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Developmental changes in the relationship between type 2 synapses and spiny neurons in the monkey visual cortex

This study continues an exploration of synaptic development in the primary visual cortex of the monkey (Macaca nemestrina). In a prior study (Mates and Lund, '83a), we observed that type 2 synapses on the cell bodies of spiny stellate neurons of lamina 4C appeared not only to increase in number during early postnatal development but also subsequently decreased during maturation. Using quantitative, stereological electron microscopic methods, we examined the maturation of this synapse population from embryonic day 159 to adult, on spiny stellate neurons of 4Cα and β and, for comparison, on pyramidal neurons in upper and lower lamina 6. Tissue was also taken for comparison from two animals reared to 8 weeks of age with binocular eyelid closure from birth. We confirmed that a marked increase and subsequent decrease occurred in this somal type 2 synapse population on both neuron populations. However, due to the infrequency of the smooth dendritic neurons (approximately 5% of the neuron population) giving rise to the type 2 contacts, and due to expansion of the neuropil during maturation increasing intercell distances against constant volume of the type 2 axon arbors, it is concluded that the decrease in, type 2 somal synapses may represent a redistribution to dendrites rather than loss from the neuropil. Cells of lamina 4Cβ (receiving input from the parvocellular lateral geniculate nucleus‐LGN) show a slower initial accumulation of type 2 contacts compared to neurons of lamina 4Cα (receiving input from magnocellular LGN), or to pyramidal neurons of lamina 6. The type 2 synapse population of the 4Cβ neurons also showed no reduction in the animals visually deprived to 8 weeks of age, unlike the populations of type 2 synapses in 4Cα and 6, which were fewer than normal.