The genesis of efferent connections from the visual cortex of the fetal rhesus monkey.

The prenatal development of the cortical projections to the dorsal lateral geniculate nucleus (LGN), superior colliculus (SC) and pulvinar was studied by autoradiography of orthogradely transported 3H-proline injected into the occipital cortex of fetal rhesus monkeys aged from embryonic day 63 (E63) to E95. Differentiation of pyramidal neurons situated in the infragranular strata of the cortical plate (prospective layers 5 and 6, which give rise to these efferent projections) was also examined in Golgi preparations prepared from specimens of corresponding embryonic ages. In autoradiographs of the E63 fetus, no radioactive label was seen in subcortical structures. In two specimens injected around E70, label was present in the prospective magnocellular layers of the LGN and within the immediately surrounding cell-poor zones. At these young fetal ages, the presence of topographic order in the corticogeniculate projection could not be determined due to the large size of the injection sites relative to the small cerebral vesicles. By E84 the portion of the prospective parvocellular layers adjacent to the white matter also contained label which was characteristically wedge-shaped and appropriately located with respect to the site of the cortical injection, suggesting that topographic order is established. In the oldest fetus (E95) label in the LGN assumed a configuration similar to that seen in the adult. The cortical projection also invades the SC and pulvinar around E70. In the SC, label was initially confined to the stratum opticum, but by E84 it extended into the superficial gray. Thus, all known classes of efferent pathways from the visual cortex to subcortical structures are present by the middle of the 165-day gestational period in rhesus monkey. The one-month period, E63-E97, during which these efferent visual connections are established is characterized by the considerable growth and increased complexity of the dendritic arborization of pyramidal cells in the infragranular cortical layers of area 17. Thus the development of visual cortical efferents occurs in rough synchrony with the genesis of the afferent pathway from the LGN (Rakic, '76a; '79) and with the onset of morphological differentiation of pyramidal neurons in the infragranular cortical layers.