DEVELOPMENT OF MICROGLIA IN THE QUAIL OPTIC TECTUM

The development of microglia in the quail optic tectum from embryonic day 6 to adulthood was studied by using the QH1 monoclonal antibody. I n youngest tecta, microglial cells were scarcely present, but their nu mber rose in subsequent stages. A clear pattern of microglial cell dis tribution was observable in embryos of 9-16 days. (1) Round cells appe ared close to the ventricular layer. (2) Large numbers of ameboid and round labeled cells were seen in the stratum album centrale during dev elopment. A gradient of cell density was observable in this layer, as fewer labeled cells appeared in medial regions of the tectum than in l ateral regions. (3) Maturing ramified cells were found in layers exter nal to the stratum album centrale, where they increased in number and in branching complexity during development. In adult tecta, almost all microglial cells were of the mature ramified type and were distribute d homogeneously in the different tectal layers, although in some layer s they had particular morphological features. The distribution of micr oglia in the developing tectum and in adjacent regions provided insigh t into the routes of microglial cell invasion of the tectum during dev elopment. Apparently, a proportion of microglial cells reached the tec tal parenchyma from the meninges and from the ventricular lumen, but t he majority of them migrated along nerve fiber tracts from their entry point at the pial surface of the ventromedial caudal tectum. After th ey reached the stratum album centrale, microglial cells continued thei r migration toward more external layers, where they differentiated int o ramified microglia. (C) 1994 Wiley-Liss, Inc.