The adrenal (suprarenal) glands secrete both steroid hormones and catecholamines. They have a flattened triangular shape and are embedded in the perirenal fat at the superior poles of the kidneys.
The adrenal glands are covered with a thick connective tissue capsule from which trabeculae extend into the parenchyma, carrying blood vessels and nerves. The secretory parenchymal tissue is organized into cortical and medullary regions (Fig 20.16):
• The cortex is the steroid-secreting portion. It lies beneath the capsule and constitutes nearly 90% of the gland by weight.
• The medulla is the catecholamine-secreting portion. It lies deep to the cortex and forms the center of the gland.
Parenchymal cells of the cortex and medulla are of different embryologic origin
Embryologically, the cortical cells originate from mesodermal mesenchyme, whereas the medulla originates from neural crest cells that migrate into the developing gland (Fig. 20.17). Although embryologically distinct, the two portions of the adrenal gland are functionally related (see below). The parenchymal cells of the adrenal cortex are controlled, in part, by the anterior lobe of the pituitary gland and function in regulating metabolism and maintaining normal electrolyte balance (Table 20.9).
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