GANGLION-CELL NEUROGENESIS, MIGRATION AND EARLY DIFFERENTIATION IN THE CHICK RETINA

Neurogenesis, migration and maturation of ganglion cells in the posterior pole of chick retina have been studied using embryonic incorporation of[H-3]thymidine, immunocytochemistry and retrograde labeling. Unlike previous studies, we have examined the neurogenesis of independently identified ganglion cells that have survived the period of naturally occurring cell death (embryonic days 11-16). Embryos were labeled with [H-3]thymidine at different embryonic ages (embryonic days 3, 5 and 7). After the chicks hatched, ganglion cells were retrogradely labeled with rhodamine microspheres and the retinas were processed for autoradiography and fluorescent microscopy. The results indicate that 40% of the ganglion cells in the posterior pole undergo a final mitosis by embryonic day 3 and that more than 25% of the ganglion cells are born on or after embryonic day 7. These results also suggest that naturally occurring cell death does not preferentially affect ganglion cells born on specific embryonic days. Using immunocytochemistry with an antibody against neuron-specific beta-tubulin and retrograde labeling with the carbocyanine dye DiI we show that ganglion cells begin to differentiate before the completion of their migration to the presumptive ganglion cell layer. These results suggest the following developmental sequence. (1) Ganglion cells of the posterior pole undergo their final mitosis near the ventricular margin between embryonic days 2 and 8. (2) They maintain contacts with both retinal surfaces and their nuclei move toward the ganglion cell layer. At this time they start to differentiate, expressing a form of neuron-specific tubulin and growing axons that can reach the optic chiasm. (3) Once migration is completed dendritic development commences.