Control of cell number in the developing neocortex. I. Effects of early tectal ablation.

Target availability is an important factor in the early control of neuron number in many structures in the developing vertebrate nervous system. In early neocortical development, the role of target availability in the survival of subcortically projecting neurons is not yet understood, particularly because these cells' axons are widely distributed and highly branched. In this study, we have looked for alterations in the pattern of early cell death, adult cell density and adult morphology of pyramidal cells in layer V of visual cortex consequent to removal of one of their principal targets, the ipsilateral superior colliculus. After neonatal tectal ablation, there was no difference in the incidence of pyknotic cells in the cortex overall, or in layer V during the period of normal cell death in the cortex. Neither in adulthood, nor at any point in development did the density of layer V cells or cortical cell density overall differ from normal in Nissl material. Soma size of cells in layer V overall did not differ from normal in Nissl material. In addition, the soma size of the subpopulation of cells labelled with horseradish peroxidase (HRP) from midbrain injections was unaltered. In summary, this cell population appears unresponsive in both number and morphology to deletions of a major component of its target pool. This observation has some interesting implications for reasons of constancy of cell number in layer V across cytoarchitectonic areas.