Transient populations of presumptive macrophages in the brain of the developing hamster, as indicated by endocytosis of blood-borne horseradish peroxidase.

During postnatal development, clusters of cells associated with the mononuclear phagocytic system appear within the white matter of rodents and cats. We studied the distribution and morphology of these cells in the hamster's brain during the first 2 weeks after birth. In animals of different ages, horseradish peroxidase was injected into the heart. After 3-6 h survival, the animals were perfused with aldehydes and had their brains removed, cut and reacted. In another series, fixed brain sections from horseradish peroxidase-injected and non-injected animals were reacted for a non-specific esterase expressed by monocytes and macrophages. The horseradish peroxidase reaction-product was seen throughout the nervous tissue at the first postnatal day, appearing more concentrated in certain brain sectors from postnatal day 3 through 10, to finally become restricted to a few regions at postnatal day 16. Horseradish peroxidase-labeled cells appeared in increasing numbers from postnatal day 1 to 8, decreasing thereafter to disappear completely at postnatal day 16. Some labeled cells were roundish or elliptical with few, if any, processes; others had several clearly detectable processes. Horseradish peroxidase-labelled cells formed clusters within the dorsal subventricular zone, dorsal cortical white matter, corpus callosum and several other prosencephalic fiber tracts. The morphology of esterase-reactive cells was less clearly outlined but their distribution and relative density correlated with those of horseradish peroxidase-labeled cells. Also, many horseradish peroxidase-labeled cells were esterase-positive in most clusters. We conclude that (1) some cells in the developing brain selectively endocytose and accumulate blood-borne horseradish peroxidase in their cytoplasm, (2) these cells do not appear to be neurons but a particular cell type associated to the mononuclear phagocytic system and (3) they cluster transiently in particular sectors of the cortical and subcortical white matter during the first 2 weeks after birth.