Structural development of the lateral geniculate nucleus and visual cortex in monkey and man

This study concerns the development of the primary visual pathway of the primate. The lateral geniculate nucleus (LGN) is the principal thalamic relay to the visual cortex (area 17), and its neurons have similar morphological characteristics in both monkey and man, as identified by Golgi impregnation. The commonest neuron is the multipolar with a radiate or tufted dendritic tree; next is the bipolar neuron with two or three diametrically opposed dendritic trunks. Less frequent are neurons with beaded dendrites and others with fine, axon-like dendritic processes, possibly interneurons. The dendritic tree of all neurons remains generally within a lamina, but some dendrites cross interlaminar zones. LGN neurons are identifiable before birth and differ from their adult form by the presence of immature features, especially numerous dendritic and somatic spines, most frequent at birth in monkeys and at about 4 months postnatally in man. They disappear almost completely by 3 months in monkeys and 9 months in man. The human LGN has reached its ‘adult’ volume by this age.Two stages in the development of the human area 17 can be defined. The first is marked by a rapid growth to its ‘adult’ volume by about 4 months, and by intense synaptogenesis beginning in the foetus and reaching a maximum around 8 months. The second stage is one of stabilization in the volume of area 17 and loss of synapses to reach ‘adult’ synaptic density around 11 years, at about 60% of the maximum values.The formation of transitory morphological features in the first weeks or months of life coincides with a period of visual plasticity in infant monkeys and humans. Our observations can be correlated with experimental evidence for visual development in monkeys and with clinical evaluation of visual activity during the human preverbal stage, a period of great importance in the establishment of visual acuity, of stereopsis and of oculomotor function, all very sensitive to the numerous forms of visual deprivation.