Although considerable evidence implicates increased fetal adrenal function as a major factor in the initiation of parturition in the sheep, the mechanism responsible for this increased activity has not yet been determined. We have investigated the development of the function of fetal lamb adrenal cortical cells dispersed in vitro. There was no change in the sensitivity of the cells to synthetic ACTH (ACTH1-24), as demonstrated by the concentration of ACTH1-24 producing a 50% maximum response in corticoid secretion. This finding does not support the suggestion that there is a qualitative change in fetal adrenal receptor function as term approaches. No stimulation of corticoid was observed after the administration of ±MSH or PRL in vitro at any gestational age or of αMSH in vivo in four fetuses at 125130 days gestation. Both the maximum output and the 50% maximum response in corticoid secretion of adrenal cells from term fetuses were similar to those of adrenal cells from adult ewes. A significant increase in the maximum output of corticoids per cell in response to ACTH124 occurs as early as 107 days gestation and continues steadily to term.
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