GLIOGENESIS IN POSTNATAL RAT OPTIC-NERVE - LC1 PLUS MICROGLIA AND S100-BETA PLUS ASTROCYTES

Lipocortin 1 (LC1) and S100-beta, two Ca2+-binding proteins that serve as specific markers for microglia and astrocytes, respectively, have been used to study postnatal gliogenesis in the rat optic nerve. Compu terized image analysis was used to quantify and map the stained and un stained glia in transverse sections (10 mu m thick) taken 1-2 mm from the chiasm in optic nerves from rat pups at postnatal day 0 (P0), P7, P14, P21, P28, P38 and adults. The number of astrocytes was remarkably constant (100 per section) at all ages. Because the area of the nerve increases 10-fold from P0 to adult, the population density of astrocy tes begins at > 5000 mm(-2) and drops to 400 mm(-2) in the mature nerv e; however, because the nerve length increases two-fold, the number of astrocytes doubles over the same period. In contrast, the number of L C1 + cells per section initially is sparse (4 at PO), increases rapidl y up to 36 at P21 and levels off at 49 in adults. The microglia popula tion density is relatively stable throughout development (200-300 mm(- 2)) except during the peak of oligodendroblast apoptosis (P21) when it rises to 450 mm(-1). Neonatally, LC1 immunoreactivity predominantly l abels spherical-ameboid cells; but by P28 they are replaced by mature ramified microglia. The number of unstained cells (putative oligodendr ocytes) per section increases from 11 at P0 to a peak of 308 at P21, a nd declines slightly to 269 in adults. While generally confirming conc epts of astrocyte and oligodendrocyte ontogeny from the literature, th e present report adds considerable detail regarding microglia, which o ften have been ignored. Microglia identified by LC1 immunoreactivity c omprise 12% of the glia in adult optic nerve near the chiasm.