Studies of the distribution and functional roles of transitory amoeboid microglial cells in developing rat brain using exogenous horseradish peroxidase as a marker.

When HRP was injected intraperitoneally (i. p.), labelled amoeboid microglial cells (AMC) were consistently localized in the subcortical white matter and circumventricular zones in early postnatal (1 and 7 days old) but absent in late postnatal (14-day-old) rats. The ingested HRP disappeared from the labelled cells 5 days after IP injection. Subcutaneous injection of HRP had also resulted in the labelling of amoeboid microglial cells in the corpus callosum of early postnatal rats. When the injected HRP was followed ultrastructurally over a time course sequence in intravenously (i. v.) injected rats, it was first detected in the invaginations on the luminal side of endothelium and in the endothelial cytoplasm 30 min after injection. HRP was present both in the endothelium and amoeboid microglial cells 3 hours later. With time, the tracer was progressively accumulated in the cytoplasm of AMC and it was sequestered in the vacuoles and lysosomes. It is concluded from this study that when injected i. p., i. v. or subcutaneously in the early postnatal rats, HRP is circulated to the blood vessels in the subcortical white matter and circumventricular zones where it gains access into the nervous tissues by transendothelial transport. The extravasated tracer is then phagocytosed by the residential AMC and subsequently degraded. The absence of labelling of AMC is attributed to the maturation of BBB which impedes the entry of exogenous tracer by the 14th postnatal day.