The Journal of Neuroscience, August 1, 1998, 18(15):5766-5776
The Development of Topography in the Hamster
Geniculo-Cortical Projection
Kristine
Krug,
Adam L.
Smith, and
Ian D.
Thompson
University Laboratory of Physiology, Oxford University, Oxford OX1
3PT, United Kingdom
Precise point-to-point connectivity is the basis of ordered maps of
the visual field. The immaturity of the newborn hamster's visual
system has allowed us to examine emerging topography in the
geniculo-cortical projection well before thalamic axons have reached
their cortical target, layer IV. Using anterograde transneuronal labeling with wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP), we visualized the ingrowth of the whole population of geniculate fibers in the neonatal hamster. Two days after
birth (P2), the bulk of the fibers is in the deep cortical layers and
the subplate. At the same age, injections of paired retrograde tracers
(red and green fluorescent latex microspheres) into area 17 reveal an
unordered projection from the dorsal lateral geniculate nucleus (dLGN)
to cortex. Individual labeled cells are found throughout the dLGN, and
quantitative analysis reveals no segregation of the red and the green
populations. At P6, when the pattern of geniculate back label appears
ordered and essentially adult-like, geniculate fibers have reached
layer IV. The role of selective cell death in this process was
investigated by making a tracer injection at P2 and allowing the
animals to survive to P6 or P12, when the map is mature. The results
show early labeled neurons that made inappropriate connections when the
projection was scattered surviving through the period of geniculate
cell death. We conclude that the geniculo-cortical map develops from an
initially unordered projection to the subplate and the lower cortical
layers. Selective cell death appears not to contribute significantly to
this process.
Key words:
topography; cortical maps; rodent; geniculocortical; map formation; cell death; retrograde tracing; hamster; terminal
retraction
Copyright © 1998 Society for Neuroscience 0270-6474/98/18155766-11$05.00/0
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