Variation with age in the labelling of amoeboid microglial cells in rats following intraperitoneal or intravenous injection of a fluorescent dye

Abstract

Amoeboid microglial cells (AMC) in the corpus callosum were selectively labelled following a single intraperitoneal (i.p.) injection of the fluorescent dye, rhodamine isothiocyanate (RhIc) into postnatal rats. The frequency of RhIc-labelled cells varied with age, with the largest number occurring in 7-d-old animals. Thereafter, the labelled cells declined drastically in number and fluorescence and were barely detectable in 12-d-old injected rats. Labelled cells were absent in 13-d or older rats given an RhIc injection. When the injected RhIc was followed over a time course sequence, it was first detected in the cerebral blood vessels and their lining endothelia within 5 min after the injection. A variable number of AMC emitting a weaker fluorescence were closely adherent to the outer walls of the blood vessels. With time, the fluorescence in the AMC was progressively enhanced, but that in the blood vessels showed a concomitant reduction. In the rats that received an intravenous (i.v.) injection of RhIc, the labelling pattern of AMC, both in terms of its variation with age and in temporal sequence, paralleled that in rats given i.p. injections. In 12-d-old rats subjected to a stab wound coupled with an i.p. injection of RhIc, a considerable number of AMC not normally labelled at this age were activated. The cells exhibited an intense fluorescence and expressed MHC surface antigen immunoreactivity. It is concluded from this study that when injected i.p. or i.v., RhIc is readily circulated to the cerebral vessels, where it enters brain tissue by transendothelial transport. The tracer which permeates the corpus callosum is endocytosed by the AMC on surveillance. The entry of RhIc, however, is impeded by the maturation of the blood-brain barrier which occurs between 12 and 13 d of age.