Transient lectin binding by white matter tract border zone microglia in the foetal rabbit brain

Axonal growth cones of developing white matter tracts are guided through th e cerebrum by interactions with cell surface and extracellular matrix molec ules expressed by glial cells that mediate cell adhesion and contact-depend ent inhibition. Specific carbohydrates are considered essential for the pro per functioning of these molecular complexes. We studied developmental aspe cts of complex carbohydrate expression by white matter glia in the foetal r abbit brain using the tomato lectin Lycopersicon esculentum, which has affi nity for components of the extracellular matrix proteins and cell surface p roteins (N-acetylglucosamine) and activated lysosomal membrane glycoprotein s (N-acetyllactosamine). Concentrations of the lectin-positive glia were tr ansiently found immediately adjacent to developing white matter tracts of t he foetal rabbit brain from 22 to 32 days' gestation. The number of positiv e cells markedly diminished by the fourth post-natal day and in the adult b rain. The lectin-positive glia did not react with antibody to glial fibrill ary acidic protein. However, they did express the macrophage surface antige n, Mac-1, indicating that the lectin binding reflected the presence of micr oglial activated lysosomal membranes. These data suggest that, in addition to their role as central nervous system scavengers, microglia are involved in a specifically timed function in the neurodevelopmental programme of whi te matter tract formation. (C) 1998 Chapman & Hall.