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

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-