A light and electron microscopic study of proliferation and maturation of fibrous astrocytes in the optic nerve of the human embryo.

Optic nerves from perfusion-fixed human embryos of 28,50,75,105,120 and 165 mm crown-rump length were examined in the electron microscope. The number of glial cells per section was found to increase steadily from 10 weeks post-conception to 18 weeks and a close correlation (r = 0.92) was found between the percentage vascularity and the glial population. Mitotic figures were present in all optic nerves examined. From 14 weeks onwards all glial cells, except pericytes, were found to be fibrous astrocytes. The human fibrous astrocyte appears to pass through the following stages of development: (1) Astrocytic precursors (dark glioblasts) have a dense cytoplasmic matrix with few organelles, although a single cilium is frequently present.(2) Concomitant with the increase in vascularization of the optic nerve found between 12 and 14 weeks glycogen granules increase in the cytoplasm of astrocytic precursors, followed by microfibrils, which appear first in the processes and later extend into the perikaryon. (3) With the appearance of glycogen granules the cytoplasmic organelles, particularly mitochondria, increase in amount and the cytoplasmic matrix gradually becomes less dense. (4) With increasing age fewer organelles are found in astrocytic processes, which become thinner and densely packed with microfibrils.