Neuronal composition and development in lamina 4C of monkey striate cortex.

Lamina 4C (Lund, '73) of the monkey, Macaca nemestrina, visual striate cortex occupies a key position as a principal recipient zone of axons from the lateral geniculate nucleus (LGN). Synaptic maturation in lamina 4C is of particular interest since it involves a competitive interaction between thalamic axons for postsynaptic territory: an interaction which is strongly influenced by afferent activity (Hubel et al., '77). As an initial step toward understanding the normal process of synapse maturation in 4C, this study examines Golgi material to define the adult neuron populations of subdivisions 4C alpha (receiving afferents from magnocellular LGN) and 4C beta (receiving afferents from parvocellular LGN). Three groups of spine-bearing neurons are described--one relatively confined to either alpha or beta subdivision, the other two bridging the depth of 4C; four groups of smooth dendritic neurons interact with the spine-bearing population. Electron microscopy of normal and Golgi-impregnated tissue is used to define key features of synapse populations on these neurons. From embryonic day 159 through adulthood the smooth and spiny neurons occur in the same constant proportions in the neuropil (5% smooth, 95% spiny). Changes in the distribution of synapses on the spiny neurons are analyzed qualitatively; type 1 axon terminals (asymmetric apposition--round vesicles) shift from dendritic shafts to spine tips during maturation. Each spine is found to bear a type 1 contact at all ages; these results allow us to conclude that the figures of Boothe et al. ('79) on changes in spine populations during maturation can now be interpreted as changes in type 1 synapse populations. It is shown that type 2 synapses (symmetric appositions--pleomorphic vesicles) arise from axons of the smooth dendritic neurons. These synapses are found to increase in number on the spiny cell somata in early postnatal development, and this is followed by a decrease in number to the adult level.