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

Volume 14
Number 10
Rat optic nerve 741
Fig. 13. Postnatal tenth day. A, serpentine double-cell membrane which, led by a small
cytoplasm of the glia cell (°), extends over a few axons (arrows). x40,000. B> axons are
surrounded by a relatively large glia cell element (*) and by the double membranes (double
arrows). x58,400.
tain sparse micro-organelles. They consist
of the electron lucent cytoplasm and
are distinguishable from those of the astrocyte
which contain rich filaments. These
thin processes appear to produce myelin
sheaths at a later date,
Myelination. Myelination of the optic
nerve begins at a relatively later stage.
During the first postnatal week, fine processes
of the oligodendroglia cell begin to
extend profoundly in the space between
axons. On the eighth and ninth days, when
the diameter of the optic nerve measures
about 200 ju and the development of the
retinal photoreceptor elements is considerably
advanced, the first sign of myelination
is noted in the area immediately behind
the globe and in the chiasm. The
extending thin-cell processes begin to surround
groups of axons. These thin processes
become a double-cell membrane
structure, eliminating the cytoplasmic matrix
in several locations (Fig. 12). These
seem to be the earliest myelin membrane.
The early myelin membrane seems to attach
to a group of axons instead of an individual
fiber. The tip portion of the process
which becomes the mesoaxon seems to preserve
a small amount of the cytoplasm
(Figs. 12 and 13, A). Also, the usual myelination
process in which a few axons are
surrounded by a large cytoplasm of the glia
cell is occasionally found (Fig. 13, B).
During the next few days the number
of double-membrane structures increases
markedly. Many axons are surrounded
completely by these membranes and by
layers of lamellar membranes. By the end
of the second week, almost all axons are
found to be surrounded by myelin sheaths
(Fig. 14). The nodes of Ranvier, the area
where cytoplasms of two oligodendroglia
cells meet, are frequently found in this
stage (Fig. 15). By the end of the third
week, all axons become myelinated. Fine
processes of the astrocytes then begin to
fill in the spaces between the myelinated
nerve fibers.