Development of the optic nerve of the rat - Investigative ...

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Volume 14 Number 10 Rat optic nerve 737 Fig. 7. The peripheral portion of the optic stalk of a 15-day-old fetus. Elongating stalk cells form large intercellular spaces which are filled with small axons and glia processes. The cell in the center (*) is differentiating into an astrocyte. The cytoplasm contains microtubules and filaments. xl0,000. Insert shows wavy microtubules and vesicles in the tip of a growing axon. x25,200. 15-day-old fetuses, there is a considerably slender portion in the central part of the nerve in which axon fibers are sparsely distributed. Glia cells. Glia cells appear to develop from the stalk wall cells. The differentiation process of the stalk cell has been described above (Figs. 2 through 4). On the fifteenth day, the optic nerve becomes oval in cross-section. A cluster of cells which are surrounded by axons is present in the upper off-central zone, especially of the retrobulbar segment. The cells in the cluster proliferate with a moderate mitotic activity for about two days. After the disappearance of the neuro-
738 Kuwabara Investigntive Ophthalmology October 1975 Fig. 8. High magnification of the growing axons. A, lS-day-old fetus. The base of the stalk cells form the peripheral limit of the optic nerve. The arrow indicates the basement membrane. x32,000. B, 16-day-old fetus. Axons contain regularly spaced microtubules. x51,200. epitheliar lumen, the apical junctions of the stalk cells disappear. The proliferated cells begin to disperse into the extending axon bundles. The cytoplasm of these early glia cells contains abundant rough endoplasmic reticulum and a moderate number of mitochondria. Some original stalk cells which have elongated initially appear to move their nuclei to the basal portion. These cells contain relatively sparse membranous micro-organelles (Fig. 8). Also, cells with similar cytologic structure are present occasionally among the axons and other glia cells. The cells which proliferate in the earlier stage appear to become the astrocytes. The astrocytes begin to extend large branches into the axon bundles. The cytoplasm begins to contain numerous filaments, microtubules, and various microorganelles (Fig. 9). The extending process of the astrocytes often shows prominent junctions between them (Fig. 9, insert). At a slightly later stage, cells with darker cytoplasm begin to appear in the central zone of the cluster (Fig. 10). The cytoplasm of these cells contains abundant rough endoplasmic reticulum, free ribosomes, and microtubules. The cells disperse into the nerve tissue also, but their number is small until the second to third postnatal day. These cells appear to become oligodendroglia cells. At the time of birth, both glia cells are distributed diffusely throughout the nerve, but they are often arranged in strand-like fashion between the axon bundles (Fig. 11). Oligodendroglia cells appear to proliferate gradually with a moderate mitotic activity. Fine branches of the oligodendroglia cells appear to extend after the astrocytes have divided the axon bundles into groups. This occurs during the first week after birth. Despite the rich micro-organelles in the somar cytoplasm, the extending fine processes of the oligodendroglia cells con-
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Development of the optic nerve of the rat - Investigative ...

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