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. In youngest tecta, microglial cells were scarcely present, but their number rose in subsequent stages. A clear pattern of microglial cell distribution was observable in embryos of 9-16 days. (1) Round cells appeared close to the ventricular layer. (2) Large numbers of ameboid and round labeled cells were seen in the stratum album centrale during development. A gradient of cell density was observable in this layer, as fewer labeled cells appeared in medial regions of the tectum than in lateral regions. (3) Maturing ramified cells were found in layers external 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 distributed homogeneously in the different tectal layers, although in some layers they had particular morphological features. The distribution of microglia in the developing tectum and in adjacent regions provided insight into the routes of microglial cell invasion of the tectum during development. Apparently, a proportion of microglial cells reached the tectal parenchyma from the meninges and from the ventricular lumen, but the majority of them migrated along nerve fiber tracts from their entry point at the pial surface of the ventromedial caudal tectum. After they reached the stratum album centrale, microglial cells continued their migration toward more external layers, where they differentiated into ramified microglia.