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 the cerebrum by interactions with cell surface and extracellular matrix molecules expressed by glial cells that mediate cell adhesion and contact-dependent inhibition. Specific carbohydrates are considered essential for the proper functioning of these molecular complexes. We studied developmental aspects of complex carbohydrate expression by white matter glia in the foetal rabbit brain using the tomato lectin Lycopersicon esculentum, which has affinity for components of the extracellular matrix proteins and cell surface proteins (N-acetylglucosamine) and activated lysosomal membrane glycoproteins (N-acetyllactosamine). Concentrations of the lectin-positive glia were transiently found immediately adjacent to developing white matter tracts of the foetal rabbit brain from 22 to 32 days' gestation. The number of positive cells markedly diminished by the fourth post-natal day and in the adult brain. The lectin-positive glia did not react with antibody to glial fibrillary acidic protein. However, they did express the macrophage surface antigen, Mac-1, indicating that the lectin binding reflected the presence of microglial 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 white matter tract formation.