The number, dendritic morphology, and
retinal distribution of displaced
ganglion cells were studied in two anuran species, Xenopus laevis and Bufo marinus.
Horseradish peroxidase or
cobaltic lysine complex was applied to the cut end of the optic nerve, and the size, shape, and
retinal position of retrogradely filled
ganglion cells displaced into the inner nuclear layer were determined in
retinal wholemount and sectioned material. Approximately 1% of
ganglion cells in Xenopus and 0.1% in Bufo were found to be displaced. In both species, many of the previously described orthotopic
ganglion cell types (Straznicky & Straznicky, 1988; Straznicky et al., 1990) were present among displaced
ganglion cells. In Xenopus more displaced
ganglion cells were found in the
retinal periphery than in the
retinal center, and they formed 3 or 4 distinct bands around the optic nerve head. In Bufo the incidence of displaced
ganglion cells was higher along the visual streak than in the dorsal and ventral peripheral retina. These results indicate that the distribution of displaced
ganglion cells approximates the
retinal distribution of orthotopic
ganglion cells. One of the likely mechanisms to account for this developmental paradox may be that the formation of the inner plexiform layer, adjacent to the ciliary margin, acts as a mechanical barrier by preventing the entry of some of the late developing
ganglion cells into the
ganglion cell layer.