The cells between the villi form the germinal area known as the crypts of Lieberkühn. The main functions of the crypts are cell renewal and water, ion, exocrine and endocrine secretion. The crypt epithelium consists of at least 4 different cell types:
a) the Paneth cells which secrete large amounts of protein-rich materials, and in the rat are known to phagocytose selected protozoa and bacteria. Human Paneth cells contain lysozyme, IgA and IgG.
b) goblet cells which secrete mucus. These cells are able to tolerate a higher osmotic stress than the enterocytes and are more firmly attached to the basement membrane. Exposure to toxins or hypertonic vehicles leads to accumulation of goblet cells at the apex of the villus, goblet cell capping, and a hypersecretion of mucus presumably as a protective response1.
c) undifferentiated cells, the most common type, whose major function is in the renewal process of the epithelium. Cells in the enterocyte lineage divide several more times as they migrate up the crypts. As they migrate onto the villi, they will differentiate further into the mature absorptive cells that express all the transport proteins and enzymes characteristic of those cells.
d) endocrine cells which produce hormones and peptides such as gastrin, secretin, cholecystokinin, somatostatin, enteroglucagon, motilin, neurotensin, gastric inhibitory peptide, vasoactive peptide and serotonin.
Stem cells in the crypts divide to form daughter cells. One daughter cell from each stem cell division is retained as a stem cell. The other becomes committed to differentiate along one of the four pathways mentioned above.
The volume of intestinal secretions formed by the cells in the crypts is around 1800 ml per day and is almost pure extracellular fluid, with a pH of between 7.5 and 8.0. The fluid is rapidly absorbed by the villi and provides the watery vehicle required for the absorption of substances from chyme.
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