Figure 1624

Photomicrograph of intestinal glands showing Paneth cells. This photomicrograph shows the base of intestinal (jejunal) glands in a H&E preparation. The gland on the right is sectioned longitudinally; the circular cross-sectional profile of another gland is seen on the left. Paneth cells are typically located in the base of the intestinal glands and are readily seen in the light microscope because of the intensive eosin staining of their granules. The lamina propria contains an abundance of plasma cells, lymphocytes, and other connective tissue cells. Note several lymphocytes in the epithelium of the gland (arrows). x240. Inset. This high magnification of the area indicated by the rectangle shows the characteristic basophilic cytoplasm in the basal portion of the cell and large accumulations of intensely staining, eosinophilic, retractile secretory granules in the apical portion of the cell. An arginine-rich protein found in the granules is probably responsible for the intense eosinophilic reaction. x680.

lates insulin release in the pancreas, and motilin initiates gastric and intestinal motility. Although other peptides produced by enteroendocrine cells have been isolated, they are not considered hormones and are therefore called candidate hormones (page 488). Enteroendocrine cells also produce at least two hormones, somatostatin and histamine, which act as paracrine hormones (see page 488), i.e., hormones that have a local effect and do not circulate in the bloodstream. In addition, several peptides are secreted by the nerve cells located in the subrnu-cosa and muscularis externa. These peptides, called neu-rocrine hormones, are represented by VIP, bombesin, and the enkephalins. The functions of these peptides are listed in Table 16.2.

M cells convey microorganisms and other macromolecules from the intestinal lumen to Peyer's patches

M cells are epithelial cells that overlie Peyer's patches and other large lymphatic nodules; they differ significantly from the surrounding intestinal epithelial cells. M cells have microfolds rather than microvilli on their apical surface, and they take up microorganisms and macromolecules from the lumen in endocytotic vesicles. The vesicles are transported to the basolateral membrane where they discharge their contents into the epithelial intercellular space in the vicinity of CD4+ T lymphocytes. Thus, substances that gain access to the body from the intestinal lumen via M cells come into contact with cells of the immune system as they reach the basolateral surface. Antigens that reach lymphocytes in this manner stimulate a response in the GALT that is described below.

Intermediate cells have characteristics of both immature absorptive cells and goblet cells

Intermediate cells constitute most of the cells in the lower half of the intestinal gland. These cells are still capable of cell division and usually undergo one or two divisions before they become committed to differentiation into either absorptive or goblet cells. These cells have short, irregular microvilli with long core filaments extending deep into the apical cytoplasm and numerous macular (desmo-somal) junctions with adjacent cells. Small mucin-like secretory granules form a column in the center of the supranuclear cytoplasm. Intermediate cells that are committed to becoming goblet cells develop a small, rounded collection of secretory granules just beneath the apical plasma membrane; those that are committed to becoming absorptive cells lose the secretory granules and begin to show concentrations of mitochondria, rER, and ribosomes in the apical cytoplasm.

GALT is prominent in the lamina propria of the small intestine

As noted above, the lamina propria of the digestive tract is heavily populated with elements of the immune system; approximately one fourth of the mucosa consists of a loosely organized layer of lymphatic nodules, lymphocytes, macrophages, plasma cells, and eosinophils in the lamina propria. Lymphocytes are also located between epithelial cells. This GALT serves as an immunologic barrier throughout the length of the gastrointestinal tract. In cooperation with the overlying epithelial cells, particularly M cells, the lymphatic tissue samples the antigens in the epithelial intercellular spaces. Lymphocytes and other antigen-presenting cells process the antigens and migrate to lymphatic nodules in the lamina propria where they un

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