A quantitative study of developing axons and glia following altered gliogenesis in rat optic nerve

Abstract

Axonal and glial cell development within rat optic nerve in which gliogenesis was altered by systemic injection of 5-azacytidine (5-AZ) was examined by quantitative electron microscopy. In neonatal (0–2 days) rat optic nerves, all fibers are premyelinated, and they exhibit a fairly uniform diameter (∼0.22 μm). These fibers occupy ∼55% of the optic nerve volume. At this early age, glia within the optic nerve consist only of cells af astrocytic lineage and progenitor cells. These glia occupy ∼28% of the optic nerve volume, and there are ∼80 glial cells/optic nerve cross section. In 14-day-old normal optic nerves, myelinated and ensheathed fibers comprise ∼17% and 9%, respectively, of the total number of axons. Mean axonal diameter of myelinated fibers is ∼0.75 μ m, while mean diameter for ensheathed axons is ∼0.50 μ m. By volume, these fibers occupy ∼25% of the nerve, which is similar to the volume occupied by premyelinated axons in these nerves. At 14 days of age, there are ∼300 glial cells/optic nerve transverse section, and these glia occupy ∼37% of the volume in normal optic nerve. Oligodendroglia represent ∼40% of total glial cells present, while astroglia and progenitor cell each comprise ∼30% of the cells. In optic nerves from 14-day-old rats treated with 5-AZ, few myelinated fibers are present and the number of oligodendroglia is markedly reduced. Axonal diameter of premyelinated fibers is similar to that of age-matched controls. Myelinated and ensheathed fibers comprise ∼2% of the total fibers present in 5-AZ-treated optic nerves, with the remaining fibers being premyelinated. The few myelinated and ensheathed fibers present in 5-AZ-treated optic nerves display similar axonal diameters to corresponding fibers from age-matched control tissue. Glial cells occupy ∼40% of the nerve volume, and there are ∼200 glia/nerve cross section in 5-AZ-treated rats. Astroglia comprise ∼63% of the total glial cells, while ∼12% of the cells are oligodendroglia. These results demonstrates that 5-AZ is a potent inhibitor of oligodendrogliogenesis, with a concomitant marked reduction in the number of myelinated fibers.