Section 2: Epithelial Tissue

Ronald A. Bergman, Ph.D., Adel K. Afifi, M.D., Paul M. Heidger, Jr., Ph.D.
Peer Review Status: Externally Peer Reviewed

Plates

The layer of cells that covers the outer, and lines the inner, body surfaces is designated as epithelium. In general, many of these cells have a free surface, which is actually or potentially exposed to the external environment (skin, and the respiratory tract), or to a moist environment continuous with the external environment (digestive, reproductive, and urinary tracts). Other epithelial cells, comprising glands found in underlying connective tissue, are in continuity with the surface epithelium by epithelial duct cells. The glandular epithelium secretes diverse products, which are carried to the external surface. The products of these glands include sweat, bile, urine, reproductive cells and associated glandular secretions, mucus, milk, digestive enzymes, hydrochloric acid, and so on. Some epithelial cells have migrated away and have lost contact with the free surface. These cells form distinctive cellular masses, which are termed endocrine glands. The secretory products of these cellular masses are delivered into the vascular system to be carried to their specific sites of activity by the blood stream. The endocrine system will be considered in Section 15 of this atlas.

It is important to remember that everything that enters or leaves the body is either modified or synthesized by epithelial cells or has diffused or has been transported through this tissue. The various functions of epithelium include protection, secretion, excretion, digestion, absorption, lubrication, sensory reception, and reproduction. Such a diversity of functional activity depends upon structurally diverse cell types and cell groupings.

Epithelia are classified by histologists according to cell layering and cell shape. On this basis, three distinct types of epithelium are recognized: (1) simple, which is a single cell layer; (2) pseudostratified, which is a single cell layer but appears to have two or more layers; and (3) stratified, which is composed of several to many cell layers. Only the simple and stratified epithelia have important subgroupings, which are classified according to the shape of the cells that are exposed to the free surface. The simple epithelia are described as squamous (sheets of flattened cells), cuboidal (in which the cells are roughly equal in height and width when seen in sections-they are actually five or six-sided when seen in cross section [Plate 17]), and columnar (in which the cells are greater in height than width when seen in most sections-these, too, are actually five- or six-sided in cross section). The stratified epithelia include stratified squamous, in which the superficial cells on the free surface are flattened; stratified cuboidal, in which the superficial cells on the free surface are cuboidal; and stratified columnar, in which the superficial cells on the free surface are columnar.

From a functional point of view, the simple epithelia carry out the most diverse activities, which include absorption, excretion, synthesis, secretion, and sensory reception, whereas the stratified epithelia have protective functions, serve as conduits or ducts, and produce reproductive cells. In order to serve their distinctive functional roles, epithelial cells often display distinctive cell membrane or surface modifications and appendages.

The epithelial types shown in this section represent the morphological varieties of simple and stratified epithelia. The structural features of many other epithelial cell types and groupings are found in other sections of this atlas, where their functional role will be considered in the context of organ function.

A classification of epithelial cell types and some of their locations in the body follows.

1. Simple Epithelium

    

   1. Squamous
      Innermost lining of blood and lymph vessels and the heart (endothelium). Lining of the pleural, cardiac, and abdominal cavities. Initial segments of ducts of glands. Air sacs or alveoli of the respiratory system. Renal glomeruli and corpuscles. Kidney tubules (thin segment of loop of Henle of the nephron).
   2. Cuboidal
      "Germinal" epithelium covering the ovary. Ducts of many glands. Ciliary body of the eye.
   3. Columnar Stomach, intestines, and gallbladder of the digestive system. Small bronchi of the respiratory system.
      Uterine tubes. The secretory cells of many glands (endocrine and exocrine) vary from cuboidal to columnar. Size and shape may vary with the functional state (e.g., thyroid gland).
   4. Pseuclostratified
      Pharynx, trachea, and large bronchi. Male excurrent ducts (epididymis and vas deferens). Parts of the female and male urethra.
   5. Specialized
      Glands of intestinal tract, nasal cavity, bronchi, uterine tubes, and accessory sex glands.
   6. Pigmented
      Epithelium of retina.
   7. Neuroepithelium
      Receptor cells of taste, hearing, and balance.

    

2. Stratified Epithelium
   1. Stratified squamous
      Keratinized and non-keratinized epithelium of skin, palpebral conjunctivum, oral cavity, esophagus, and anus. Urethra near the external orifice. Vagina.
   2. Stratified cuboidal
      Ducts of sweat and sebaceous glands of the skin. Graafian follicles of ovary.
   3. Stratified columnar
      Pharynx, larynx, urethra, and portions of the excretory ducts of salivary and mammary glands.
   4. Transitional (urothelium)
      Renal calyces and pelvis, ureter, and urinary bladder.

Several specializations of epithelial cells found on their free or exposed surface include the brush or striated border of the absorbing cells of the intestine and kidney, motile cilia of the pseudostratified epithelium of the respiratory system, and non-motile stereocilia of the pseudostratified epithelium lining the epididymis. Specializations that structurally and functionally link adjacent cells together include the "terminal bars" illustrated in Plate 23 and the "intercellular bridges" or clesmosomes associated with prickle cells found in stratified squamous epithelium (Plate 137). Marked infoldings of the basal cell membrane, termed basal striations, are seen in certain active transport cells such as the proximal convoluted tubule cells of the kidney and ducts of certain glands (Plates 16 and 211). Between the basal surface of epithelial cells and the underlying connective tissue is the basement membrane, which varies markedly from place to place and in certain disease states. This extracellular structure has been shown by electron microscopy to have several components that are produced by both epithelial cells and the underlying connective tissue fibroblasts (Plates 20 and 21).

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