Genesis of neurons in the retinal ganglion cell layer of the monkey

We have analyzed the genesis of various neuronal classes and subclasses in the ganglion cell layer of the primate retina. Neurons were classified according to their size and the time of their origin was determined by pulse labeling with 3H‐thymidine administered to female monkeys 38 to 70 days pregnant. All offspring were sacrificed postnatally, and their retinas processed for auto radiography. The somata of cells in the retinal ganglion cell layer generated on embryonic day (E) 38 ranged from 9 to 14 μm in diameter. Between E40 and E56, the minimum soma diameter remained around 8–9 μm, while the maximum gradually increased to 22 μm. As a consequence, the means of the distributions of labeled cells also increased with age, from 11.8 μm diameter for cells generated on ESS to 14.6 μm diameter at E56. Over this period the percentage of labeled cells in the 10.5–16.5 μm and > 16.5 μm diameter range gradually increased. The proportion of the labeled cells in the < 10.5 μm diameter range decreased from ESS to E45, but subsequently increased rapidly. At the end of neurogenesis in the retinal ganglion cell layer, around E70, most labeled cells were considerably smaller (7–9 μm) than those generated earlier. Our results indicate that within the ganglion cell layer of the macaque, neurons of small caliber are generated first, followed successively by medium sized cells. Large, putative Pα cells are generated late. The production between E56 and E70 of cells with the smallest somata suggests that the last‐generated neurons in the ganglion cell layer are predominantly displaced amacrine cells. Within the same sector of retina, different classes of neurons in the ganglion cell layer of the rhesus monkey appear to have a sequential schedule of production. © 1992 Wiley‐Liss, Inc.