Growth and cytodifferentiation of the fetal lamb adrenal cortex prior to parturition.

Growth patterns and cytodifferentiation of the fetal lamb adrenal cortex, from 136 days of gestation to birth at normal term, have been examined. The gland almost doubles in weight over this period (0.45--0.80 g) and the inner cortical zone (zona fasciculata+zona reticularis) nearly quadruples in thickness (0.33--1.25 mm). The phase of rapid growth has begun by 136 days, and the growth patterns are essentially linear. The inner cortical zone consists of light and dark cells, the former increasing in number proportionately more than the dark cells. This change in the cell ratio is linked with increased fetal production of cortisol. As growth and differentiation proceed the light cells increasingly possess a more vesicular SER; mitochondria of the orthodox configuration, with predominantly vesicular cristae; a well developed Golgi complex with numerous small, bristle coated vesicles; and dark granules near the Golgi complex, in the peripheral cytoplasm, and external to the cells. The sixfold increase in the volume of the inner cortical zone and the ultrastructural evidence of cytodifferentiation appear adequate to account for the known increase in the fetal synthesis of cortisol during this stage of pregnancy.