Vascularization of the human embryonic optic nerve

Vascularization was examined in perfusion-fixed optic nerves from human embryos aged from 8 to 18 weeks post-conception. The volume density of blood vessels increased from 0.7% at 8 weeks to 24% at 18 weeks; the surface area of blood vessels increased from 2.1 mm2/mm3 at 8 weeks to 49.9 mm2/mm3 at 18 weeks. At all ages endothelial cells were joined by zonulae occludentes. Capillary basement membranes were present at all ages and blood vessels not lying in developing fibrous septa were invested with a complete glial sheath from 8 weeks of age. Only extremely rarely were parts of small capillary walls not covered by glial processes. Arterioles were absent from the optic nerves until 18 weeks post-conception. At first vascularization was most obvious in the central part of the nerve but there was no evidence for the presence of a central artery of the optic nerve. There did not appear to be a direct correlation between the degree of vascularization and increase in glial or axonal number.