Small blood vessels and the origin of phagocytes in the rat cerebral cortex following heavy particle irradiation

Abstract

Alterations in the structure of small cerebral vessels were studied with the electron microscope at various intervals following ionizing particle irradiation. The disruption of small vessels required heavier doses of irradiation than is required for destruction of neural or glial elements. The appearance of large phagocytes which might be identified with the compound granular corpuscle is dependent upon and correlated with perivascular reaction. These cells appear to be derived from the pericytes of small vessels and not from any glial element within the brain parenchyma. The appearance of dense bodies or lamellar inclusions in cytoplasm cannot constitute evidence of phagocytosis. The only cerebral element which appears to display macrophage activity seems to be derived from the vascular pericyte. It is suggested that mesodermal elements only extend beyond the basal lamina of vessels under conditions of tissue disruption. The implications of these findings with respect to the designation “microglia” is discussed.