Electron microscopic study of the developing capillaries of human brain.

The ulstrastructural characteristics of the cerebellum and olfactory bulb capillaries were studied in 12 human specimen measuring 25-200 mm. Type I capillaries, whose lumina were wider than 8 mu, contained no pores of fenestrations. The basement membrane was sometimes discontinous. In that case, junctions without quintuple-layered zones could be observed. Such capillaries may correspond to primary vessels in which the sinusoid character has disappeared. Elsewhere, the basement membrane was continuous and the interendothelial junctions always contained quintuple-layered zones. Type II capillaries, whose lumina measured between 2 and 8 mu had basement membranes that were either discontinuous or continuous. They were identical to the immature capillaries described previously in numerous species including man. Type III caillaries, rarely observed, were characterized by the presence of several endothelial cells with abundant cytoplasm, which limited the mina from 0.5 --3 mu in diameters. They had continuous basement membranes. Type IV and V capillaries has small or non-patent lumina. The basement membrane was absent or rudimentary. These capillaries appeared to correspond to simple and complexendothelial sprouts. In Types II, III, V and most Type IV capillaries, the interendothelial junctions contained quintuple-layered zones. No mature capillaries were observed. The hypothetical pathways of development of cerebral capillaries and the degree of permeability of immature interendothelial junctions are discussed. The long duration of maturation in man probably accounts for the diversity of capillary formations observed as compared with animals. The co-existence over along period of time of several capillary types with probably varying permeability may play a significant role in the maturation of the blood-brain barrier.