Development of the visual cortex in a wallaby--phylogenetic implications.

The visual cortex of one of the smallest macropod marsupials, a wallaby, the quokka Setonix brachyurus, was examined at maturity and during development from postnatal day 1 to 150 in Nissl-stained or Golgi-stained sections. Injections of horseradish peroxidase into the primary visual centres in adults identified cortical neurons projecting to these regions. The pattern of cell generation was determined by tritiated thymidine/autoradiography. The adult visual cortex was composed of the usual six layers of cells with dendritic morphologies similar to those seen in eutherian (placental) mammals. The margin of Layer 6 overlapped slightly with the white matter, and pyramidal cells projecting to subcortical regions were located at all depths of Layer 6. Cortical development was similar to that of eutherian mammals, with the formation of transient cellular layers below the developing cortical plate. Autoradiographic studies confirmed that the cortical plate was composed of two layers: a compact zone of densely packed cells located pial to a striated layer containing more loosely packed cells. Postnatal cell generation of Layers 6-2 took place from postnatal day 3 to day 85, with Layer 4 neurons, the main target of incoming visual axons, being generated around postnatal day 40. Cells located at maturity in the white matter, mostly of glial morphology, were generated from postnatal day 40. A subplate zone containing early differentiating cells was not evident, and postnatal 3H-thymidine labelling did not identify a population of early generated neurons below the cortical plate. In the tammar wallaby Macropus eugenii, the majority of target neurons for incoming geniculate axons are generated about 20 days after the first axons enter directly into the cortical plate without a waiting period in a subplate. Geniculate axons in the wallabies have a relatively longer 'wait' for their target neurons than do those in cats or monkeys. Therefore, if geniculate axons in marsupials make temporary synapses, while they wait for their target neurons to be generated, their temporary connections must, unlike those in cats or monkeys, be with cells already in the cortical plate.