[Mg++-ATPase activity in circumventricular capillaries during fetal development of rats].

We performed an ultracytochemical study of Mg++-ATPase as a marker of maturation in circumventricular capillaries at the developmental stage of rats, from 19 gestational days to 18 postnatal day, comparing with hippocampal capillaries. During the course of perinatal development, the predominant site of the Mg++-ATPase activity in circumventricular capillaries was shifted from the luminal cell membrane to the antiluminal cell membrane and the total enzyme activity was markedly increased. In all hippocampal capillaries observed, the predominant site of the Mg++-ATPase activity was the antiluminal cell membrane. It was suggested that immaturity of circumventricular capillaries may be one of the causative factors leading to intraventricular and subependymal hemorrhages in neonates.