Endocrine regulation of fetal development and its relation to parturition in the rhesus monkey.

Rhesus monkey fetuses were surgically prosencephalectomized (Type 2) or functionally hypophysectomized (Type 1) at 75 days gestation, then returned to the uterus until elective Caesarean section on day 145--150 (term 167 days). Deprivation of fetal hypothalamic releasing factors in Type 2 and fetal pituitary tropic hormones in Type 1 significantly delayed the ontogeny and functional development of fetal endocrine tissues. Bone ossification and growth were significantly retarded in Type 1 only, not in Type 2. In Type 1 the body and all organs except the endocrine glands were about half normal weight. The adrenals, thyroids, ovaries and testes were histologically abnormal and about one-tenth normal weight. Non-endocrine organs were histologically similar to 110-130-day fetuses. Thyroxine (T4) concentrations were significantly depressed in Type 1 fetal and maternal plasma at Caesarean section but normal in Type 2. Cortisol concentrations were normal in Type 1 fetal and maternal plasma. Types 1 and 2 plasma oestradiol concentrations were significantly lower in mothers but normal in fetuses. Type 1 placentas produced significantly less progesterone in vitro than normal. Fetal endocrine autonomy is indicated (thyrotropin-releasing factor excepted). Many of the hypothalamic and anterior pituitary hormones do not pass in effective amounts from mother to fetus. Fetal endocrine autonomy is a prerequisite for the control of both development and parturition.