Cell kinetics and secretion of mucus in the gastrointestinal mucosa, and their diurnal rhythm.

The surface lining cells in the gastric and the intestinal mucosa, arising from the proliferative cell zone at the neck region of glands and in the crypts, respectively, are renewed in 3-5 days. There is a diurnal rhythm in the proliferation of the precursor cells; more surface lining cells arise in the daytime. This results in sloughing of more cells. In the gastrointestinal mucosa, there are two modes of mucus secretion. One is a slow secretion of jelly-like visible mucus, which is produced in the gastric surface epithelial cells and in the intestinal goblet cells. It is secreted when the cells reach the surface and become sloughed off into the lumen; the secretion time is equal to their life span, on the order of 3-5 days. The other mode is a rapid but persistent secretion of invisible mucus, which can be traced by autoradiography with 35SO4 and [3H]glucose; the mucus is secreted every 2-3 h. This is seen in gastric foveolar cells, in pyloric gland cells, in Brunner's gland cells, and in intestinal absorptive cells. More mucus is produced and secreted in the daytime.