A correlative SEM/TEM examination of the endothelium surface in neural capillaries.

The modifications of the endothelial surfaces were analyzed in growing neural microvessels by scanning and transmission electron microscopes in the optic tecta of chick embryos and chickens. The endothelial inner aspect appears regular and smooth in the early stages of the vessel growth (7th incubation day). Later (14th incubation day) both the abluminal and luminal surfaces of the endothelium follow a very sinuous course and the luminal ones appear extremely rich in pleomorphic microprojections. When the endothelium differentiation is concluded (5-day-old chicken), the cells are very thin and again exhibit regular and smooth surfaces. These findings reveal a great mobility of the cell membrane of the endothelial cells when they are growing longer and thinner by a moulding process. Moreover, the presence of a number of pinocytotic pits in the embryo vessels would indicate that the neutral vessels, provided with a typically low pinocytotic activity in the adult life, are engaged in this function during development.