Journal of Neuroscience, Vol 5, 2570-2580, Copyright © 1985 by Society for Neuroscience
Evidence for shifting connections during development of the chick retinotectal projection
SC McLoon
The pattern in which optic axons invade the tectum and begin synaptogenesis
was studied in the chick. The anterogradely transported marker, horseradish
peroxidase, was injected into one eye of embryos between 5 and 16 days of
development (E5 to E16). This labeled the optic axons in the brain. The
first retinal axons arrived in the most superficial lamina of the tectum on
E6. They entered the tectum at the rostroventral margin. During the next 6
days of development the axons grew over the tectal surface. First they
filled the rostral tectum, the oldest portion of the tectum, and then they
spread to the caudal pole. Shortly after the first axons entered the tectum
on E6, labeled retinal axons were found penetrating from the surface into
deeper tectal layers. In any given area of the tectum, optic axons were
seen penetrating deeper layers shortly after arriving in that area.
Electron microscopic examination showed that at least some of the labeled
axons in rostral tectum formed synapses with tectal cells by E7. These
results show two things which contrast with results from previous studies.
First, there is no delay between the time the retinal axons enter the
tectum and the time they penetrate into synaptic layers of the tectum.
Second, the first retinotectal connections are formed in rostral tectum and
not central tectum. Retrograde tracing showed the first optic axons that
arrived in the tectum were from ganglion cells in central retina. Previous
studies have shown that the ganglion cells of central retina project to the
central tectum in the mature chick. This opens the possibility that the
optic axons from central retina, which connect to rostral tectum in the
young embryo, shift their connections to central tectum during subsequent
development. As they enter the tectum the growth cones of retinal axons
appear to be associated with the external limiting membrane. During the
time that connections would begin to shift in the tectum a second
population of axons appears at the bottom of stratum opticum, some with
characteristics of growth cones. This late-appearing population may
represent axons shifting their connections. These results have implications
for theories on how the retinotopic pattern of retinotectal connections
develops.
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