Naturally occurring neuron death in the ganglion cell layer of the neonatal rat: morphology and evidence for regional correspondence with neuron death in superior colliculus

Abstract

Light and electron microscopic examination of the inner layers of the normal infant rat retina reveal naturally degenerating cell bodies in the ganglion cell layer, degenerating axons in the optic fiber layer, and degenerating processes in the inner plexiform layer. The degenerating ganglion cell bodies are characterized by condensed nuclear chromatin, an abundance of free monosomes, and few cisternae of rough endoplasmic reticulum. The cells correspond to one type of degenerating neuron described in normal bird embryos but are surprisingly quite different from the morphology of the majority of naturally degenerating neurons in the infant rat superior colliculus. The reason for this difference appears related to the relative immaturity of the ganglion cells when they die. By inspection of the present material and from previous results 14 it is clear that the timing of cell death in the ganglion cell layer and colliculus overlaps considerably. Furthermore, counts of degenerating profiles in 6-and 7-day-old rats (the days when cell death appears maximal in both structures) show that caudal superior colliculus and nasal ganglion cell layer, which are normally connected, have significantly more degenerating profiles than rostral colliculus and temporal ganglion cell layer, which are also connected. It may be that these corresponding patterns of cell death reflect a reciprocal control of neuron survival in the two structures. In the course of our study of degenerating cells we also find evidence for two populations of normal neurons in the developing ganglion cell layer. One is a large pale cell which is relatively rich in intracytoplasmic organelles and the other is a small, more densely staining cell which has relatively sparce organelles and coarse nuclear chromatin (at least at the earlier postnatal ages). The possibility exists that this second cell type represents an interneuron located in the ganglion cell layer.