Development of the meninges of the human embryonic optic nerve

From 8 to 14 weeks post-conception the human embryonic optic nerve is surrounded by loosely packed layers of connective tissue. From 8 weeks onwards the innermost layers of this sheath consist of fibroblast-like cells whereas the cells of the middle and outer layers, although similar in appearance, contain increasing amounts of glycogen, until by 14 weeks very large amounts of glycogen are present in these cells. A collagen-rich dural layer first appears at 15 weeks post-conception. This layer contains few cells. The outer-most cells have the appearance of fibroblasts and are joined to each other by long sheet-like processes. Cell in the middle of the dura are glycogen-containing cells, apparently actively engaged in collagen production. These cells are similar to those found in the adjacent arachnoid. By 18 weeks of age the meninges are well-differentiated. The endothelial cells of all capillaries found in the meninges from 8 weeks post-conception, including dural capillaries, are sealed by zonulae occludentes. At 15 and 18 weeks a few fenestrated capillaries were observed at the junction of the dura and the surrounding loose connective tissue. Peripheral nerves are closely related to the meninges from 8 weeks post-conception and peripheral nerve bundles are present in the dura from 15 weeks. At 18 weeks the first signs of myelination were observed in these bundles.