Small intestine

The most proximal and widest part of the small intestine is the duodenum. The jejunum and ileum are similar to the duodenum in histology and composition. A distinctive feature of the jejunum and ileum is the lack of glands in the submucosa.

Characteristic of the small intestine, the mucosa forms fingerlike projections called villi that extend into the intestinal lumen. These are epithelial folds lined by two types of cells, enterocytes and goblet cells. Enterocytes are simple columnar cells with basal elongated nuclei and an apical brush border. The brush border is the microscopic representation of small protrusions of the cell membrane, microvilli, which greatly increase the surface area of the cell enhancing absorptive capacity. The other cell type is mucus secreting goblet cells that can be recognized by the presence of an apical mucous cup. The core of the villus is part of the lamina propria. The most numerous cells in the lamina propria are immune cells, most of which are lymphocytes.

Underlying the villi are the intestinal glands, also called the crypts of Lieberkühn. Enterocytes, goblet cells, paneth cells that secrete antibacterial enzymes (recognized by eosinophilic granules in their apical cytoplasm) and enteroendocrine cells also line the crypt. A thin layer of smooth muscle marks the end of the mucosa, the muscularis mucosae.

Underlying the mucosa is the submucosa, containing larger vessels and nerves. In the jejunum and ilium contains aggregations of lymphoid follicles termed Peyer's patches.While, the submucosa of duodenum, includes numerous pale stained glands, namely the Brunner’s glands. These are branched tubular or alveo-tubular glands lined with columnar secretory epithelium. Brunner’s glands secrete large amounts of alkaline mucous that neutralize the acidic contents from the stomach.

The muscularis externa consists of an inner circular layer and outer longitudinal layer of smooth muscle.