Equivalent cell density in three areas of neonatal rat cerebral cortex.

Using the disector method, equivalent cell densities (number of cells per unit volume) were found in the frontal (motor), parietal (somatosensory) and occipital (visual) cortex of neonatal rat (1.2 x 10(6) cells/mm3). As determined with GABA post-embedding immunocytochemistry on semithin sections, at least 11% of these cells expressed GABA. Because neonate frontal cortex is thicker than the parietal or the occipital cortex, the number of cells under an unit area of pial surface was greater in this cortex. This clearly differed from the adult pattern, where the greatest number of cells beneath an unit pial area is that of the somatosensory versus the frontal and the occipital cortex. It suggests that the attainment of the adult number of cells in a cortical column is achieved through differential cell death in the different areas and that the amount of thalamic input could be a crucial determinant of the adult cytoarchitecture.