Epithelia

Blood and lymphatic vessels are lined with a special type of simple squamous epithelium called endothelium. Examine slide #70 (nasal mucosa) for blood vessels and locate their endothelial lining. Blood vessels are usually recognized by their content of red blood cells (small red staining spheres). In these preparations nuclei of the squamous endothelial cells are prominent, but cell boundaries cannot be made out because the cells are so flattened

This type of epithelium consists of a single layer of cells which is thicker than the squamous endothelial cells. Cuboidal epithelial cells are approximately as wide as they are tall and can be found in the kidney tubules, glands and ducts. A good example is the thyroid gland, slide #67 which is composed of follicles that are discrete areas of colloidal material surrounded by a simple cuboidal epithelium. Note, that there are some variations in cell thickness.

Pseudostratified ciliated epithelium is typically found lining the respiratory system and is also found in other tissues such as the epididymis. Three types of cells make up this epithelium - basal, columnar and fusiform cells. The first two are easy to see, but fusiform cells are difficult to demonstrate due to their shape. All cells rest on the basement membrane, but vary in height, and, as a result, the nuclei are at different levels giving the appearance of stratification.

Stratified squamous epithelium is found in the skin and in the passageways leading to the exterior, such as the mouth, anus and vagina.

Examine the esophagus (slide #50) and note the epithelium lining the lumen. In the odd-numbered boxes, it is cut in cross-section and the epithelium lines the lumenal surface of the tube; in the even-numbered boxes it is a longitudinal piece of tissue with the epithelium on one surface. By visual inspection, most of the tissue is pink. The epithelium will be the most basophilic material in the slide. Inspection of the epithelial layer at low-power indicates that the basal surface of the epithelium is indented by connective tissue papillae. If these papillae are cut in cross-section, they appear as circular areas within the epithelium.

Slide #43 (Thick skin), is a section through the plantar surface of a foot. The pink material is connective tissue and the epithelium is the blue layer next to it. The thick outer layer is composed of the keratinized cells mentioned above. It may be partially or completely detached in some slides. If so, examine your neighbor's slide.

This epithelium is found in urinary tract. Its appearance varies depending on whether it is in a stretched or relaxed state. Examine slide #76 (human bladder) and slide #9 (rat bladder) made from relaxed bladders. The cells in the outermost layer characteristically have a large amount of cytoplasm, which may give the cell a swollen, balloon shape appearance ("cap" cells). In slide #9, many cells in the outer layer appear to be covered by a dense-layer of material which is not part of the epithelium.

Epithelial cells have polarity in terms of their morphology and orientation. The plasma membrane nearest the basement membrane is the basal membrane; the apical membrane lines the free surface, and the lateral membrane is between adjacent cells. The various membranes may be modified to perform specialized functions. Projections from the cell-surface may be found on different cells depending on their function. These can be visualized in the light microscope but their structure is only apparent in electron micrographs (E.M.s). Examine the E.M.s in your textbook. Non-motile processes called microvilli may project from the surface of cells. Microvilli vary in size from extremely small processes which can only be seen with the electron microscope, to larger structures which can be seen with the light microscope. In the small intestine (slide #3), individual microvilli are not seen but the brush border they form is visible. Explain why. What is the function of microvilli?

Motile processes called cilia are present in the pseudostratified epithelium of the trachea (slide #72). Observe these and then study the E.M.s noting the characteristic 9 + 2 microtubule arrangement. Notice in the thin section EM of a simple columnar ciliated epithelium numerous cilia cut in both cross section and longitudinal section can be observed at the apical membrane. The cilia are attached to basal bodies in the apical cytoplasm

Thin section EM of a tight junction on the lateral membrane of adjacent epithelial cells. Using this procedure, the tight junction appears as punctate membrane fusions.

In freeze-fracture EM, these membrane fusions appear as intramembrane ridges. The function of tight junctions is to seal the intercellular space between epithelial cells and prevent transepithelial diffusion. Tight junctions maintain epithelial cell surface polarity.

Desmosomes are observed on the lateral membrane between epithelial cells. They are involved in cell-cell adhesion and are abundant in organs that undergo severe mechanical stress such as the skin. Hemidesmosomes are found on the epithelial basal membrane and anchor the epithelium to the basal lamina.

Gap junctions appear as plaques of tightly packed intramembrane particles in freeze-fracture electron microscopy. For comparison, the intramembrane particles of the adjacent plasma membrane (labelled M in the picture) are less densly packed. Gap junctions are involved in cell-cell communication.