IMMUNOHISTOCHEMICAL DETECTION OF THROMBOSPONDIN IN MICROGLIA IN THE DEVELOPING RAT-BRAIN

The development of microglia involves the expression of a phenotype di splaying phagocytic behaviour termed brain macrophage or amoeboid micr oglial cell. We have previously shown that rat brain macrophages purif ied in vitro secrete thrombospondin, an extracellular matrix protein, which acts on cultured neuronal cells by promoting neurite growth. In the present study, the expression of thrombospondin was investigated i n tissue sections of the developing rat forebrain in relation to the d istribution of microglia. These cells were identified using anti-macro phage antibodies and the isolectin B4 from Bandeiraea simplicifolia. I mmunocytochemical detection of thrombospondin clearly outlined a cell population displaying the morphologies and distribution of brain macro phages, from the 17th day of embryonic life up to the end of the secon d postnatal week. These cells were most numerous in cortical and subco rtical regions of developing fibre tracts such as the corpus callosum or the internal capsule. The localization of thrombospondin in brain m acrophages was confirmed by double immunostaining using ED1 monoclonal anti-macrophage antibodies. Ramified microglial cells were also label led transiently by anti-thrombospondin antibodies during early postnat al life. These results provide in situ evidence supporting the notion that microglial cells could favour axonal growth by producing thrombos pondin during development.