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Journal of Neuroscience, Vol 4, 1130-1152, Copyright © 1984 by Society for Neuroscience
Does timing of axon outgrowth influence initial retinotectal topography in Xenopus?
CE Holt
The question of whether timing is involved in generating the topographic
organization of the earliest embryonic projection from the retina to the
tectum has been examined in Xenopus laevis. First, the normal schedule of
axonal outgrowth from the retina to the tectum was characterized. Groups of
axons originating from either dorsal or ventral extremes of the retina were
labeled by in vitro incubation of sectors (one-quarter to one-third) of eye
primordia in [3H]proline and their time courses of outgrowth were
determined using light microscope autoradiography. Comparisons of the
growth of dorsal and ventral nerve fiber populations showed that those from
the dorsal retina leave the eye first, grow along the optic pathway, and
reach the tectum roughly 6 hr ahead of those from the ventral retina. This
stereotyped sequence of outgrowth schedules the development of the initial
retinotectal map: first the ventrolateral tectum receives input from the
dorsal retina (stage 37/38), and then the dorsomedial part receives input
from ventral retina (stage 40). Second, to test whether the accurate timing
of axon outgrowth and target invasion defines the spatial ordering of the
earliest connections, the normal schedule of retinal fiber outgrowth was
altered by substituting dorsal halves of young stage 21 eye primordia,
labeled with [3H]proline, for those in older stage 27 embryos. These
heterochronic transplants resulted in retarded outgrowth from the dorsal
retina such that the original pioneer fibers reached the tectum at least 9
hr later than normal, arriving after ventral retinal fibers, thereby
reversing the normal sequence of tectal invasion. Despite this, the initial
pattern of tectal innervation remained spatially normal. It is concluded
that the retinotectal map is generated not by the temporal sequence of
retinal axon outgrowth but, rather, by selective means of neuronal
recognition.
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