The small intestine (jejunum and ileum) measures about 6 meters and absorbs nutrition, water and electrolytes. It is 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 mucosa as well as 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 HE 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.
The general structure of all parts of the GI-tract is
1) tunica serosa /adventitia - Loose connective tissue with elastic and collagen fibers, nerves and vessels, covered by a single layer of flat mesothelial cells. Where there is no mesothelial cover the outermost layer is called adventitia.
2) tela subserosa - thin layer of loose connective tissue separating the serosa and muscle layer.
3) tunica muscularis - which for most parts is composed of an inner circular and outer longitudinal smooth muscle layer. Between the muscle fibers the myenteric plexus of Auerbach can be identified.
4) tela submucosa - a thick layer of loose connective tissue with numerous of blood and lymphatic vessels. Here is where the ganglion cells of the submucosal plexus of Meissner might be seen.
5) tunica mucosa - the innermost layer that comes in contact with the gastrointestinal content. It has secretory and absorptive function. The mucosa consists of the innermost epithelium that forms surface cells and glands, embedded in the lamina propria containing mainly of loose connective tissue with small blood vessels and immune cells. A thin layer of smooth muscle, lamina muscularis mucosae, demarcates the division of the mucosa and submucosa.