Developmental changes in basal and circadian fluctuations in plasma dehydroepiandrosterone sulfate (DHEAS) and cortisol (F) from birth until 3 months of life were studied in conjunction with morphological characteristics of the fetal and neonatal adrenal cortex in baboons (Papio anubis). These studies were complimented by measurements of in vitro production of DHEAS and F (basal and ACTH stimulated) by adrenal tissue slices. Cortisol, DHEAS, estrone, estradiol, and progesterone were determined in plasma and incubation medium by specific RIA.

After delivery, an initial rise in plasma DHEAS was sustained for 4 days, followed by a precipitous decline, which reached a nadir between days 10–12 postpartum. Thereafter, plasma DHEAS and F concentrations stabilized with minor fluctuations. A significant (P < 0.05) and persistent diurnal rhythm in DHEAS and F secretion was evident by the end of the first week after birth. Administration of estrone acetate and progesterone in oil maintained neonatal plasma concentrations of estrone, estradiol, and progesterone at levels comparable to those in utero, but had no quantitative or qualitative effect on the pattern of plasma DHEAS or F in the neonate. The relative production of F to DHEAS by tissue slices in vitro (the calculated F/DHEAS ratios) indicates that DHEAS secretion by the fetal adrenal is 10-fold higher than that of F near parturition; ACTH stimulation does not alter this relationship. By postnatal day 10, the basal production rates of F and DHEAS are equivalent, and the response to ACTH stimulation favors production of F. With advancing neonatal age (at 30 and 100 days), there is an increase in the F/DHEAS secretion ratio both during the basal state and in response to ACTH.

The baboon adrenal glands increased in weight during the last month of gestation and then stabilized during the early postnatal period; a gradual increase in weight was observed after 30 days postpartum. Within 2 weeks after parturition, the relative width of the fetal zone decreased dramatically to occupy less than one third of the total cortex. During involution, we observed a decrease in cell size and a reduction in cytoplasmic vacuolation. A zone of closely packed cells with numerous areas of cell death (the dense band) separated the zona fasciculata from the fetal zone. Cell proliferation was observed in the upper regions of the definitive cortex.

We conclude the following. 1) The hypothalamic-hypophyseal mechanisms that regulate the diurnal adrenal secretory rhythm are established in the early neonatal period in the baboon. 2) The unique postnatal changes in adrenal morphology and secretion are probably a response to the different degrees of ACTH stimulation that result from changes in the MCR of F. 3) Circulating placental estrogens and progesterone may not contribute to the relative 3β-hydroxysteroid dehydrogenase deficiency in the baboon fetal adrenal. 4) The fetal zone involutes gradually after birth with areas of focal necrosis but without hemorrhage. We suggest that the zona fasciculata develops by progressive proliferation of new cells within its upper margins and not by transformation of preexisting cells in the fetal zone.