Morphology

The adrenal glands are bilateral structures situated above the kidneys. They are comprised of an outer region or cortex, which normally makes up more than three-quarters of the adrenal mass, and an inner region or medulla (Fig. 1). The medulla is a modified sympathetic ganglion that, in response to signals reaching it through cholinergic, preganglionic fibers, releases either or both of its two hormones, epinephrine and norepi-nephrine, into adrenal venous blood. The cortex arises from mesodermal tissue and produces a class of lipid-soluble hormones derived from cholesterol and called steroids. The cortex is subdivided histologically into three zones. Cells in the outer region, or zona glomer-ulosa, are arranged in clusters (glomeruli) and produce the hormone aldosterone. In the zona fasciculata, which comprises the bulk of the cortex, rows of lipid-laden cells are arranged radially in bundles of parallel cords (fasces). The inner region of the cortex consists of a tangled network of cells and is called the zona reticularis. The fasciculata and reticularis, which produce both cortisol and the adrenal androgens, are functionally separate from the zona glomerulosa.

The adrenal glands receive their blood supply from numerous small arteries that branch off the renal arteries or the lumbar portion of the aorta and its various major branches. These arteries penetrate the adrenal capsules and divide to form the subcapsular plexus from which small arterial branches pass centrip-etally toward the medulla. The subcapsular plexuses also

FIGURE 1 Histology of the adrenal gland.

Medulla

FIGURE 1 Histology of the adrenal gland.

and, to a lesser extent, corticosterone are the physiologically important glucocorticoids and are so named for their ability to maintain carbohydrate reserves. Gluco-corticoids have a variety of other effects as well. At high concentrations, aldosterone may exert glucocorticoid-like activity and, conversely, cortisol and corticosterone may exert some mineralocorticoid activity (see later discussion). The adrenal cortex also produces androgens, which as their name implies have biological effects similar to those of the male gonadal hormones (see Chapter 45). Adrenal androgens mediate some of the changes that occur at puberty. Adrenal steroid hormones are closely related to steroid hormones produced by the testis and ovary and are synthesized from common precursors. In some abnormal states the adrenals may secrete any of the gonadal steroids.

Adrenocortical Hormones

All of the adrenal steroids are derivatives of the polycyclic phenanthrene nucleus, which is also present in cholesterol, ovarian and testicular steroids, bile acids, and precursors of vitamin D. Use of some of the standard conventions for naming the rings and the carbons give rise to long loops of capillaries that pass between the cords of fascicular cells and empty into sinusoids in the reticularis and medulla. Sinusoidal blood collects through venules into a single large central vein in each adrenal and drains into either the renal vein or the inferior vena cava.

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