Dynamics of microglia in the developing rat brain

Entrance of mesodermal precursors into the developing CNS is the most well-accepted origin of microglia. However, the contribution of proliferation and death of recruited microglial precursors to the final microglial cell population remains to be elucidated. To investigate microglial proliferation and apoptosis during development, we combined proliferating cell nuclear antigen (PCNA) immunohistochemistry, in situ detection of nuclear DNA fragmentation (TUNEL), and caspase-3 immunohistochemistry with tomato lectin histochemistry, a selective microglial marker. The study was carried out in Wistar rats from embryonic day (E) 16 to postnatal day (P) 18 in cerebral cortex, subcortical white matter, and hippocampus. Proliferating microglial cells were found at all ages in the three brain regions and represented a significant fraction of the total microglial cell population. The percentage of microglia expressing PCNA progressively increased from the embryonic period (25-51% at E16) to a maximum at P9, when the great majority of microglia expressed PCNA (92-99%) in all the brain regions analyzed. In spite of the remarkable proliferation and expansion of the microglial population with time, the density of microglia remained quite constant in most brain regions because of the considerable growth of the brain during late prenatal and early postnatal periods. In contrast, apoptosis of microglia was detected only at certain times and was restricted to some ameboid cells in white matter and primitive ramified cells in gray matter, representing a small fraction of the microglial population. Therefore, our results point to proliferation of microglial precursors in the developing brain as a physiological mechanism contributing to the acquisition of the adult microglial cell population. In contrast, microglial apoptosis occurs only locally at certain developmental stages and thus seems less crucial for the establishment of the final density of microglia.