TABLE Physiologic Actions of Other Hormones in the Gastrointestinal Tract 17.2 Modified from Johnson LR, ed. Essential Medical Physiology. Philadelphia: Lippincott-Raven, 1998. GI, gastrointestinal. Major Action Hormone Site of Synthesis Stimulates Inhibits Candidate hormones Pancreatic polypeptide PP cells in pancreas Gastric emptying and gut motility Pancreatic enzyme secretion Pancreatic bicarbonate secretion Peptide YY L cells in ileum and colon Electrolyte and water absorption in the colon Gastric acid secretion Gastric emptying Food intake Glucagon-like peptide-1 (GLP-1) L cells in ileum and colon Insulin release Gastric acid secretion Gastric emptying Paracrine hormones Somatostatin D cells in mucosa throughout GI tract Gastrin release Gastric acid secretion Release of other GI  hormones Histamine Mucosa throughout GI tract Gastric acid secretion Neurocrine hormones Bombesin Stomach Gastrin release Enkephalins Mucosa and smooth muscle throughout GI tract Smooth muscle contraction Intestinal secretion Vasoactive inhibitory peptide (VIP) Mucosa and smooth muscle throughout GI tract Pancreatic enzyme secretion Intestinal secretion Smooth muscle contraction Sphincter contraction
The lumen is relatively wide, and the secretory cells are similar in appearance to the surface mucous cells, suggesting a relatively viscous secretion. Enteroendocrine cells are found interspersed within the gland epithelium along with occasional parietal cells. The glands empty into deep gastric pits that occupy about half the thickness of the mucosa (Fig. 17.15).
Epithelial Cell Renewal in the Stomach
Surface mucous cells are renewed approximately every 3 to 5 days.
The relatively short lifespan of the surface mucous cells, 3 to 5 days, is accommodated by mitotic activity in the isthmus, the narrow segment that lies between the gastric pit and the fundic gland (Fig. 17.16). The isthmus of the fundic gland contains a reservoir of tissue stem cells that undergo mitotic activity, providing for continuous cell renewal. Most of the newly produced cells at this site become surface mucous cells. They migrate upward along the wall of the pit to the luminal surface of the stomach and are ultimately shed into the stomach lumen.
The cells of the fundic glands have a relatively long  lifespan.
Other cells from the isthmus migrate down into the gastric glands to give rise to the parietal cells, chief cells, mucous gland cells, and enteroendocrine cells that constitute the gland epithelium. These cells have a relatively long lifespan. The parietal cells have the longest lifespan, approximately 150 to 200 days. Although parietal cells develop from the same undifferentiated stem cells, their lifespan is distinctly different. Recently, it has been hypothesized that parietal cells may have originated from a bacterium called Neurospora crassa that previously existed in a symbiotic relationship with the cells of the human stomach. The basis for this hypothesis is that the human proton pump (H/K-ATPase) found in parietal cells bears a strong genetic similarity to proton pumps found in this bacterium. The bacterial DNA is thought to have been translocated and subsequently incorporated into the nucleus of the stem cells, probably with the help of a virus.
The chief and enteroendocrine cells are estimated to live for about 60 to 90 days before they are replaced by new cells migrating downward from the isthmus. The mucous neck cell, in contrast, has a much shorter lifespan, approximately 6 days.