Fine structure of the free cell surface in developing mouse intestinal mucosa

Dimensions of the longest intestinal microvilli were measured in normal mice from the 14 day embryo through weaning. Major changes are associated with birth and weaning. Microvilli increase in height and volume before birth and after birth increase in height but keep the same volume by decreasing in diameter. There is thus an increase in surface area associated with birth and with weaning. This correlates with the increases in alkaline phosphatase activity of mouse intestine at these times as described by Moog. Location of reaction product suggests that phosphatase is associated with the inner layer of the unit membrane. Attempts to reproduce the normal shape changes of microvilli by injecting 9 to 10 day old mice with cortisone failed. The microvilli increased in height but did not decrease in diameter.