Development of visual system - comparison of monkey and man.

In Golgi preparations of the lateral geniculate nucleus of adult humans and monkeys several types of neuron are described. Multipolar neurons have a "radiate" or "tufted" dendritic arbor. The next commonest class is the bipolar neuron with two or three thick dendrites arising from opposite poles of the soma. A few examples of rare medium-sized neurons with beaded dendrites are found. There are also small neurons with fine "axon-like" dendritic processes. Some have long, untapered dendrites and others shorter dendritic arbors. A class of large, capsular neurons is found in the circumgeniculate capsule. Maturation of lateral geniculate nucleus neurons in baby humans and monkeys was studied in Golgi preparations. They pass through post-natal stages characterised by dendritic growth cones and a profusion of spine-like protuberances on dendrites and somata. The mature form is found by the second month in monkey and about one year in man. The morphological changes in the thalamus are paralleled by synaptogenesis in the visual cortex. In man synaptic density reaches a maximum at about one year of age, declines later in childhood and stabilises at adult levels by about 11 years of age. In both monkey and man the period of morphological maturation in the visual pathways corresponds to a time of increasing visual acuity when visual deprivation is most likely to have permanent harmful effects.