Development of the fetal pituitary-adrenal axis in the sheep

In the ovine fetus, plasma levels of corticosteroids are very low between 60 and 130 days of gestation, then increase dramatically before birth. ACTH appears to be an important regulating hormone for the fetal adrenal cortex, the sensitivity of which to this hormone increases during late gestation. However, the relationship between immunoreactive ACTH and corticosteroids in the fetus is unclear. We review herein recent work performed in our laboratory on the regulation of ACTH secretion by ovine fetal pituitary cells and on the biochemical modifications responsible for the enhancement of the steroidogenic response to ACTH of fetal adrenal cells. It is suggested that qualitative together with quantitative changes in the pituitary drive to the fetal adrenal has to be taken in account to explain the rise of corticosteroids in prepartum animals. Also extra pituitary hormones may be operating during intrauterine life to regulate fetal adrenal function.