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Response of amoeboid microglia/brain macrophages in fetal rat brain exposed to a teratogen

This study examined the time course response of amoeboid microglia/brain macrophages in the rat fetus induced by a single intraperitoneal injection of cyclophosphamide, a teratogen, into the mother rat at 13 days of gestation. Compared to the normal fetal brain, a marked increase in amoeboid microglia was observed in the telencephalon and diencephalon of experimental fetuses, especially in those killed at embryonic day 15. Conglomerations of microglia occurred in the dorsal and superior neuroepithelium of diencephalon, basal telencephalon, cortical neuroepithelium, and hippocampal formation as identified with OX‐42, OX‐18, and ED‐1 by immunohistochemistry. Rhodamine isothiocynate (RhIc) as a tracer was injected via the tail vein into the pregnant rat to assess the phagocytic capability of these cells. Following the tracer injection, none of microglial cells in normal fetal brain was detectable. RhIc, however, was readily taken up by amoeboid microglia in fetal brain with injury insult. Double labeling has shown that the RhIc‐labeled cells were immunoreactive with ED‐1, OX‐42, OX‐18, and OX‐6, confirming their microglial nature. Microglial proliferation was assessed by immunohistochemistry using bromodeoxyuridine, which showed a marked increase in mitotic activity. Confocal microscopic analysis revealed that a varying number of microglia coexpressed iNOS, macrophage colony‐stimulating factor (M‐CSF), and ICAM‐1. RT‐PCR analysis showed increased expression of M‐CSF mRNA. Furthermore, colony‐stimulating factor‐1 receptor mRNA was localized in microglia by in situ hybridization. The present results suggest that NO along with M‐CSF and ICAM‐1 is involved in microglial proliferation in prenatal brain injury. J. Neurosci. Res. 64:79–93, 2001. © 2001 Wiley‐Liss, Inc.