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 nuclei possessing multiple nucleoli and by a sparse neuropil. During the subsequent age periods studied progressive maturational changes lead to neurons having round nuclei with a single nucleolus and well-developed 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 increasing gestational age, the synapses increase in size, maturity, types and complexity; an acquisition of complex synaptic arrangement (triad) occurs by 20-21 weeks. Excitatory synapses appear earlier than do the inhibitory ones. Formation of retinogeniculate contacts precedes that of the corticogeniculate type. The synaptic density and total synapse 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, significant increase in synaptogenesis, presence of transient contacts on soma and large number of free postsynaptic membrane densities (PSD). The period thereby represents a critical period in the development of synapses in LGN. The numerical values obtained by by the age of 34-35 weeks are still low as compared to the values reported for other areas of brain.