A quantitative histological study of cell division and changes in cell number in the meningeal sheath of the embryonic human optic nerve.

Mitotic cells are present in all layers of the meninges of the human optic nerve between 8 and 18 weeks post-conception. The number of meningeal cells per section remains constant between 8 and 12 weeks before rising rapidly from 234 at 12 weeks to 747 at 18 weeks. The mitotic index is only 0.17% at 8 weeks but rises to 1.02% at 10 weeks before falling gradually to 0.29% at 18 weeks. A comparison of the results of this study with a previous one on gliogenesis in the same nerves (Sturrock, 1975) leads to the tentative conclusion that mitotic activity in the layers of the meninges is probably sufficient to account for the increase in meningeal cell number between 8 and 18 weeks. The substantial increase in thickness of the meninges found between 14 and 15 weeks is due to a large increase in the amount of collagen in the developing dural layer rather than the modest increase in cell number. During mitosis meningeal cells do not retract their processes and cell division usually occurs tangentially.