The quantitative effects of dark-rearing and light exposure on the laminar composition and depth distribution of neurons and glia in the visual cortex (area 17) of the rat

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Abstract
The effects of dark-rearing and light-exposure on the distribution of neurons and glial cells types in the rat visual cortex (area 17) have been investigated. Three groups of animals were studied: i) rats reared in the dark until weaning at 21 days post natum (21 DPN) and subsequently light-exposed for 31 days (Group 21/31); ii) rats darkreared until 52 DPN and then exposed to light for 3 days (Group 3 dL); and iii) rats totally dark-reared until 52 DPN (Group 52 dD). Semithin sections tangential to the pial surface were obtained at sampling intervals 50 μm apart throughout the depth of the left visual cortex. The volume numerical densities of neurons, astroglia, oligodendroglia, and microglia, at each sampling strata in the cortex were calculated using stereological techniques. The laminer density and distribution of neurons was not significantly different between the three groups. In comparison with group 21/31 there was a marked reduction in the densities of astroglia, oligodendroglia, and microglia in lower layer 5 of groups 3 dL and 52 dD. Additionally, the density of microglia in thalamorecipeint layer 4 was greatly increased in group 3 dL compared with groups 21/31 and 52 dD. These results indicate specific alterations in the glial cell composition of the rat visual cortex following periods of dark-rearing and light-exposure. Furthermore, changes in the density of glial cells in layer 5 may reflect functional modifications in neurons projecting to the superior colliculus.

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