MICROGLIA DEVELOPMENT IN THE QUAIL CEREBELLUM

We used the QH1 antibody to study changes in the morphological feature s and distribution of microglial cells throughout development in the q uail cerebellum. Few microglial precursors were present in the cerebel lar anlage before the ninth incubation day (E9), whereas many precurso rs apparently entered the cerebellum from the meninges in the basal re gion of the cerebellar peduncles between E9 and E16. From this point o f entry into the nervous parenchyma, they spread through the cerebella r white matter, forming a 'stream' of labeled cells that could be seen until hatching (E16). The number of microglial cells in the cerebella r cortex increased during the last days of embryonic life and first po sthatching week, whereas microglial density within the white matter de creased after hatching. As a consequence, the differences in microglia l cell density observed in the cerebellar cortex and the white matter during embryonic life diminished after hatching, and microglia showed a nearly homogeneous pattern of distribution in adult cerebella. Amebo id and poorly ramified microglial cells were found in developing stage s, whereas only mature microglia appeared in adult cerebella. Our obse rvations suggest that microglial precursors enter the cerebellar anlag e mainly by traversing the pial surface at the basal region of the ped uncles, then migrate along the white matter, and finally move radially to the different cortical layers. Differentiation occurs after the mi croglial cells have reached their final position. In other brain regio ns the development of microglia follows similar stages, suggesting tha t these steps are general rules of microglial development in the centr al nervous system. (C) 1997 Wiley-Liss, Inc.