Adrenal gland: some evidence for the structural and functional zonation of the steroidogenic tissues

There are two regions of steroidogenic tissue in the bird adrenal gland: a subcapsular zone (SCZ) 40-60 cells thick consisting of cells with irregularly shaped nuclei, relatively little smooth endoplasmic reticulum, and mitochondria with shelflike cristae that surrounds an inner zone (IZ) of tissue comprised of smaller cells with rounded nuclei, a more abundant endoplasmic reticulum, and mitochondria with tubular cristae. The cristae in the mitochondria of IZ cells, but not the SCZ cells, seem to be quite labile and only assume the tubular configuration when subjected to corticotropic stimulation. The properties of the steroidogenic cells in the two zones are quantitatively distinct, the cells of the SCZ producing relatively more aldosterone and relatively less corticosterone than the cells of the IZ. The corticotropic responsiveness of the IZ cells is dependent on the synthesis of protein in the cytoplasm and a normal pattern of microfilament assembly. The dose responsiveness of both the IZ and the SCZ cells in vitro is a complex quadratic function of the corticotropin concentration to which they are exposed; the semilogarithmic linear response to corticotropin is restricted to a narrow midrange of concentrations, and at high concentrations cells from both zones of the gland respond submaximally. Throughout the dose-response range, however, the steroidogenic cells of the IZ are more sensitive and more responsive to corticotropic stimulation than are the cells of the SCZ.