Synaptogenesis in visual cortex of normal and preterm monkeys: evidence for intrinsic regulation of synaptic overproduction.

Abstract

We used quantitative electron microscopy to determine the effect of precocious visual experience on the time course, magnitude, and pattern of perinatal synaptic overproduction in the primary visual cortex of the rhesus monkey. Fetuses were delivered by caesarean section 3 weeks before term, exposed to normal light intensity and day/night cycles, and killed within the first postnatal month, together with age-matched controls that were delivered at term. We found that premature visual stimulation does not affect the rate of synaptic accretion and overproduction. Both of these processes proceed in relation to the time of conception rather than to the time of delivery. In contrast, the size, type, and laminar distribution of synapses were significantly different between preterm and control infants. The changes and differences in these parameters correlate with the duration of visual stimulation and become less pronounced with age. If visual experience in infancy influences the maturation of the visual cortex, it must do so predominantly by strengthening, modifying, and/or eliminating synapses that have already been formed, rather than by regulating the rate of synapse production.

Thick Visual Cortex in the Early Blind

“…The synaptic density of the visual cortex reaches a maximum of ϳ150% of adult levels between 4 and 12 months through synaptogenesis, and then gradually decreases to adult levels by ϳ5 years of age through "synaptic revision," "regression," or "pruning" (Johnson, 1997). Although the absence of retinal input does not affect the schedule and magnitude of synaptogenesis (Winfield, 1981;Bourgeois and Rakic, 1996), the synaptic revision in the visual cortex has been shown to depend on visual experience (Stryker and Harris, 1986;Bourgeois et al, 1989). This process of synaptic regression appears to be interrupted by early visual deprivation.…”

“…Synaptic density in the visual cortex increases to a maximum during the first postnatal year and then gradually decreases to the adult level at ϳ11 years of age (Huttenlocher and de Courten, 1987). Although the initial increase in synaptic density by synaptogenesis does not depend on visual experience (Winfield, 1981;Bourgeois and Rakic, 1996), the subsequent pruning of synapses has been shown to depend on visual input (Bourgeois et al, 1989). Thus the interruption of synaptic elimination by early visual deprivation may prevent the visual cortex from thinning.…”

Structural and functional brain development and its relation to cognitive development

“…prefrontal cortex) in primates are relatively protracted compared to others (e.g. visual cortex) (Bourgeois et al, 1989Huttenlocher, 1990Huttenlocher, , 1997. These regions of prolonged development are perhaps most interesting when considering the developing child throughout adolescence and into adulthood.…”

Maturation of Cerebral Connections and Fetal Behavior

“…It was demonstrated experimentally that precocious visual stimulation in prematurely born rhesus monkeys did not change the rate of accretion of synapses in the visual cortex. 30 However, the same authors observed changes in the length of synaptic profiles. These findings strongly suggest that neurogenetic events which normally enfold before birth in preterm primates are predominantly enfolding according to intrinsic (genetic) programs, but upon sensory stimulation display a certain level of structural plasticity.…”