Embryonic chick retinal ganglion cells identified "in vitro". Their survival is dependent on a factor from the optic tectum.

When HRP is injected into the optic tecta of embryonic or newly hatched chicks, the ganglion cells in the contralateral retina can be successfully dissociated into culture and identified at any time by appropriate histochemical staining. Histological examination of whole mounts of retinae both ipsilateral and contralateral to an injection site indicated that no HRP diffused out of an injected tectum, and that the only reaction product that could be visualized was restricted to the ganglion cell layer of the contralateral eye. Because retinal ganglion cells are the only retinal neurons to project to the optic tectum, the intraxonal retrograde transport of HRP to these cells allows their unequivocal identification from amongst the heterogeneous population of retinal neurons present after dispersal into single cells in monolayer culture. The presence of HRP in the cell bodies did not appear to impair their ability to survive, grow or express neurites. Counts of labeled cells from progressively aged birds confirmed that the peak number of generated ganglion cells occurs on embryonic day 10,and that is a 40% decline in the number these neurons over the following 3 days. However, when labelled ganglion cells from 10 day embryos were grown in culture with optic tectum, all the ganglion cells survived over the following 4 days, including those destined to die in vivo. This trophic effect cannot be induced by cerebellum, but is partly induced by media first conditioned over tectal cells. The trophic effect exerted by optic tectum appears therefore to be specific and chemically mediated. We suggest that the death of retinal ganglion cells in vivo may be a consequence of the inability of some cells to establish adequate supplies of a growth factor from the optic tectum.