Similarly, the stratifed squamous epithelium of skin is replaced in most sites during a period of approximately 28 days. Cells in the basal layer of the epidermis, appropriately named the stratum basale (germinativum), 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 ex-foliated 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 the liver. However, loss of significant amounts of liver tissue through physical
In two general locations, surface epithelium and its un-derlying connective tissue are regarded as a functional unit called a membrane. The two types of membrane are mucus membrane and serous membrane. The term membrane as used here should not be confused with the biologic membranes of cells, nor should the designations mucus and serous be confused with the nature of the gland secretion as discussed above. Mucus membrane, also called mucosa, lines those cavities that connect with the outside of the body, namely, the alimentary canal, the respiratory tract, and the geni-tourinary 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 peri-toneal, 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 con-sists of a lining epithelium, the mesothelium, a support-ing connective tissue, and a basement membrane between the two. Serous membranes do not contain glands, but the fluid on their surface is watery.
 FOLDER 5.5 Functional Considerations: Mucus and Serous Membranes
FIGURE 5.42 • Autoradiograph of intestinal gland (crypt).
Autoradiograph of crypts in the jejunum of a rabbit that had been injected with tritiated thymidine 8 hours before death and fixation. Nearly all of the epithelial cells in this replicative zone of the intestinal mucosa are labeled, indicating that they were synthesizing DNA at the time the animal was injected. 600. (Reprinted with permission from Parker FG, Barnes EN, Kaye GI. The pericryptal fibroblast sheath. IV. Replication, migration, and differentiation of the subepithelial fibroblasts of the crypt and villus of the rabbit jejunum. Gastroenterology 1974;67: 607–621.) 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.