Maturation of the ileal epithelium in the young rat.

Normal maturation in the rat ileal epithelium (ileal 'closure') occurs between 18 and 21 days, probably by a process of cellular replacement. A single dose of cortisone acetate induced a precocious ileal 'closure' within 6 days. The morphological changes observed were similar to those seen during normal 'closure'. Morphometric measurements made of ileal villi during both normal and induced 'closure' suggest that there was a gradual change in the type of cells produced by the crypt. This was perhaps related to the duration of exposure of the cryptal cells to the 'closure' stimulus. Ten days after cortisone acetate treatment vacuolated cells were again seen on the villus in some animals. The variation in their distribution on the villus was perhaps due to differing rates at which the villi had recommenced pinocytosis because of the physiological state of the animal. A single dose of corticosterone failed to promote any morphological changes indicative of ileal 'closure' when observed at the electron microscope level.