Synapse elimination in the developing visual cortex: a morphometric analysis in normal and dark-reared cats.

The elimination of synapses from developing visual cortex, occurring after day 70 in cat, is thought to result from competition among redundant axons innervating area 17. The dependence of this elimination on visual experience was tested by comparing normal cats at 70 and 220 days of age with littermates dark-reared from day 70 to day 220. Thickness of all cortical laminae, numerical density (NV) of neurons in layers I-III and NV of synapses in layers I-III were measured in two regions of area 17, corresponding to central (0-5 degrees) and peripheral (15-20 degrees) representations of the binocular visual field. In the central region, normal development was characterized by a 25% decrease in thickness of layers I-III with a corresponding increase in the NV of neurons. The NV of synapses in layer I decreased by 29%, with a 52% decrease in the number of synapses under one mm2 of pial surface. In layers II-III the NV of synapses decreased by 31%, with a 46% decrease in the number of synapses under 1 mm2 of pial surface. These developmental changes were unaffected by dark-rearing. In the peripheral region, normal cats exhibited no significant changes in laminar thickness or in NV of synapses, although the number of synapses under 1 mm2 of pial surface decreased by 25% in layer I and by 18% in layers II-III. In dark-reared cats the thickness of layers I-III decreased by 26% with a corresponding increase in the NV of neurons. The NV of synapses in layer I decreased by 18% and the number of synapses under 1 mm2 of pial surface by 51%. In layers II-III the NV of synapses decreased by 22% and the number of synapses under 1 mm2 of pial surface by 39%. Thus, in normal development the loss of synapses is greater from central than from peripheral representations of area 17. Dark-rearing does not alter the elimination of synapses in central cortex, but accelerates this loss in peripheral cortex.