Cellular Organization in Rat Somatosensory Cortex: Effects of Sex and Laterality

Layer IV of rodent somatosensory cortex contains distinct arrangements of cells characterized as barrels. When barrels first form in rats, each barrel consists of a cell-dense "wall" and a cell-sparse "hollow." With age, the distinction of the boundary between barrel walls and hollows diminishes. Cellular arrangements within barrels were quantified to test whether the barrels are influenced by sex and laterality during cortical development. A computer-assisted method was developed to measure cell densities in relation to barrel boundaries. The boundaries between barrel walls and hollows were determined in tissue double-stained for Nissl substance and cytochrome oxidase histochemistry. The distinction between barrel walls and hollows revealed by Nissl stains differed significantly between anterior and posterior barrels. This distinction declined significantly in anterior barrels from Postnatal Day 10 (P10) to P30. The area of cortex containing barrels was estimated from composites of Nissl-stained sections. At P20 the detectable barrel cortex area was larger on the right in females and on the left in males resulting in a significant sex difference in barrel cortex asymmetry. This sex difference in barrel cortex laterality was detected only in Nissl-stained tissue; there were no differences attributable to sex or side in barrel cortex area analyzed for cytochrome oxidase reactivity. We hypothesize that sex-dependent differences in barrel cortex structure result from lateralized differences in cellular organization.