Regional specialization in retinal ganglion cell projection to optic tectum of Dipsosaurus dorsalis (Iguanidae).

Abstract

Regional variation in the composition of the ganglion cell layer is a characteristic feature of vertebrate retinas. This suggests that corresponding patterns of regional variation might be expected in CNS structures which receive a direct, topographic projection from the retina. To investigate this possibility, I studied the retinotectal projection in an iguanid lizard (Dipsosaurus dorsalis), which shows a marked degree of regional variation in its retinal ganglion cell layer. Results of these experiments indicate that in D. dorsalis the optic tectum is differentiated horizontally into five regions which can be distinguished on the basis of cytoarchitecture and the patterning of retinal afferents, as visualized with silver degeneration and autoradiographic tracing techniques. These five areas are named according to their location: 1) Ventral Rim, 2) Lateral Wall, 3)Dorsolateral Area, 4) Dorsal Wall and 5) Medial Rim. Following enucleation, the Medial and Ventral Rim are characterized by fascicles of coarse degenerated fibers in layers 13, 12, and 9, by a relative absence of fine particulate debris in reduced silver material, and by the absence of silver grains in autoradiographs following intraocular injections of tritiated proline. They differ in the density of large calibre fibers and in the temporal course of degeneration. In the Lateral Wall the cellular layers are best differentiated, and nine retino-recipient layers can be distinguished below the stratum opticum. Degeneration is particularly heavy over the plexiform layers 9, 11, and 13. There is a superficial to deep gradient of degeneration density and of silver grains within layers 13 and 11. In the lower half of layer 11, argyrophilic debris and silver grains are aligned in vertical columns. The Dorsolateral Area is a circular region which covers the dorsolateral convexity of the tectum just caudal to mid tectal levels. It is characterized by a relatively low density of degenerated fibers in layers 13, 12, and especially in layer 9. Finally, the Dorsal Wall is similar to the Lateral Wall in the laminar distribution of degeneration and silver grains, except that there is an additional band of fine debris over layers 12 and 13 which is not visible in the Lateral Wall. Rostrally, the Dorsal Wall is greatly expanded (the Dorsal Expansion) and in this region a well developed plate of neurons lies immediately subjacent to the stratum opticum. This is the only tectal area which shows substantial degeneration at short survival times. These data suggest that in D. dorsalis there is horizontal variation in the morphology and patterning of retinal ganglion cell axons to the superficial tectal layers. Since Dipsosaurus retina exhibits a substantial degree of regional variation in the ganglion cell layer, and since the retina projects topographically onto the tectum, it is suggested that regional variation in the optic tectum may reflect regional variation in the ganglion cell layer of the retina.