The small intestine (in the Human Protein Atlas represented by jejunum and ileum) measures about 6 meters and absorbs nutrition, water and electrolytes. The jejunum and ileum are similar to the duodenum in histology and composition. The permanent transverse submucosal fold extending into the lumen of the intestine are termed plica circularis. The plica circularis consist of a mucosa, as well as a submucosa. The core is the submucosa composed of loose connective tissue, blood vessels, nerves and dispersed lymphoid tissue. A distinctive feature of the jejunum and ileum is the lack of glands in the submucosa.
The mucosa is characterized by numerous fingerlike villi that protrude into the lumen of the intestine. Enterocytes, which are columnar epithelial cells with basally located oval nuclei and an apical brush border mainly line the villi. The enterocytes located to the villi have mainly absorptive function. Interspersed between the enterocytes are goblet cells, which are recognized by their content of a large mucous globule, resembling a small "empty bubble" within the epithelial lining. The goblets cell is connected to the basement membrane with a thin cytoplasmic strand that is difficult to distinguish in hematoxylin and eosin staining. Underlying the intestinal villi are the intestinal glands. They are straight tubular glands that are slightly dilated at their bottom. Intestinal stem cells line the proximal part of the glands. The stem cells give rise to all the cells in the epithelium, which are the paneth cells, the enterocytes, the goblet cells and the enteroendocrine cells. The paneth cells secrete antibacterial enzymes and are located in the lower portion of the intestinal glands. Paneth cells are recognized by eosinophilic granules in the cytoplasm.
The lamina propria forms the core of the intestinal villi and surrounds the intestinal glands. It consists of loose connective tissue, vessels, nerves and numerous immune cells, most of which are lymphocytes. The muscularis mucosae demarcate the end of the mucosa.
Underlying the mucosa is the submucosa, containing larger vessels and nerves. In the jejunum, the submucosa shows no typical characteristics whereas the ileum contains aggregations of lymphoid follicles termed Peyer's patches.
The muscularis externa consists of an inner circular layer and outer longitudinal layer of smooth muscle. The jejunum and ileum have a mesentery and a serous lining of flat mesothelial cells.