The pattern of neurogenesis in the retina of the rat

Summary

The morphological features of the retina of neonatal rats have been analyzed with the rapid Golgi method in an attempt to provide some embryological observations crucial to the study of neuronal specificity in the visual connections. The retinal neurons and photoreceptors form from primitive epithelial cells that assume the characteristics of neuroblasts. Initially they extend from the internal to the external limiting layers. There is no evidence of free cellular migration or of cells resembling the neuroblast of His. While their perikarya are situated deep within the matrix zone, the first signs of differentiation appear at the external and internal limiting membranes, where the receptor inner segments and ganglion cell axons begin to form. Subsequently the perikarya move through the primitive epithelial processes to the prospective outer nuclear or ganglion cell layers. In the receptor cells, this is accompanied by the differentiation of the rods and cones and of the receptor fibers. In the ganglion cells, the perikaryal translocation is followed by the differentiation of the dendrites and the internal plexiform layer. The amacrine and bipolar cells follow a similar sequence of changes. The receptor outer segments form in conjunction with the processes of pigmented epithelial cells; the differentiation of the ganglion cell dendrites occurs in association with the formation of the amacrine and inner bipolar processes. The amacrine and ganglion cells begin to differentiate first, followed closely by the receptor cells and the bipolar cells. Müller's cells and astrocytes differentiate last. Horizontal cells were not studied. There is a gradient of differentiation, such that the axons and dendrites of the ganglion cells near the optic nerve head differentiate earlier than those located more peripherally. The implications of the findings for the analysis of the mechanisms controlling growth, differentiation, and neuronal specificity in the visual system are discussed.