Perinatal development of the small intestine of the sheep.

Soon after birth, the villi in distal regions of the small intestine of the sheep rapidly decrease in size so that by day 6 the normal proximal-distal size gradient (tallest villi proximally) is established. This is the reverse of the gradient found throughout fetal life, where distal villi are taller than proximal villi. While crypt depth increases in both regions, the increase is more marked in proximal regions. The proportion of crypt cells labelled with a pulse of (3H) thymidine increases dramatically during day 1. Vacuolated cells are present in distal regions at day 2, but are absent by day 5. It is possible that the rapid loss of villus tip cells in distal regions is the means by which closure is achieved in this species. There is evidence to suggest that endogenous cortisol plays an important role in these changes.