Appearance and distribution of fetal brain macrophages in mice. Immunohistochemical study with a monoclonal antibody.

A study on the localization of fetal and neonatal brain macrophages of mice from embryonic day 10 (E10) to postnatal day 21 (P21) was carried out immunohistochemically using a monoclonal antibody against a macrophage differentiation antigen (Mac-1) and the labeled avidin-biotin technique. In the central nervous system, the macrophages recognized first were mainly located in the choroid plexuses of the fourth and lateral ventricles at E14. Their number increased at E17-P3 and gradually decreased thereafter. In the cerebral parenchyma, a few macrophages appeared at E14 in the matrix cell layer. They were also detected in the migrating zone at E15, E17 and in the cortical plate at E19. Mapping of positive cells at the stage of neuroblast formation (E15, E17, E19) disclosed the precise distribution of cerebral macrophages. The macrophages that appeared first in the choroid plexuses at E15 may be derived from the subarachnoid vessels, which extend into the stroma of the choroid plexuses when the matrix cell layer invaginates into the lateral ventricle to form the choroid plexuses. Almost all of the macrophages recognized in the cerebral parenchyma disappeared at P9 when the cytoarchitecture seemed to be completed. In the cerebellum, which develops later than the cerebrum, macrophages appeared after birth and were located mainly in the internal granular layer. The brain macrophages always appeared in the regions where cell proliferation and brain remodeling are most active at each stage. These findings suggest that fetal and neonatal brain macrophages may play an important role in scavenging degenerated cells and cell debris during histogenesis of the central nervous system.