Figure 1633

Mucosa of the large intestine, a. This photomicrograph of a H&E preparation shows the mucosa and part of the submucosa. The surface epithelium is continuous with the straight, unbranched, tubular intestinal glands (crypts of Lieberkuhn). The openings of the glands at the intestinal surface are identified (arrows). The epithelial cells consist principally of absorptive and goblet cells. As the absorptive cells are followed into the glands, they become fewer in number, whereas the goblet cells increase in number. The highly cellular lamina propria contains numerous lymphocytes and other cells of the immune system. b. Scanning electron micrograph of the human mucosal surface of the large intestine. The surface is divided into territories by clefts (arrows). Each territory contains 25 to 100 gland openings. x140. (From Fenoglio CM, Richart RM, Kaye Gl. Gastroenterology 1975;69: 100-109.)

tract, it demonstrates some additional structural features and greater development of some others. These include

• The collagen table, a thick layer of collagen and proteoglycans that lies between the basal lamina of the epithelium and that of the fenestrated absorptive venous capillaries. This layer is as much as 5 /xm thick in the normal human colon and can be up to 3 times that thickness in human hyperplastic colonic polyps. The collagen table participates in regulation of water and electrolyte transport from the intercellular compartment of the epithelium to the vascular compartment.

• Well-developed GALT, which is continuous with that of the terminal ileum. In the large intestine GALT is more extensively developed; large lymphatic nodules distort the regular spacing of the intestinal glands and extend into the submucosa. The extensive development of the immune system in the colon probably reflects the large number and variety of microorganisms and noxious end products of metabolism normally present in the lumen.

• A well-developed pericryptal fibroblast sheath, which constitutes a fibroblast population of regularly replicating cells. They divide immediately beneath the base of the intestinal gland, adjacent to the stem cells found in the epithelium (in both the large and small intestines). The fibroblasts then differentiate and migrate upward in parallel and synchrony with the epithelial cells. Although the ultimate fate of the pericryptal fibroblast is unknown, most of these cells, after they reach the level of the luminal surface, take on the morphologic and his-tochemical characteristics of macrophages. Some evidence suggests that the macrophages of the core of the lamina propria in the large intestine may arise as a terminal differentiation of the pericryptal fibroblasts • Absence of lymphatic vessels in the lamina propria. There are no lymphatic vessels in the core of the lamina propria between the intestinal glands. Lymphatic vessels form a network around the muscularis mucosae, as they do in the small intestine, but no vessels or associated smooth muscle cells extend toward the free surface from that layer. The absence of lymphatic vessels from the lamina propria is important to understanding the slow rate of metastasis from certain colon cancers. Cancers that develop in large adenomatous colonic polyps may grow extensively within the epithelium and lamina propria before they even have access to the lymphatic vessels at the level of the muscularis mucosae. Lymphatic vessels are found in the

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