The distribution within the brain of ferritin injected into cerebrospinal fluid compartments. II. Parenchymal distribution

Abstract

The electron lucent channels, about 200 Å‐wide, between the plasma‐lemmas of adjacent cell processes within the brain are true pericellular spaces along which ferritin molecules can move. The micelles, visualized electronmicroscopically, move through the spaces until they encounter an interglial fusion or its myelin counterpart. The interspace between glial processes is abruptly sealed by these inter‐membranous fusions or occasionally distended by a dense filler capable of trapping ferritin. The cerebral interspace is thus highly variable in width and content. Molecules initially enter the interspace by passing across the basement membrane of the glial border fronting the subarachnoid space whence they are pinocytosed by the underlying glial processes and, at the subarachnoid border of the anterior medullary velum, by also passing directly between adjacent ependymal extensions. Once having entered the parenchymal interspace, micelles can be pinocytosed by neuronal somata and processes and, to a greater degree, by glial fibers. Pinocytosis by the thicker rather than the attenuated portions of glial fibers suggests that the plasmalemmas on the opposite sides of a process must be separated by a critical distance before pinocytotic indentations can be formed. The thinner portions thus offer greater barriers to the movement of micelles than do thicker regions.