Figure 1617

Photograph of the mucosal surface of the small intestine. This photograph of a segment of a human jejunum shows the mucosal surface. The circular folds (plica circulares) appear as a series of transversely oriented ridges that extend partially around the lumen. Consequently, some of the circular folds appear to end (or begin) at various sites along the luminal surface (arrows). The entire mucosa has a velvety appearance because of the presence of villi.

• Microvilli of the enterocytes provide the major amplification of the luminal surface. Each cell possesses several thousand closely packed microvilli. They are visible in the light microscope and give the apical region of the cell a striated appearance, the so-called striated border. Enterocytes and their microvilli are described below.

The villi, intestinal glands, along with the lamina propria, associated GALT, and muscularis mucosae, constitute the essential features of the small intestinal mucosa

Villi, as noted, are projections of the mucosa. They consist of a core of loose connective tissue covered by a simple columnar epithelium. The core of the villus is an extension of the lamina propria, which contains numerous fibroblasts, smooth muscle cells, lymphocytes, plasma cells, eosinophils, macrophages, and a network of fenestrated blood capillaries located just beneath the epithelial basal lamina. In addition, the lamina propria of the villus contains a central, blind-ending lymphatic capillary, the lacteal (Fig. 16.19). Smooth muscle cells derived from the muscularis mucosae extend into the villus and accompany the lacteal. These smooth muscle cells may account for reports that villi contract and shorten intermittently, an action that may force lymph from the lacteal into the lymphatic vessel network that surrounds the muscularis mucosae.

The intestinal glands, or crypts of Lieberkiihn, are simple tubular structures that extend from the muscularis mucosae through the thickness of the lamina propria, where they open onto the luminal surface of the intestine at the base of the villi (see Fig. 16.18). The glands are composed of a simple columnar epithelium that is continuous with the epithelium of the villi.

As in the stomach, the lamina propria surrounds the intestinal glands and contains numerous cells of the immune system (lymphocytes, plasma cells, mast cells, macrophages, and eosinophils), particularly in the villi. The lamina propria also contains numerous nodules of lymphatic tissue that represent a major component of the GALT. The nodules are particularly large and numerous in the ileum, where they are preferentially located on the side of the intestine opposite the mesenteric attachment (Fig. 16.20). These nodular aggregations are known as aggregated nodules or Reyer's patches. In gross specimens, they appear as aggregates of white specks.

The muscularis mucosae consists of two thin layers of smooth muscle cells, an inner circular and an outer longitudinal layer. As noted above, strands of smooth muscle cells extend from the muscularis mucosae into the lamina propria of the villi.

At least five types of cells are found in intestinal mucosal epithelium

The mature cells of the intestinal epithelium are found both in the intestinal glands and on the surface of the villi. They include

• Enterocytes, whose primary function is absorption

• Goblet cells, unicellular mucin-secreting glands

• Paneth cells, whose primary function is to maintain mucosal innate immunity by secreting antimicrobial substances

• Enteroendocrine cells, which produce various paracrine and endocrine hormones

• M cells (microfold cells), modified enterocytes that cover enlarged lymphatic nodules in the lamina propria

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