Microglia in the human embryonic optic nerve.

Microglial cells were identified electron microscopically in optic nerves from 8, 10, 12, 14, 15 and 18 weeks post-conception human embryos. In 8 and 10 weeks embryos, most microglia were relatively undifferentiated and the cytoplasm contained numerous large vacuoles. These were similar to the tuberous and amoeboid microglia described by earlier authors. In older embryos well differentiated microglia were present. At all ages microglia contained cellular debris and evidence of lipid droplets. Although there was a statistically significant correlation between vascularity and the number of microglia it is postulated that this is coincidental rather than biologically significant. Most microglia were found within bundles of axons and there was no preferential distribution in relation to blood vessels or to the pial surface at any stage of development. The percentage of microglia present increased from 1.3% at 8 weeks to 2.7% at 18 weeks.