The role of retinogeniculate afferents in the development of connections between visual cortex and the dorsal lateral geniculate nucleus

The role of retinogeniculate afferents in the development of patterns of connections between visual cortex and the lateral geniculate nucleus (LGN) was addressed by studying the effect of bilateral enucleation at birth on those patterns of connections in tree shrew. In normal adult tree shrews there are six LGN cell layers separated by cell-sparse interlaminar spaces. The reciprocal connections between the LGN and visual cortex are restricted to a column running across all six LGN layers; the geniculocortical projection arises from the cell layers while the corticogeniculate projection terminates primarily in the interlaminar spaces. At birth, when the experimental animals were bilaterally enucleated, the retinogeniculate fibers have begun to segregate by eye but neither the cytological characteristics of individual layers nor the interlaminar spaces have yet formed, and the corticogeniculate fibers have not entered the nucleus. Bilateral enucleation does not prevent the development of the cytological characteristics of individual layers but the interlaminar spaces do not develop. The results of [3H]proline/HRP injections into visual cortex in animals bilaterally enucleated at birth indicate that in the absence of retinogeniculate fibers, and thus interlaminar spaces, the corticogeniculate fibers do not concentrate at the laminar borders but instead spread across all six LGN cell layers. Despite the failure of this projection to concentrate at laminar borders, the corticogeniculate fibers do terminate within a restricted projection column.