Restricted mobility and endocytosis of anionic sites on newborn rat jejunal brush border membranes.

The distribution and mobility of anionic sites on the microvillous surface of newborn rat jejunal absorptive cells were studied using polycationic ferritin (PCF) as a visual probe and were compared with anionic sites previously described for adult jejunum. Segments from 5- to 26-day-old rats were incubated in PCF for 5 minutes either before or after fixation for electron microscopy. From days 5 to 20, anionic sites were distributed diffusely along the lengths of the microvilli and did not show random translational mobility. In contrast, microvilli examined from animals at weaning (2l to 26 days) resembled those from adults in which most binding sites were capable of lateral mobility and were induced by PCF to culster into discrete patches. The diffuse pattern was altered by cortisone administration, paralleling a premature reduction in the endocytic apparatus of the cell. The difference in mobility of anionic sites with age coincides with differences in absorptive function. Evidence is presented showing that in the neonate binding of PCF to the microvilli was followed with time by endocytosis into an apical system of tubules for intracellular transport, incorporation into coated vesicles, and release through the lateral cell surface. The results suggest that endocytosis is accomplished by a mechanism that includes a directionally controlled movement for the selective internalization of PCF binding sites from the membranes of the microvilli to those of the tubular cytoplasmic channels.