Prenatal development of the visual system in rhesus monkey

Autoradiographic evidence from juvenile rhesus monkeys that had been exposed to a pulse of [1 *3H]thymidine at different embryonic (E) and early postnatal (P) days indicates that all neurons which compose the visual system of this primate have been generated two months before birth. The first retinal ganglion cells (RGG) are generated around E30 preceding by a few days the onset of genesis of neurones destined for the dorsal lateral geniculate body (LGd) and superior colliculus (SC) both of which begin at E36. Production of neurons destined for the primary visual cortex (area 17) begins at approximately E43 and ends by E l02. Neurons destined for layer IV, the major target of axons from the LGd, are generated between E70 and E85. The prenatal development of visual connections was studied by the autoradiographic method of anterograde axoplasmic transport in foetuses killed 14 days after unilateral eye injection of a mixture of [ 3 H]proline and [ 3 H]fucose. Initially, in the LGd and in the SC projections from both eyes overlap. Segregation of the axons and/or terminals from the two eyes occurs in the LGd and SC during the middle period of gestation. Transneuronal transport of tritium shows that although LGd axons form the optic radiation before E78, these fibres do not yet enter the developing cortical plate at this foetal age. During the second half of gestation, geniculocortical axons carrying input from each eye invade the cortex but are not yet segregated into ocular dominance columns. Rather, grains are distributed uniformly over the entire layer IV at E l24. Three weeks before birth, at E l44, segregation of afferents into sublayers IVA and I VC is apparent, and the first hint of ocular dominance columns is displayed by slight differences in grain counts in alternating areas of layer IV. These results show that all neurons in the primate visual system have been generated, reached their final positions and formed their basic connections subserving ocular dominance before birth, i.e. before visual experience. In the SC and LGD, monocular segregation is well established during the middle period of gestation, whereas in the cortex it has begun, but is not fully developed at birth.