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The small intestine consists of the duodenum, the jejunum, and the ileum, which share the same wall structure formed by, from inside to outside, a mucosa, a submucosa, a muscularis interna, a muscularis externa, and a serosa. The mucosa of the small intestine, comprising simple columnar epithelium and a lamina propria, forms finger-like projections, villi, which protrude into the lumen, and deep cavities, the crypts of Lieberkühn, between the villi. The predominant cell in the epithelium is the absorptive enterocyte with microvilli on its apical membrane. Interspersed between the enterocytes are the oval, mucous goblet cells. Deep in the crypts the epithelium contains enteroendocrine cells with granules in the cell portion facing the lamina propria. The lamina propria of the small intestine consists of loose connective tissue and blood vessels, while a central lymph vessel, the lacteal, is present in the lamina propria of each villus. Large lymphoid aggregates, Peyer’s patches, occur in the submucosa throughout the intestines; M cells form part of the epithelium covering the Peyer’s patches.
The jejunum and ileum are histologically identical, except for their villi and the presence of Paneth cells. The villi of the jejunum are tall and cylindrical, while they are short and cylindrical in the ileum. Paneth cells have eosinophilic cytoplasmic granules and occur in clusters at the bases of crypts. In the mouse, the Paneth cells are especially found in the jejunum and have very prominent granules.
The micrographs present a section of the jejunum. In the 4X and 10X micrographs a submucosal Peyer’s patch is visible. Together the 4X, 10X, and 20X micrographs show the various components of the jejunal wall. The 40X micrograph depicts the Paneth cells, which are characteristic of the mouse jejunum. The 40X micrograph also shows several cells in mitosis, indicative of the continuous cell proliferation in the intestinal crypts.
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