BIRTH-DATES OF NEURONS IN THE RETINAL GANGLION-CELL LAYER OF THE FERRET

The present study determined the temporal and spatial patterns of gene sis for neurons of different sizes in the retinal ganglion cell layer of the ferret. Fetal ferrets were exposed to tritiated thymidine on em bryonic trays E-22 through E-36. One to 3 months after birth, they wer e perfused and their retinae dissected, and autoradiographs were prepa red from resin-embedded sections throughout the entire flattened retin al ganglion cell layer. Soma size differences in conjunction with sepa rate retrograde labeling and calbindin immunocytochemical studies were used as criteria for identifying different retinal ganglion cell subt ypes in juvenile and adult ferrets. Neurons of different sizes in the ganglion cell layer were generated at different stages during developm ent. Medium sized cells were generated primarily between E-22 and E-26 ; the largest cells were generated between E-24 and E-29; small cells were generated between E-26 and E-32; and very small cells were genera ted between E-29 and E-36. The former three groups were interpreted to be three subtypes of retinal ganglion cells, while the latter group w as interpreted to be displaced amacrine cells. This temporal order of the genesis of ganglion cell classes is consistent with the spatial or dering of their fibers in the mature optic chiasm and tract, and it is consistent with the developmental change in decussation pattern recen tly shown in the optic pathway of embryonic ferrets. The spatial patte rn of genesis suggests that ganglion cells of a particular class are a dded to the ganglion cell layer in a centroperipheral fashion initiate d in the dorsocentral retina nasal to the area centralis. No evidence was found for a wave of ganglion cell addition that proceeded in a spi ralling pattern around the area centralis, as has been reported in the cat. (C) 1994 Wiley-Liss, Inc.