Journal of Neuroscience, Vol 4, 1052-1063, Copyright © 1984 by Society for Neuroscience
An evaluation of the hypothesis of shifting terminals in goldfish optic tectum
SS Easter Jr and CA Stuermer
It has been proposed that the terminal arbors of retinal ganglion cells
continually shift positions in the optic tectum as a natural consequence of
growth in larval amphibians and teleost fish. Here, we develop a detailed
anatomical model of this process, based on recent observations in goldfish.
The model predicts some unusual structural features of optic axons in the
tectum. It was tested in two ways by use of the tracer, horseradish
peroxidase (HRP). In the first, optic axons in the tectum were visualized
by anterogradely transported HRP applied in the nerve or retina. As
predicted, the axons had three parts: (1) fascicular segment (in the
stratum opticum), (2) extrafascicular segment (in the synaptic neuropil),
and (3) terminal arbor (also in the neuropil). The extrafascicular segment
extended up to 1 mm caudally from the fascicle through which it entered the
tectum. At its terminus, the arbor was restricted to a single lamina in
which it branched extensively over an elliptical area with major axes
between 100 and 200 micron long. In the second test, retinal ganglion cell
bodies were labeled by retrogradely transported HRP applied to the tectum
in a manner intended to label all axons and terminals in the vicinity. The
pattern and locations of labeled cells varied, depending on where the HRP
was applied, in detailed accord with the model's predictions. The close
agreement between the model and the observations supports the hypothesis of
shifting terminals.
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