Postnatal development of retinogeniculate, retinopretectal and retinotectal projections in the opossum

The postnatal development of retinogeniculate, retinopretectal and retinotectal projections has been studied by the anterograde transport of proline-labeled proteins in 20 pouch young opossums aged from 10 to 60 days. Radioautographical findings suggest delayed development of uncrossed as compared to crossed projections. There is a phase of overlapping projections from both eyes in thalamic and tectal target sites. Partial segregation of projections to the dorsal lateral geniculate nucleus (GLD) is preceded by differential distribution of crossed and uncrossed terminal fields along its dorsoventral axis (at age 23 days). The quasilaminar pattern of projections in the dorsocaudal region of GLD pars alpha is incipient by 42 days and is fully established at 60 days of age, as eye opening starts. The mature pattern of projections to the ventral lateral geniculate nucleus (GLV) is established much earlier, at 23 days of age. The development of retinopretectal projections is assessed mostly from the analysis of the olivary pretectal nucleus (PO). Distribution of silver grains into discrete areas coextensive with PO is relatively delayed (by 23 days of age) as compared to the nuclei of the lateral geniculate body. Soon after, however, the mature pattern of projections to PO is established (at 33 days of age). The early development of retinotectal projections (from 10 to 23 days) is compatible with an initial tangential course of crossed optic fibers in the superior colliculus (CS) but other alternatives remain open, such as a sequential outside-in arrangement of terminal fields of deeply coursing fibers. Arborization of uncrossed fibers is delayed at extreme rostromedial and caudolateral portion of the territory of the main uncrossed retinotectal projection. Segregation of uncrossed projections at different depths of CS is nearly complete by 42 days. Differences in the development of terminal fields in different target nuclei or in regions of a given target site are discussed in relation to retinal and local factors.