An autoradiographic analysis of neurogenesis in the chick retina in vitro and in vivo.

Patterns of [3H]thymidine incorporation during neurogenesis of the embryonic chick retina have been compared in vitro and in ovo. Pieces of posterior, undifferentiated retinas were dissected from embryos on day 6 of incubation (E6) and cultured in the presence of [3H]thymidine. Label was added to the medium for 3 h on day 1, 2, 3 or 4 in culture. The retinas were fixed on the fifth day, embedded in epon, sectioned and processed for autoradiography. In parallel experiments, in ovo injections were made on embryonic day 6, 7, 8 or 9 (E6-E9). On E12 the embryos were fixed and a piece of the posterior retina from each eye was dissected and processed for autoradiography as above. Results show that the retinal explants develop well in culture and all of the layers of the neural retina differentiate. However, the cultured retinas are thinner than those grown in ovo. [3H]Thymidine labeling indicates that nearly all retinal neurons undergo their final mitotic divisions between E6 and E9. In addition the patterns of labeling in culture are similar to those in ovo. Most neurons, including the majority of cells in the ganglion cell layer and outer nuclear layer, are labeled on the first three days in culture and in E6-E7 embryos, while labeled cells are restricted to the inner nuclear layer in older specimens. Counts of labeled and unlabeled neurons in the ganglion cell layer suggest that the temporal pattern of neurogenesis in culture lags behind that in the embryo by about one day but that the spatial patterns of cell migration are the same.