Remodeling of the rat small intestinal mucosa during the suckling period.

The development of the suckling rat small intestine has been reexamined to investigate changes to cell kinetics and morphology more fully. In this period of rapid growth and reorganization, villi and crypts steadily increase in size in proximal regions; however, in distal regions a dramatic shortening of villi occurs at about 15 days. The result of this remodeling is that the normal adult proximodistal gradient of villus height is established At 15 days, migration rates in both regions increase dramatically. In proximal regions, the extent of 24-h migration of thymidine-labeled enterocytes is dependent on villus height. In distal regions, the increase in migration rate can be explained by the shortening of the villi. The extent of 24-h migration was not related to villus height. On the basis of these results, it is suggested that care must be taken in drawing conclusions relating any parameter to assumed distal region cell renewal patterns. This is because changes in the relative proportions of cell types present apparently can be brought about by villus deletion, independent of cell renewal.