Cells that line surfaces:
Epithelium is the name given to the cells that line a surface. The outer surface of the body is lined by an epithelium called the epidermis. The inside of the body cavity is also lined by an epithelium, as is the outer surface of many internal organs. The inner surfaces of all the tubes and ducts of the body are also lined by epithelia, whether they open to the outside (eg. digestive system, reproductive system) or are enclosed (eg. cardiovascular system). Glands (eg. sweat glands, salivary glands, liver) are also formed by epithelial cells.
Barrier and other functions:
A major function of all epithelia is to form a barrier. Thus, epithelial cells are always contiguous with one another and are usually joined by special junctions. A discussion of the types of junctions is found in Ross et al., pg. 66-72. Generally, the intercellular space between epithelial cells is very limited. The functions of epithelia differ markedly. Although all form a barrier, some are much more impermeable than others. The epidermis of the skin is a more or less impermeable barrier. On the other hand, the epithelium lining the intestine, while providing a barrier, must also absorb nutrients. Some epithelia have a secretory function (eg. stomach, glands), and some are actively involved in the synthesis of a large number of metabolic substances (eg. the endothelial cells lining blood vessels).
Free surface and basement membrane:
All epithelia have a free surface, that is to say a surface to which no cellular or extracellular formed elements adhere. The free surface may contain specialized modifications, such as microvilli or cilia. Such modifications enhance the function of the epithelium. Microvilli in the intestine greatly increase the absorptive surface, while cilia in the respiratory tract beat mucous secretions and entrapped particles away from the lungs toward the pharynx. A discussion of cell surface modifications is found in Ross et al., pg. 72-76.
All epithelia rest on a basement membrane, through which it is attached to the underlying tissue. The basement membrane is generally not identifiable with the light microscope using standard hematoxylin and eosin (H&E) staining. That is because it is thin and also stains with eosin, which makes it blend in with the connective tissue immediately adjacent to it. When a "basement membrane" appears unusually thick, as in the trachea, it is due to a dense layer of closely spaced and aligned collagen fibrils belonging to the underlying connective tissue. The basement membrane can be identified in light microscopy with periodic acid-Schiff (PAS) staining, which reacts with the sugar moieties of its proteoglycans.
With the electron microscope, the basement membrane can be seen to consist of several components. An electron dense layer, called the basal lamina (or lamina densa) consists of a network of fine filaments. Other filaments in a layer called the lamina lucida join the basal lamina to the plasma membrane of the epithelial cells, while anchoring fibrils or microfibrils connect it to reticular fibres of the underlying connective tissue. For a more detailed discussion of the basement membrane, see Ross et al., pg. 61- 66. Although the term basement membrane was originally used with light microscopy, and the term basal lamina referred only to the electron dense layer seen with the electron microscope, the two terms are now used interchangeably by many people.
Types of epithelium:
Epithelia are classified according to the number of layers of cells they contain and the shape of the cells. If the cells are arranged in a single layer, the epithelium is called simple; if the cells are arranged in two or more layers, the epithelium is called stratified. If the cells have a flattened shape, the epithelium is called squamous; if the cells have about the same height, width and depth, it is called cuboidal; if the height of the cells exceeds their width, it is called columnar. Thus, an epithelium with a single layer of flattened cells is called a simple squamous epithelium. If the shape of the cells is not identical in the various layers of a stratified epithelium, it is the outermost layer (ie at the free surface) that determines the classification. For example, the epidermis is called a stratified squamous epithelium even though the cells in the basal layers are cuboidal, because the outermost layer of cells is squamous. Images of different types of epithelia are shown below.
Simple squamous epithelium
A simple squamous epithelium consists of a single layer of flattened cells. Figure 1 shows the simple squamous epithelium lining Bowman's capusule which surrounds the glomerulus of the kidney. The arrows indicate the basophilic nuclei of the epithelial cells, the eosinophilic cytoplasm is difficult to distinguish from the surrounding kidney tubules. (There is no need to be concerned with the other structures in the kidney right now.)
Figure 2 shows a high power view of the luminal part of an aorta, which, like all of the vascular system, is lined by a layer of flattened cells called endothelial cells (arrows). The basophilic nuclei of the endothelial cells can easily be distinguished. The surrounding cytoplasm is eosinophilic and tends to blend in with the underlying tissue. The cell boundaries are not distinct.
Figure 3 shows a surface view of mesothelium, the simple squamous epithelium that lines the body cavities (abdominal, thoracic, pericardial) and covers the outer surface of many organs. The cells, whose outlines are clearly seen, look like a pan full of fried eggs.
Simple cuboidal epithelium
A simple cuboidal epithelium consists of a single layer of cells whose dimensions are more or less equal in all directions. Figure 4 shows parts of several thyroid follicles with cuboidal cells. The follicles are arranged in a roughly circular fashion and the secretory material (colloid) is at the centre of the follicles. The rounded nuclei of the follicular cells can easily be identified and in some cases the boundaries between the cells are fairly clear (asterisks)
Figure 5 (from slide #59 of your collection) shows kidney tubules. The proximal and distal convoluted tubules both consist of a simple cuboidal epithelium. The proximal tubules are larger, stain more intensely and have a more irregular shape than the distal tubules. The outline of a few cells in the distal tubule in the middle of the field of view can be seen. Note the more or less square shape of the cells.
