After 10-15 weeks of gestation, the human fetal adrenal cortex undergoes rapid growth due to enlargement of a specialized cortical compartment known as the fetal zone (FZ). Soon after birth, the FZ regresses and the adult zonation pattern develops at least in part from cells derived from the persistent definitive zone (DZ), a thin layer of tightly packed cells surrounding the FZ. We postulated that growth of the fetal adrenal cortex involves zone-specific cellular hyperplasia, whereas the postnatal involution of the FZ is due to apoptosis. Therefore, we investigated the pattern of cellular proliferation and death in the FZ and DZ of the human fetal and postnatal adrenal cortex using immunohistochemical staining for proliferating cell nuclear antigen as a marker of mitosis and in situ detection of DNA fragmentation as a marker of apoptosis. Between 10-14 weeks' gestation, the mitotic indexes (percentage of proliferating cell nuclear antigen-positive cells) in the DZ (26.46 +/- 2.95%) and in the FZ (21.26 +/- 2.57%) were not significantly different. Between 15-20 weeks gestation, the mitotic index increased significantly (P < 0.05) in both zones (FZ, 33.84 +/- 5.21%; DZ, 67.45 +/- 7.58%) relative to levels before 15 weeks. This increase persisted between 21-24 weeks gestation (FZ, 39.5 +/- 4.22%; DZ, 58.63 +/- 6.83%). Interestingly, after 14 weeks, the mitotic index of the DZ was significantly greater (P < 0.05) than that of the FZ. In adrenal specimens obtained from infants born prematurely and treated in utero with glucocorticoid, the mitotic indexes in the FZ and DZ were significantly decreased. At all stages of gestation, no apoptotic nuclei were detected in the DZ. However, scattered apoptotic nuclei were detected in the central portions of the FZ. The number of apoptotic nuclei in the inner FZ increased with advancing gestation and was maximal during the first postnatal month. To identify factors that may regulate apoptosis, primary cultures of midgestation FZ cells were treated with activin A and transforming growth factor-beta (TGFbeta). Activin A and TGFbeta both induced apoptotic cell death, as assessed by internucleosomal DNA cleavage (DNA laddering). Induction of apoptosis by activin A was prevented by concomitant addition of follistatin, an activin-binding protein. Taken together, these data indicate that 1) growth of the human fetal adrenal cortex involves cellular hyperplasia, mainly in the DZ and to a lesser extent in the FZ, which is probably dependent on ACTH; and 2) apoptosis occurs predominantly in the inner cortical compartment and may be responsible for the rapid regression of the FZ after birth, a process that may be regulated by activin A and/or TGFbeta.