Cytodifferentiation and developing neuronal circuitry in the human lateral geniculate nucleus

Abstract

The developing neural substrate in dorsal lateral geniculate nucleus of human fetuses and premature, full-term and postnatal infants, has been analysed using rapid Golgi impregnation, computerized image analysis, electronmicroscopy, and immunocytochemical method for localization of γ-aminobutyric acid. Nuclear and cytoplasmic neuronal maturation is observed to extend over the entire period studied. Axodendritic synapses, presumably of retinal origin, are occasionally seen at 13-14 fetal weeks. They become increasingly apparent at 18-19 weeks. Dendrodendritic contacts are visualized at 15 weeks. Cortical terminals and occasional triadic contacts are evident around 21 weeks. The inhibitory interneurons containing γ-aminobutyric acid are present in small numbers at 15-16 weeks; their numerical density increases considerably at 17 weeks but decreases thereafter. The presence of γ-aminobutyric acid containing nerve cells and synaptic triads is indicative of the formation of inhibitory circuitry. At 15-16 weeks neurons are mostly bipolar although different forms of multipolar cells may be seen. By 24 weeks the radiated and bitufted multipolar neurons, neurons with beaded dendrites and neurons with axon-like dendritic processes are identifiable. There are no apparent differences in differentiation of neurons between the cranial and caudal parts of dorsal lateral geniculate nucleus. At 15-16 weeks, however, the cells of prospective magnocellular zone appear to be more mature than do the cells in the parvocellular zone. The neuronal soma increases continuously in size. Dendrite development starts at 15-16 weeks of gestation, thereafter the increase in number of their branches and their length is observed. Between 15-16 and 24 weeks, spines and filiform processes appear first on the proximal shafts of the dendrites and subsequently on their distal portion. There is increase in the number of filiform processes and hair-like appendages on geniculate neurons of premature infants born at 32 and 37 weeks of gestation and of a 4-day postnatal infant. Computerized quantitative data substantiate the progressive increase in growth parameters. The significance of comparative and functional aspect of the data is discussed.