There are two adrenal glands, one at the upper pole of each kidney. The gland is a composite of two distinct structural and functional components: a cortex and a medulla. The cortex develops from mesoderm and secretes steroid hormones; the medulla develops from neuroectoderm of the neural crest and secretes catecholamines.
The cortex is divided into three zones according to the type and arrangement of its parenchymal cells. These are designated zona glomerulosa, zona fasciculata, and zona reticularis. The zona glomerulosa constitutes 15% of the cortical volume. It secretes mineralocorticoids (aldosterone and deoxycorticosterone). The zona fasciculata constitutes nearly 80% of the cortical volume. It secretes the glucocorticoids (Cortisol, cortisone, and corticosterone) and a small amount of adrenal androgens. The zona reticularis (5 to 7% of cortical volume) produces most of the adrenal androgens.
The zona fasciculata and the zona reticularis are regulated by adrenocorticotropic hormone (ACTH) secreted by the ade-nohypophysis in response to corticotropin-releasing factor (CRF) produced by the hypothalamus. The zona glomerulosa is not regulated by ACTH but is under feedback control of the renin-angiotensin system that also regulates blood pressure.
Figure 1, adrenal gland, human, H&E x45.
This low-magnification micrograph of a section through the partial thickness of an adrenal gland shows the outer capsule (Cap), the cortex (Cort) from one surface of the gland, the underlying medulla (Med), and a very small portion of the cortex from the other surface of the gland (Cort, bottom center). The cortex has a distinctly different appearance in both structural organization and staining characteristics.
From the inner portion, the medulla, note the lighter appearance of the medullary tissue. A small amount of adipose tissue (AT) in which the gland is partially embedded is seen at the upper center of the micrograph. The corticomedullary boundary (dashed lines) has a wave-like contour, a reflection of the irregular shape of the gland. Within the medulla are a number of relatively large blood vessels (BV). These are the medullary veins that drain both the cortex and the medulla.
Figure 2, adrenal gland, human, H&E x180.
This is a higher magnification of a portion of the capsule and the full thickness of the cortex from an area in Figure 1. The capsule consists of dense connective tissue in which the larger arteries (A) travel to give rise to smaller vessels that will supply the cortex and medulla. The zona glomerulosa (ZG) is located at the outer part of the cortex, immediately under the capsule. The parenchyma of this zone consists of small cells that appear as arching cords or as oval groups of cells.
The zona fasciculata (ZF) consists of radially oriented cords and sheets of cells, usually two cells in width, that extend toward the medulla. The cells of the outer part of the zona fasciculata are generally larger than those of the inner portion of this zone and typically stain poorly because of the large number of lipid droplets that they contain. The cells of the zona reticularis (ZR) are relatively small and contain little or no lipid droplets and, consequently, stain prominently with eosin. Because of their small size, the nuclei are in close proximity to one another, much like the cells of the zona glomerulosa.
Figure 3, adrenal gland, human, H&E x245.
This is a higher magnification of the area inscribed by the left rectangle in Figure 2. It shows the zona glomerulosa (ZG) and the outer portion of the zona fasciculata (ZF). Note the smaller size of the cells in the zona glomerulosa than those in the zona fasciculata. In addition, cells of the zona glomerulosa contain fewer lipid droplets than those of the zona fasciculata. Typically, the cells in this part of the zona fasciculata are filled
Figure 4, adrenal gland, human, H&E x245.
This is a higher magnification of the area inscribed by the right rectangle in Figure 2. This deep portion of the zona fasciculata (ZF) reveals smaller cells, although they are still arranged in cords and contain lipid droplets, though with lipid droplets, thus, the very poor staining characteristic of their cytoplasm. Delicate connective tissue trabeculae (arrows) extend from the capsule to surround the glomerular groups of cells and extend between the cords of cells in the zona fasciculata. Capillaries and arterioles are located within the connective tissue trabeculae. Usually, the capillaries are collapsed and, without the presence of red blood cells in their lumina, are thus difficult to identify.
in lesser amounts. The cells of the zona reticularis (ZR) are arranged in irregular anastomosing cords and contain at best only a small amount of lipid and, consequently, their cytoplasm stains with eosin.
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