Retinofugal projections in the short-tailed opossum (Monodelphis domestica)

In the current investigation, retinofugal projections to midbrain and thalamic nuclei of Monodelphis domestica were investigated using wheat-germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Large intraocular injections of WGA-HRP were placed into the eye, and patterns of labeled axon terminals were related to nuclear boundaries in tissue that was stained for Nissl or reacted for cytochrome oxidase (CO). Our results demonstrate that the major projection from the retina is to the contralateral dorsal lateral geniculate nucleus (LGNd) and the superior colliculus (SC). Connections were also observed with the contralateral pretectal nucleus (PRT), the lateral posterior nucleus (LP), and the ventral division of the lateral geniculate nucleus (LGNv). Ipsilateral connections were with the LGNv and LGNd. These findings are consistent with reports in other marsupials as well as with studies in a number of eutherian mammals. Thus, there appears to be a common pattern of retinofugal projections that all mammals share, probably due to retention from a common ancestor. However, some features such as a lack of ipsilateral input to the SC (which are absent only in certain species like Monodelphis, platypus, and echidnas) may represent a primitive state retained from a common ancestor. When comparisons of retinofugal connections and LGNd organization are made across taxa, three types of organization are observed: a homogenous LGNd with a high degree of binocular overlap of projections; a partially differentiated LGNd with some segregation of eye-specific inputs; and a fully segregated structure with a large degree of segregation of eye-specific inputs. We discuss the factors that contribute to the organization observed in extant mammals and conclude that phylogeny and lifestyle appear to be the underlying factors contributing to the organization of the LGNd.