EPITHELIAL TISSUE

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Essentials of Histology 

     Almost all body surfaces are covered by continuous sheets of cells termed epithelial membranes or epithelium. Such cellular sheets are also able to invaginate into the underlying connective tissue, forming constitutive structures, known as epithelial glands. To understand the different types of epithelial membranes and glands, it is necessary to discuss and examine sections of organs and structures, which incorporate various other tissues, as well.
    They present many characteristics:

• such membranes contain no capillaries. Therefore, nutrients and oxygen have to reach by diffusion the epithelial cells from capillaries, which are located in the underlying loose connective tissue.
• epithelial cells are attached to the underlying connective tissue by a component known as basement membrane.
• continuous sheets of cells with contiguous cell borders present specialized sites of close contact, called cell junctions.
• is provided with afferent nerve endings that are sensitive to pain.
• the luminal surfaces exhibits  microvilli, stereocilia, cilia or stratum corneum. The vast numbers of microvilli or stereocilia greatly expand the area over which absorption takes place. The cilia are motile, hairlike processes, which are found in the upper part of the respiratory system. The ciliary action plays a key role in the trapping and disposal of particles that enter the airways in the inhaled air. Finally, the stratum corneum constitutes an effective barrier against pathogenic organisms. As this layer is almost waterproof, the body does not become desiccated in a dry atmosphere.
• presents relatively scant amounts of intercellular substances, which contains the molecules called caderins and integrins. These molecules are very important to the process of intercellular attachment.

• number of cell layers. An epithelial membrane that consist of only a single layer of cells is termed a simple epithelia. An epithelium that presents two or more cell layers is called stratified. If some cells extend from the base of the membrane to the epithelial free surface, but others only are located in the basal portion, the epithelium is called pseudostratified.
• basic morphology of cells – squamous, cuboidal and columnar epithelial, or special morphology, as goblet cells and others.
• type of specialization present in the luminal surface : microvilli, stereocilia, cilia and stratum corneum.

• number of ducts – simple and compound glands.
• type of epithelium observed in the ducts – cuboidal, columnar and pseudostratified.
• number of secretory units – branched or unbranched.
• form of the secretory units – tubular, acinar or tubuloacinar.
• type of secretion – protein (serous, watery), glycoprotein (mucous, viscous) or lipid (oily secretion,  lubricate)
• how the secretory cells release their secretion – merocrine ( release by exocytosis ), holocrine (the cells died and thereupon disintegrate) and apocrine (the cells release only the apical cytoplasm).