Retinofugal pathways in two marsupials.

Autoradiographic and anterograde degeneration tracing methods were used to study and compare the organization of retinofugal pathways in two marsupial opossums, Didelphis virginiana and Marmosa mitis. Seven identical retinal targets were demonstrated for each opossum. These include: (1) the suprachiasmatic nucleus of the hypothalamus, (2) the dorsal and (3) ventral lateral geniculate nuclei, (4) the lateral posterior nucleus, (5) the pretectal complex, (6) the superior colliculus and (7) the accessory optic nuclei. While the pattern of retinal input to six of the seven targets was quite similar in the two species, the organization of the retinogeniculate pathways exhibited striking differences. In particular, our autoradiographs reveal no separation of ocular inputs within the lateral geniculate nucleus of Didelphis, i.e. the ipsilateral input is overlapped completely by the more extensive contralateral projection. In contrast, there is considerable separation, as well as overlap, of the occular inputs within the lateral geniculate nucleus of Marmosa. Our autoradiographs reveal several distinct bands of label within each geniculate nucleus, and upon superimposing the nuclei, ipsilateral and contralateral to the placement it is apparent that two of the bands overlap, while five do not (three ipsi, two contra).