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

Volume 14
Number 10
Rat optic nerve 733
brain tissue was processed as one piece in early
fetuses, and the optic nerves of animals older
than the eighteenth embryonal day were divided
into three pieces: postbulbar, central (midcanalicular),
and chiasma portions. The tissue
was postfixed in 1 per cent osmium tetroxide in
the same buffer solution for 90 minutes at 4° C.
After further trimming and orientation under a
dissecting microscope, the small pieces were dehydrated
in a series of ethyl alcohol, treated with
propylene oxide, and embedded in an epoxy resin
following Luft's method. 1 "
Sections (0.5 to 1.0 y. thick) were stained with
alkaline toluidine blue for light microscopic study.
Ultrathin sections were stained with uranyl acetate
and lead citrate and were examined by an electron
microscope with an accelerating voltage of
80 KV.
Results
The optic stalk, measuring about 200 /± in
length, becomes recognizable immediately
following the formation of the optic cup
on the thirteenth fetal day (Fig. 1). The
invaginating fold of the inner layer of the
cup extends posteriorly and becomes the
fissure which disappears rapidly as development
progresses. The whitish nerve
tissue becomes grossly visible on about the
sixteenth day. The most active differentiation
of the optic nerve appears to take
place between the sixteenth and eighteenth
day.
On the day of birth, the optic nerve is
about 2 mm. long and the configuration
of the chiasma is clearly formed. Myelination
of the nerve fibers, however, begins
on the postnatal fifth day and continues
actively until the end of the second week.
The developmental process appears to
slow down considerably by the end of the
third postnatal week and no appreciable
alteration in the cytologic appearance is
observed after this period.
Optic stalk. The tubular optic stalk is
formed at the isthmus of the neurovesicle
between the outpouching ocular tissue and
the central nervous system. The earliest
optic stalk consists of a single layer of
neuroepithelial cells. The invagination of
the optic cup extends posteriorly into the
lower portion of the stalk and becomes
Fig. 1. Early optic stalk of a 14-day-old fetus.
The inner layer of the optic cup is continuous to
the invaginated portion of the fissure. Epoxy
sections, toluidine blue. xl30.
Fig. 2. Disc area of a 14-day-old fetus. Many
necrotic cells are present in the inner layer of
the optic cup (arrow). Epoxy section, toluidine
blue. x260.