Figure 431

Autoradiograph of intestinal gland (crypt). Autoradiograph of crypts in the jejunum of a rabbit that had been injected with tritiated thymidine 8 hours prior to death and fixation. Nearly all of the epithelial cells in this implicative zone of the intestinal mucosa are labeled, indicating that they were synthesizing DNA at the time the animal was injected. X600. (From Parker FG, Barnes EN, Kaye Gl. Gastroenterology 1974;67:607-621.)

along the villi to the surface of the intestinal lumen. The migration of these new cells continues until they reach the tips of the villi, where they undergo apoptosis and slough off into the lumen.

Similarly, the stratified squamous epithelium of skin is replaced in most sites over a period of approximately 28 days. Cells in the basal layer of the epidermis, appropriately named the stratum basale (germinatiuum), undergo mitosis to provide for cell renewal. As these cells differentiate, they are pushed toward the surface by new cells in the basal layer. Ultimately, the cells become keratinized and slough off. In both of the above examples, a steady state is maintained within the epithelium, with new cells normally replacing exfoliated cells at the same rate.

In other epithelia, particularly in more complex glands, individual cells may live for a long time, and cell division is rare after the mature state is reached. These epithelial cells are characteristic of stable cell populations in which relatively little mitotic activity occurs, such as in liver. However, loss of significant amounts of liver tissue through physical trauma or acute toxic destruction is accommodated by active proliferation of undamaged liver cells. The liver tissue is essentially restored by the stimulated mitotic activity of healthy liver tissue.

chapter 4 i Epithelial Tissue 117 BOX 4.3

Functional Considerations: Mucous and Serous Membranes

In two general locations, surface epithelium and its underlying connective tissue are regarded as a functional unit called a membrane. The two types of membrane are mucous membrane and serous membrane. The term "membrane" as used here should not be confused with the biologic membranes of cells, nor should the designations "mucous" and "serous" be confused with the nature of the gland secretion as discussed above.

Mucous membrane, also called mucosa, lines those cavities that connect with the outside of the body, namely, the alimentary canal, the respiratory tract, and the genitourinary tract. It consists of surface epithelium (with or without glands), a supporting connective tissue called the lamina propria, a basement membrane separating the epithelium from the lamina propria, and sometimes a layer of smooth muscle called the muscularis mucosae as the deepest layer.

Serous membrane, also called serosa, lines the peritoneal, pericardial, and pleural cavities. These cavities are usually described as closed cavities of the body, although in the female the peritoneal cavity communicates with the exterior via the genital tract. Structurally, the serosa consists of a lining epithelium, the mesothelium, a supporting connective tissue, and a basement membrane between the two. Serous membranes do not contain glands, but the fluid on their surface is watery.

TABLE 4.4. Summary of Junctional Features

Classification

Major Link Proteins

Extracellular Cytoskeleton Ligands Components

Associated Intracellular Attachment Proteins

Functions c o

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