The gastrointestinal (GI) tract is a long, hollow tube extending from the mouth to the anal canal. It consists of a variety of cells that aid in digestion, absorption and secretion. Cell types from the mouth to the anal canal are briefly discussed.
Mouth and Esophagus
The mucous membrane in the mouth is lined by cells or epithelium called nonkeratinized stratified squamous epithelium, which continues to the pharynx or throat and into the esophagus. The epithelial type abruptly changes at the junction of the esophagus and stomach to simple columnar epithelium.
Stomach
The surface and pits of the stomach are lined by simple columnar epithelium, which is mainly comprised of surface mucous cells. These cells secrete a thick, gel-like mucous layer that protects the stomach from acid. The epithelium has tiny holes or gastric pits that are continuous with tubular glands called fundic glands and pyloric glands. The fundic glands are located in the fundus or body of the stomach and consists of chief cells, mucous neck cells and acid-forming parietal cells.
The pyloric glands are in the pyloric antrum or the bottom portion of the stomach and contain mainly mucus-secreting cells and interspersed G cells. G cells secrete the hormone gastrin that stimulates parietal cells to secrete hydrochloric acid, according to "Textbook of Medical Physiology Eleventh Edition."
Small Intestine
According to "Histology a Text and Atlas Fourth Edition," the small intestine is divided into the duodenum, jejunum and ilem. Villi and the crypts of Lieberkuhn characterize the inner lining of the small intestine. Villi are described as finger-like projections from the intestinal wall and appear as leaf-shaped in the duodenal segment. Between villi are small pits called the crypts of Lieberkuhn or intestinal glands. Each crypt consists of three types of cells, globlet cells, epithelial cells and paneth cells. The crypts and villi are lined with simple columnar epithelium consisting of two cell types, enterocytes and goblet cells. The proximal segment of the duodenum, closest to the stomach, contains mucus-secreting glands called brunner glands.
The jejunum consist mainly of villi and crypts. Villi are taller than duodenal villi and contain tiny vessels surrounding a central lacteal lymph vessel. Villi in the jejunum are lined by simple columnar epithelium.
The ileum consists of similar cell arrangements as the previous segments. The distinctive feature is the presence of mucosa associated lymph tissue called the gut-associated lymphoid tissue or GALT. In the distal portion of the ileum are nodules of lymph called peyer's patches. The GALT and peyer's patches produce immune cells in response to intestinal infections.
Large Intestine
The large intestine or colon does not have villi. The colon is pitted with deeper crypts and consist of simple columnar epithelium. The epithelium has more goblet cells than enterocytes and contains no paneth cells.
Anal Canal
The colon becomes the rectum, which ends at the boundary of the so-called pectinate line and becomes the anal canal. In this area, the epithelium transitions from simple columnar to stratified squamous epithelium. The epithelial type changes to stratified squamous keratinized epithelium at the anal opening to become continuous with the skin, according to "Histology a Text and Atlas Fourth Edition."
References
- "Textbook of Medical Physiology Eleventh Edition"; Guyton et al.; 2006
- "Histology a Text and Atlas Fourth Edition"; Kaye et al.; 2003
