The development of topography in the hamster geniculo-cortical projection.

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.