Lineage commitment and maturation of epithelial cells in the gut.

The dynamic concepts of gut epithelial cell populations which heralded the era of modern gut cell biology have been generally substantiated by recent studies and are still being correlated with functional properties. Multipotent stem cells are anchored in specific locations along the gut epithelium where decisions concerning proliferation and differentiation/migration pathways are made. Stem cells give rise to lineage precursors which transform into transit cells and sequentially express lineage specific features during their differentiation program. Morphologically and functionally mature cells along the gut epithelium are dynamically heterogeneous. 1) The squamous lineage of the esophagus forms a stratified epithelium which has an average turnover time of about 7. 5 days. 2) In the stomach, the oxyntic pit-gland unit includes pit, zymogenic and parietal cells which respectively migrate outwards, inwards, and in both directions; their turnover times average 3, 194 and 54 days, respectively. 3) The mucous units of the pyloric antrum are populated by pit cells which migrate outwards and gland cells which migrate inwards; their turnover times are about 3 and 1-60 days, respectively. 4) In the crypt-villus units of the small intestine, while both absorptive and goblet cells migrate outwards and for each the turnover time is about 3 days, Paneth cells migrate inwards and their turnover time is about 15 days. 5) In the crypts of the descending colon, both vacuolated-columnar and goblet cells migrate outwards and for each the turnover time is about 5 days. The ascending colon has an additional cell type called deep crypt secretory cells which migrate inwards and their turnover time is about 14-21 days. Finally, while the factors maintaining the gut epithelium in a steady state remain to be elucidated, this epithelium represents a remarkable system for studying the biological features of stem cells and their hierarchies.