Synaptogenesis in the occipital cortex of macaque monkey devoid of retinal input from early embryonic stages

Abstract

The role of input from the retina on the development of synaptic organization in the primate striate cortex was examined in macaque monkeys enucleated at embryonic (E) day 67 and E59. Both the prenatally operated animals and their age-matched controls were delivered at term (E165) and killed either at 3 months (at the end of the rapid phase of synaptogenesis) or 3 years (at the end of the plateau phase of synaptogenesis). As expected, in the operated animals the striate cortex had a smaller surface area but a normal thickness and complement of layers. The present study revealed that the mean densities of synaptic contacts per unit area and volume of neuropil in the striate cortex of the two operated animals were similar to those of age-matched controls (approximately 30/100 microm2 or 100/100 microm3 of neuropil). Thus, the absence of retinal input via the lateral geniculate nucleus did not affect the schedule and magnitude of synaptogenesis. Likewise, the ratio of symmetrical versus asymmetrical synapses and mean lengths of synaptic junctions were within the normal range of variation in both group of animals. The proportions of synaptic contacts situated on dendritic spines and shafts were also similar in supra- and infragranular cortical layers of normal and enucleated animals. However, the ratio of synapses situated on dendritic spines and shafts in the sublayers IVAB and IVC, which normally become reversed during late adolescence, were not reversed in the enucleates. Therefore, our study indicated that certain parameters of synaptic development, such as the density of contacts per unit volume of neuropil and the proportion of basic types and their size, in the supra- and infragranular layers of the striate cortex develop to an optimal normal level in the absence of both retinae from early embryonic stages. However, in the thalamorecipient sublayers the details of the synaptic circuits such as their localization on dendritic spines or shafts, fail to mature properly in the absence of normal functional input from the periphery.