Changes in the number of axons in the human embryonic optic nerve from 8 to 18 weeks gestation

The number of axons was estimated in optic nerves from human embryos aged 8, 10, 12, 14, 15 and 18 weeks post-conception. At 8 weeks there were 2.67 million axons. The number fell to 1.94 million at 10 weeks then increased to reach a peak of 5.86 million at 14 weeks before falling to 3.58 million at 18 weeks. Mean axon-diameter increased slowly from 0.17 micron at 8 weeks to 0.24 micron at 18 weeks. Growth cones declined in number from 34,400 at 8 weeks to 9,400 at 14 weeks and were not found thereafter. Degenerating axons were most numerous at 12 and 15 weeks.