The ultrastructure and ultraarchitectonics of the normal optic nerve of adult white rats was studied, including morphometric investigations at the electron microscopic level. The unfixed optic nerve of adult rats is about 0.45 mm thick, its length between ocular bulb and optic chiasma is about 10 mm. Mesenchymal tissue is confined to the interfascicular capillaries, the number of which decreases significantly towards the optic chiasma, quite as well the number of glial nuclei per unit volume decreases between ocular bulb and chiasma. The astrocyte processes containing abundant filaments are oriented mainly perpendicular to the direction of the myelinated fibers. The oligodendrocytes possess fine protoplasmic extensions of irregular course. In transverse sections the relative volume of myelinated fibers in electron micrographs increases between ocular bulb and optic chiasma. Provided that the diameter of the axons is invariable, this would indicate an increasing thickness of myelin sheaths towards the cerebrum. Conversely, the relative volume of tissue compartments between the myelinated fibers—mainly astrocytes and their processes—decreases between ocular bulb and chiasma, corresponding to the decreasing number of glial nuclei. These changes in the relative volume of tissue constituents along the optic nerve are statistically significant.
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