Early establishment of epithelial apoptosis in the developing human small intestine

In the adult small intestine, the dynamic renewal of the epithelium is char acterized by a sequence of cell production in the crypts, cell maturation a nd cell migration to the tip of villi, where apoptosis is undertaken. Littl e is known about enterocytic apoptosis during development. In man, intestin al architectural features and functions are acquired largely by mid-gestati on (18-20 wks);the question whether the establishment of enterocytic apopto tic processes parallels or not the acquisition of other intestinal function al features remains open. In the present study, we approached this question by examining enterocytic apoptosis during development of the human jejunum (9-20 wks gestation), using the ISEL (in situ terminal uridine deoxynucleo tidyl nick-end labelling) method. Between 9 and 17 wks, apoptotic enterocyt es were not evidenced. However, beginning at the 18 wks stage, ISEL-positiv e enterocytes were regularly observed at the tip of villi. Since the Bcl-2 family of proteins constitutes a critical checkpoint in apoptosis, acting u pstream of the apoptotic machinery, we investigated the expression of six B cl-2 homologs (Bcl-2, Bcl-X-L, Mcl-1, Bax, Bak, Bad) and one non-homologous associated molecule (Bag-l). By immunofluorescence, we found that all homo logs analyzed were expressed by enterocytes between 9 and 20 wks. However, Bcl-2 homologs underwent a gradual compartmentalization of epithelial expre ssion along the maturing crypt-villus axis, to establish gradients of expre ssion by 18-20 wks. Western blot analyses indicated that the expression lev els of Bcl-2 homologs were modulated during morphogenesis of the crypt-vill us axis, in parallel to their gradual compartmentalization of expression. A ltogether, these data suggest that regulatory mechanisms of human enterocyt ic apoptosis become established by mid-gestation (18-20 wks) and coincide w ith the maturation of the crypt-villus axis of cell proliferation, differen tiation and renewal.