The sensitivity of the fetal rat adrenal gland to adrenocorticotropic hormone in vivo and in vitro.

Rat fetuses were treated with 1 IU ACTH on day 16 or 17 of gestation and autopsied on the next day. Other fetuses of the same litter were treated with saline alone. The adrenal glands of ACTH-treated fetuses were significantly heavier than those of saline-treated fetuses and of intact controls in every experimental period. These changes in the weight of fetal adrenal glands reflect histologically on changes in the size of adrenocortical cells which were enlarged in response to injected ACTH. Adrenal glands from rat fetuses of different ages were explanted to organ cultures for 2 days, in medium with or without ACTH added. The 13-day adrenal glands were not able to respond to ACTH, but the response appeared abruptly in the 14-day adrenal glands, judging from the increased cell size in histological sections. The overall results suggest that the fetal adrenal gland begins to respond to ACTH from day 14 of gestation.