Fig. 1.

Hypothetical model of development of the midgestation HFA gland. The schema shows the HFA cortex structure, which consists of the DZ, TZ, and FZ. The medulla is not recognized as a distinct structure in the HFA throughout most of gestation. Previous studies from our laboratory and others have demonstrated that the HFA cortex is a dynamic organ: hyperplasia occurs primarily in the DZ; hypertrophy occurs primarily in the FZ; and apoptosis occurs primarily in the central portion of the FZ. Thus, cells appear to proliferate in the periphery, migrate centripetally, differentiate to form the specialized cortical compartments, and then undergo senescence when they reach the center of the cortex. Recent studies indicate that angiogenesis (formation of new capillaries from preexisting blood vessels) occurs at the periphery of the gland. C, Capsule.