Studies on the postnatal growth of the rat adrenal cortex.

In the male rat the volumes of the several zones of the adrenal cortex were measured using a point-counting technique at various ages. The resulting growth curves were fitted by the Gompertz method, and the specific growth rate calculated for each zone at 14 days of age. Also at 14 days the flash labelling index was measured in contiguous bands each three cells in thickness across the cortex; maximum indices were found in the outer cortical layers. Calculations of the birth rate for each zona were compared with the growth rates. In the zona glomerulosa, the birth rate exceeded the growth rate, giving a cell migration rate of 4 cells/1000 cells/hour. In the zona reticularis there was a net cell gain of 5 cells/1000 cells/hour; in this zone, for every 100 cells born by cell division, nearly 2000 were added by cell migration. The results support the migration hypothesis of adrenocortical cytogenesis.