adipocytes (labeled Ad in image 1.19a), in which connective tissue fibers (labeled Fi in image 1.19a) surround and organize groups of adipocytes into lobules. White adipose tissue is commonly termed as "unilocular" because each adipocyte contains a single large clear vacuole of accumulated lipid which fills most of the cytoplasmic area of the cell and pushes the nucleus to the periphery. A light microscope slide of adipose cells can be seen in image 1.19b.

Aggregations of adipocytes are often found within many organs and can dominate a slide of the tissue. Image 1.19c shows a broad field of adipose tissue. Adipose tissue commonly surrounds the kidneys and adrenal glands, and may also be associated with the mesentery, bone marrow, and omentum. Another type of adipose tissue, termed brown adipose tissue, is commonly seen in human infants and hibernating species. The brown, or "multilocular," adipose tissue differs from white adipose tissue in that lipid droplets inside the cell do not coalesce, but instead form multiple lipid vacuoles. The brown color is due to an abundant vasculature and to the presence of intracellular lysosomes containing brown pigment. The mitochondria of brown adipocytes are larger and more numerous. Since the mitochondria within brown fat are not capable of oxidative phosphorylation, they are thought of to function in generating the necessary heat required of some species during hibernation.

1.20 Skin (also see 2. Integumentary System microanatomy)

The skin, or integument, covers the body surface. In image 1.20a, a superficial epidermal (labeled Ep in image 1.20a) layer is attached to an underlying dermis (labeled De in image 1.20a) by a basement membrane. The skin accounts for approximately 15% of the body weight in humans, and the functions of the skin are closely related to its internal organization. Cells in the basal layer of the epidermis are capable of rapid mitosis and undergo keratinization to form the more superficial layers of the epidermis. A distinction is commonly made between thick skin, which covers the palms of the hands and soles of the feet, and thin skin, which covers most of the remainder of the body. Thick skin (image 1.20b) has a deeper epidermis than thin skin; the stratum corneum (composed of keratin) and stratum granulosum (the cells of which contain keratohyalin) are especially thick. The wavy black layer in the middle of the epidermis is the stratum granulosum, and above it is the stratum corneum. A layer called the stratum lucidum (which contains eleidin) is often absent in thin skin. The dermis of thin skin, however, is deeper than that of thick skin. Early in development the epidermis can, as a result of its ability to proliferate and differentiate, give rise to such appendages as hair (labeled Ha in image 1.20a), nails, sebaceous glands, and sweat glands. In image 1.20b, the structure labeled "a" represents the duct of a sweat gland which ultimately reaches the terminal coiled secretory elements (labeled "b" in image 1.20a).

Image 1.20c is a microscopic view of skin showing both oil-secreting sebaceous glands (labeled "a" in image 1.20c) and the secretory portions of sweat glands (labeled "b" in image 1.20c).

Skin serves a variety of important functions. The tough protein keratin forms the outer layer of an uninterrupted cellular covering. The keratin serves as a waterproofing material and is impermeable to many disease organisms. Melanocytes migrate into the basal layer of the epidermis and synthesize melanin pigment, which serves to protect against the harmful effects of ultraviolet light. Free nerve endings may be present in the epidermis, and various encapsulated nerve endings are present in the underlying dermis. Sweat glands are also located in the dermis of the skin, and through their activity, the integument functions in thermoregulation and the maintenance of water balance. Fat cells (labeled FC in image 1.20a) located in the dermis also provides insulation and food storage for the body.