Simple columnar epithelium
Simple columnar epithelium consists of a single layer of cells that are taller than they are wide. The cells can range from low columnar (with a height barely greater than their width) to tall columnar. Figure 6 shows the tall simple columnar epithelium of the duodenum. The elongated nuclei are located toward the middle of the cells. The free surface bears a brush border (left). The brush border is formed by microvilli - fingerlike cytoplasmic projections of the plasma membrane at the apical surface of the cells which greatly increase the absorptive surface. Some of the basement membrane shows up as a faint pink line. Below the epithelium (at the right of the image), is connective tissue.
Stratified squamous epithelium
Stratified squamous epithelium consists of several layers of cells, with the outermost layer consisting of flattened cells (usually there is more than one layer of flattened cells). This type of epithelium is found on tissues likely to encounter friction.
Figure 7 shows the stratified squamous epithelium of the esophagus (with some of the underlying connective tissue, staining pink, seen below). The epithelium consists of many layers of closely apposed cells. The cell boundaries at all levels are indistinct, but the shape of the cells can be inferred from the shape of the nuclei (purple). The nuclei of the bottom 4-5 layers of cuboidal cells are much taller and rounder than those of the overlying layers, which are flattened to accomodate to the squamous cells of those layers. Note that nuclei are identifiable in all layers.
Figure 8 shows another example of stratified squamous epithelium: the epidermis of thick skin. At the bottom left, some of the (very pale-staining) connective tissue of the underlying dermis is seen, from which dermal papillae project into the epidermis. The epidermis is divided into a number of layers, whose names need not concern us. Cell boundaries in this image are indistinct, but again the nuclei of the lower layers of cuboidal cells are seen to be more rounded. Cell division occurs in the lower layers, from which cells travel upward toward the surface where they are sloughed off. The cells can be seen to become more flattened toward the deep-staining red layer. Dark granules (brown) are evident in this layer; these are keratohyalin granules, precursors to the protein keratin with which the cells will eventually become filled. The process of keratinizatin disrupts the the nucleus and organelles of the cells. The outermost, paler layers of cells are totally flattened, have lost their nuclei and become filled with keratin. Such an epithelium is called cornified (in contrast to the non-cornified epithelium of the esophagus, where nuclei are present throughout). The thickness of the cornified layer of the skin varies greatly, being thickest on the soles and palms.
Stratified cuboidal epithelium
Stratified cuboidal epithelium consists of at least two layers of cells, with those of the outermost layer having a height, width and depth of similar of similar dimensions. The ducts of eccrine sweat glands have a stratified cuboidal epithelium, consisting of two layers of small cuboidal cells. Eccrine sweat glands are a coiled tubular structure that leads into a duct. Thus in sections, as seen in Figure 9 (from slide #25 of your collection), various profiles of the gland and its duct are seen. Here two profiles of the (pink-staining) sweat gland are seen at the centre of the field of view, and several profiles of the ducts are seen at the right. The duct is darker-staining with a smaller lumen, which is not identifiable in the profiles seen here. The asterisks indicate regions where the two layers of cuboidal cells can be seen most easily.
Stratified columnar epithelium
A stratified columnar epithelium consists of at least two layers of cells, with the cells in the outer layer being taller than they are wide. The largest ducts of some exocrine glands have areas of stratified columnar epithelium. It is also found along some areas of the anorectal junction. Figure 10 (from slide #48 of your collection) shows an excretory duct of the submandibular gland, with a layer of tall cells resting on a layer of cuboidal cells. The structure of the epithelium is seen most clearly in the area between the two arrows, where a layer of columnar cells with more elongated nuclei is resting on a layer of cuboidal cells with rounder nuclei. The material in the lumen is the secretory product of the salivary gland. The red staining material is the connective tissue underlying the epithelium. A basement membrane is not identifiable.
In addition to the above, there are two special types of epithelium called pseudostratified and transitional.
Pseudostratified epithelium has the appearance of being stratified, because it has cell nuclei at different heights within the epithelium. This is because it has some low cells which do not reach the free surface, and some tall cells, which do. However, all the cells rest on the basement membrane, so it is in fact a simple epithelium. In sections of pseudostratified epithelium, it is quite often difficult to discern that all the cells sit on the basement membrane. The distribution of pseudostratified epithelium is fairly limited. It is found in parts of the respiratory and male reproductive systems.
Figure 11 shows a section of mammalian olfactory epithelium, an example of a ciliated pseudostratified epithelium. The olfactory epithelium consists of a variety of cell types, all of which sit on the basement membrane but with nuclei at different levels. Note that it is not obvious from this section that all the cells sit on the basement membrane. Connective tissue is seen below the epithelium.
Transitional epithelium is the name given to the lining of the pelvis of the kidney, the ureter, the urinary bladder and parts of the urethra. It is a stratified epithelium that functionally accomodates well to distension and is essentially impermeable to salts and water. The thickness of transitional epithelium depends on the state of distension (full) or contraction (empty). When the ureter or bladder is empty, the epithelium can be six or more layers deep, when it is distended, the epithelium is only two or three layers deep.
Figure 12 shows a section of the inner surface of the ureter. The stratified epithelial cells are slightly elongated with clearly visible cell borders and nuclei. The epithelium here is in the contracted state.
But is it an epithelium?:
Sometimes cells may be found in close apposition similar to an epithelium, but lack a free surface (eg. Leydig cells in testis) or a basement membrane (eg. synovial cells lining synovial cavity). Such tissues are not epithelia but are called epithelioid. Epithelioid tissue may also be found in some pathological conditions.
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