MOLECULAR AND CELLULAR BIOLOGY OF THE SMALL-INTESTINE

The mucosa of the small intestine is an essential tissue with intrigui ng biology. It is a paradigm to help understand the development, growt h, differentiation, and renewal of columnar epithelial surfaces. The m ucosal epithelium is a dynamic structure that is formed during the int eraction of yolk sac endoderm with the mesoderm early during fetal dev elopment. it is continually renewed throughout adult life by multipote nt stem cells capable of differentiating into several mature lineages. The cellular processes that lead to the development of the mucosa, su ch as migration, cell fate determination, proliferation, differentiati on, and cell death, are recapitulated in the adult mouse throughout li fe. Current studies have led to a better understanding of the molecula r events that control these processes, Lineage-specific gene expressio n occurs under complex control by both positive and negative transcrip tion control elements and is undoubtedly regulated by extracellular si gnaling along the crypt-to-villus axis, The intestinal epithelial cell forms intimate contacts with T lymphocytes, which in turn may regulat e epithelial cell growth and barrier function, As mature epithelial ce lls are produced, they express a variety of intestine-specific gene pr oducts involved in digestive, absorptive, cell migratory, and cell pro tection functions. To better understand the nature and fate of epithel ial stem cells, genetic analysis of Caenorhabditis elegans and mice ha s proven to be fruitful and deserves further analysis.