Electron microscopy of endocytic elements in the cat cerebrum

Summary Small quantities of ferritin or Thorotrast were injected into the visual cortex of the cat in order to determine which cells in the cerebrum exhibited endocytic activity when exposed to foreign material. Blocks of tissue from the cortex and adjacent white matter were examined by electron microscopy 5 h and 20 h after injection.Some neurons and oligodendrocytes and all the astrocytes examined showed pinocytotic activity. Increasing the time of exposure of the tissue to the tracer did not appear to result in greatly increased uptake by individual cells, except in the case of astrocytes.Neutrophils, which invaded the tissue after 5 h, and in increased numbers after 20 h, exhibited minimal endocytic activity when they invaded the grey matter. They were more active in the white matter.Up to 20 h after injection, the three most important endocytic elements in the cerebrum were astrocytes (collectively), pale perivascular cells within the basement membrane of the blood vessels and phagocytes, which appeared to be reactive endogenous cells.Many thanks are due to Professor M. J. Blunt, School of Anatomy, University of N.S.W., for his helpful criticisms throughout the course of this study, which was supported by a research grant from the National Health and Medical Research Council of Australia to Professor Blunt. The assistance of Mrs. Ruth Mather is gratefully acknowledged.