The secretion and production of glucocorticoids and sex steroids by the zona fasciculata is under feedback control of the CRH–ACTH system. ACTH is necessary for cell growth and maintenance and also stimulates steroid synthesis and increases blood flow through the adrenal gland. Exogenous ACTH maintains the structure and function of the zona fasciculata after hypophysectomy. In animals, administration of ACTH causes hypertrophy of the zona fasciculata.
Circulating glucocorticoids may act directly on the pituitary gland, but they most commonly exert their feedback
FIGURE 21.23 • Photomicrographs of the cortex and medulla of the human adrenal gland. a. This photomicrograph shows an H&E–stained specimen of the outer cortex. It includes the connective tissue capsule, the zona glomerulosa, and the zona fasciculata. Continuous with the zona glomerulosa are the straight cords of cells that characterize the zona fasciculata. Between the cords are the capillaries and the less numerous arterioles. The red linear stripes represent capillaries that are engorged with red blood cells. 120.
b. The deep parts of the zona fasciculata, zona reticularis, and medulla are shown here. Note that the linear arrays of the cords in the zona fasciculata give way to irregular groups of cells of the zona reticularis. The medulla, in contrast, consists of ovoid groups of cells and short interconnecting cords of cells. A central adrenomedullary vein is also seen here. Note the thick longitudinally sectioned smooth muscle in part of its wall. 120.
control on neurons in the arcuate nucleus of the hypothalamus, causing the release of CRH into the hypothalamohypophyseal portal circulation. Evidence also suggests that circulating glucocorticoids and the physiologic effects that they produce stimulate higher brain centers that, in turn, cause the hypothalamic neurons to release CRH.
The zona reticularis produces glucocorticoids and androgens.
The cells of the zona reticularis are noticeably smaller than those of the zona fasciculata, and their nuclei are more deeply stained. They are arranged in anastomosing cords separated by fenestrated capillaries. The cells have relatively few lipid droplets. Both light and dark cells are seen. Dark cells have abundant large lipofuscin pigment granules, and deeply staining nuclei are evident. The cells in this zone are small because they have less cytoplasm than the cells in the zona fasciculata; thus the nuclei appear more closely packed. They exhibit features of steroid-secreting cells, namely, a well-developed sER and numerous elongated mitochondria with tubular cristae, but they have little rER.
The principal secretions of the zona reticularis are weak androgens.
The principal secretion of the cells in the zona reticularis consists of weak androgens, mostly dehydroepiandrosterone (DHEA). The cells also secrete some glucocorticoids, in much smaller amounts than those of the zona fasciculata. Here, too, the principal glucocorticoid secreted is cortisol.
The zona reticularis is also regulated by the feedback control of the CRH–ACTH system and atrophies after hypophysectomy. Exogenous ACTH maintains the structure and function of the zona reticularis after hypophysectomy.
The fetal adrenal gland consists of an outer narrow permanent cortex and an inner thick fetal cortex or fetal zone.
Once fully established, the fetal adrenal gland is unusual in terms of its organization and its large size relative to other developing organs. The gland arises from mesodermal cells located between the root of the mesentery and the developing gonad zone (see Fig. 21.19a). The mesodermal cells penetrate the underlying mesenchyme and give rise to a large