Development of the optic nerve of the rat.

Development of the rat optic nerve is studied electron microscopically. By marked bionecrosis, many neuroepithelial cells of the fissure portion of the optic stalk are eliminated immediately following optic cup formation. A small number of the surviving neuroepithelial cells forms the tubular optic stalk. The wall cells of the stalk elongate and proliferate to become a cluster of cells at the retrobulbar region. These cells differentiate into glia cells, astrocytes first and oligodendroglia cells second. Axons invade into the intercellular spaces of the elongating stalk cells which are differentiating into astrocytes. The invading axons are found first in the space at the basal portion of the stalk cells, or the peripheral zone of the optic nerve. Myelination occurs in the later stage of development. The fine processes of the oligodendroglia cells which surround groups of axons, eliminate the cytoplasm, and become the first myelin membrane.