Postnatal changes in the number of neurons and synapses in the visual cortex (area 17) of the macaque monkey: a stereological analysis in normal and monocularly deprived animals.

The surface area of the striate cortex and the thickness of its laminae were measured in a series of newborn, 3-month, 6-month, and adult macaque monkeys. The numerical densities (Nv) of neurons and synapses were measured in individual laminae. The total numbers of neurons and synapses in the striate cortex of one hemisphere were derived from these measures. Normal monkeys were compared at 3 months and 6 months of age to animals having been reared from birth with a monocular eyelid suture. No significant differences were observed between normal and monocularly deprived monkeys. The perceptual deficits and physiological abnormalities that have been reported in monocularly deprived monkeys do not appear to result from a reduction in the amount of neural circuitry in the striate cortex. The combined data from these groups, however, demonstrated several developmental changes. Cortical thickness increased from birth to 6 months and diminished to near-newborn values in the adult. The 6-month cortex was 19% thicker than that of the adult. This overshoot was greatest in layers II and III, which were 43% thicker at 6 months. Cortical surface area demonstrated a similar trend, being 23% greater at 6 months, but the differences were not statistically significant. The Nv of neurons decreased from birth to 6 months and increased to near-newborn values in the adult. The 6-month Nv was 30% less than that of adults and the greatest changes were seen in layers II and III where the Nv was 38% less than adult values. The total number of neurons in the striate cortex of one hemisphere was 16% less in adults than in newborn animals, but statistically significant neuron losses were limited to layers I, II, IVC alpha, V, and VI. The Nv of synapses increased from birth to 6 months and diminished to near-newborn values in the adult. The 6-month overshoot was 34% for the total cortex and 41% for layers I-III. The total number of synapses in the striate cortex was 95% greater at 6 months than in the adult. In layers I-III the synapses were 130-155% more numerous at 6 months. These data demonstrate an increase in neuronal connectivity in the striate cortex from birth to 6 months, especially in layers I-III, and a subsequent decrease in the adult.