DEVELOPMENT OF HUMAN LATERAL GENICULATE-NUCLEUS - AN ELECTRON-MICROSCOPIC STUDY

A qualitative and quantitative ultrastructural study has been carried out on lateral geniculate nuclei (LGN) of 21 human fetuses ranging in gestational age from 13-14 to 34-35 weeks. At the early age period of 13-15 weeks, LGN is characterized by immature cells with indented nucl ei possessing multiple nucleoli and by a sparse neuropil. During the s ubsequent age periods studied progressive maturational changes lead to neurons having round nuclei with a single nucleolus and well-develope d cytoplasmic organelles as well as to an elaborate neuropil. Synaptic contacts which are seen for the first time at the age of 13-14 weeks are of retinogeniculate type. They show features of immature synapses and are located mainly on the juxtasomatic parts of dendrites. With in creasing gestational age, the synapses increase in size, maturity, typ es and complexity; an acquisition of complex synaptic arrangement (tri ad) occurs by 20-21 weeks. Excitatory synapses appear earlier than do the inhibitory ones. Formation of retinogeniculate contacts precedes t hat of the corticogeniculate type. The synaptic density and total syna pse number show a progressive increase with increasing gestational age . The age period of 15-20 weeks of gestation is marked by presence of organelles suggestive of a high rate of metabolic activity, significan t increase in synaptogenesis, presence of transient contacts on soma a nd large number of free postsynaptic membrane densities (PSD). The per iod thereby represents a critical period in the development of synapse s in LGN. The numerical values obtained by the age of 34-35 weeks are still low as compared to the values reported for other areas of brain.