QUANTITATIVE INVESTIGATION OF SPINE AND DENDRITE DEVELOPMENT OF NEURONS IN VISUAL-CORTEX (AREA-17) OF MACACA-NEMESTRINA MONKEYS

In a previous Golgi study (Lund et al., 1977) which examined the development of the macaque monkey striate cortex (area 17) it was observed that the dendrites of neurons within the visual cortex show a marked increase in the number of spines on their surface during the first 8 wk of postnatal life. The qualitative observation was also made that all neurons then showed a marked decrease in spine numbers by the time the animal was adult. Since these spines are known to be sites of synaptic contact, changes in their numbers may reflect changes in synapse populations on these neurons. This study examines quantitatively spine frequency and total dendritic development of Golgi impregnated neurons in monkeys ranging in age from 145 days gestation to adult. Four cell types were studied: spiny stellate neurons from laminae IVC.alpha. and IVC.beta. and pyramidal neurons with soma in either lamina IIIB or upper lamina VI. After consideration of possible sources of variation in spine numbers several conclusions are made: dendritic spine development appears to be a tightly controlled process both in terms of actual numbers of spines on a neuron at any one age and in the rate of change of spine frequency. The neurons populations examined all show a gradual increase in spine numbers up to 8 wk of age. At least 2 different trends are found in spine population maturation after the 8 wk point: (A)-the spine population may remain constant at the 8 wk level for same period of time or (B)-there may be a rapid decline in spine numbers following the 8 wk peak. Those neurons associated with direct input, or early stages in the relays, from the parvocellular geniculate laminae probably show trend B, while those associated with magnocellular input, or later order combined relays within the cortex, may show trend A. Different parts of a single pyramidal neuron dendrite may show either trend A or trend B, depending on the lamina location of the dendritic segment considered. All neurons show spine population decreases between 9 mo. of age and adult (5-7 yr) suggesting continuing long term maturational changes.