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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.
At the duodenal papilla the common bile duct and one or more main excretory ducts from the pancreas open into the small intestine. The duodenum is characterized by its tall, leaf-shaped villi and the submucosal Brunner’s glands, which are also found in the duodenum of other placental mammals and are lined with cuboidal epithelium.
In the 4X and 10X micrographs the villi and crypts formed by the duodenal mucosa are seen. The 20X micrograph shows the muscle layers of the duodenum as well as the submucosal Brunner’s glands. The 40X micrograph displays the Brunner’s glands in detail.
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