Developing neuronal populations of the cat retinal ganglion cell layer.

An improved flat-mount procedure demonstrates that the developing ganglion cell layer of the cat retina contains two morphologically distinct populations of presumed neurons at all ages between embryonic day 36 (E36) and adulthood. One population resembles the adult "classical neurons" composing the ganglion cells and bar-cells of Hughes, while the remaining cells, which are smaller and possess much less Nissl substance, presumably correspond to precursors of the adult microneurons. Although the total neuron population of the retinal ganglion cell layer remains quite constant at all studied ages, its component subpopulations alter significantly during prenatal development; some 50% of classical neurons disappear before birth and the microneuron population doubles during the same period. An obvious centroperipheral gradient exists for classical neurons by stage E47, but the microneuron density gradient only becomes apparent at birth. A 2:1 centroperipheral ratio for the total neuron population is also apparent at E47. Centroperipheral neuronal density gradients continue to increase during postnatal growth. Loss of classical neurons during prenatal life as a result of cell death or transformation into microneurons, has been postulated as a mechanism for determining neuron density gradients. Cell death does occur in the ganglion cell population but it is not yet established whether microneurons of the ganglion cell layer originate from ganglion cell transformation or migrate as a differentiated class from the ventricular layer. However, it can be concluded that not all microneurons originate from ganglion cell transformation, because the total loss of classical neurons is less than the increase in microneuron numbers during development. The population magnitudes of both neuronal classes in the ganglion cell layer stabilise after birth. However, it is during the postnatal period that the adult cruciate density topography is achieved by both populations. It is concluded that differential areal growth is the prime mechanism for postnatal cell redistribution.