The Adrenal Glands Are Composed of Two Organs: the Outer Gland (Cortex) and the Inner Gland (Medulla)
The adrenal glands are two bilaterally symmetric endocrine organs located just anterior to the kidneys. Each gland is divided into two separate entities, a medulla and a cortex (Figure 34-7), each of which produces different types of hormones.
These adrenal tissues have different embryonic origins. The medulla arises from the neuroectoderm and produces amines such as norepinephrine and epinephrine. The cortex arises from the mesodermal coelomic epithelium and produces steroid hormones such as cortisol, corticosterone, sex steroids, and aldosterone. The utility of placing two such disparate tissues together is not apparent. The one common factor is that both sets of hormones are important for adaptation to adverse environmental conditions (i.e., stress).Interest in the function of the adrenal cortex was heightened in the 1930s because of the research of Hans Selye. He published a series of papers on the effects of adrenalectomy and the ability of the surgically treated animal to defend itself against injury. Selyes hypothesis was termed the general adaptation syndrome, which he divided into three parts: the alarm reaction, the stage of resistance, and the stage of exhaustion. The critical aspect of this theory was that in addition to specific responses to injury, animals responded in nonspecific ways to combat injury, and the adrenal cortex was the most important organ in leading the nonspecific response. One example of the beneficial effects of glucocorticoids in a
FIGURE 34-7 Depiction of comparable sections through the adrenal glands of A, normal rats, and B, hypophysectomized rats. Because the functional capacity of the adrenal cortex is conditioned by the release of corticotropin, hypophysectomy results in tremendous shrinkage of the cortex.The medulla is not influenced by hypophysectomy. Both sections are drawn to scale. (FromTurner CD, Bagnara JT: General endocrinology, ed 6, Philadelphia, 1976, Saunders.)
situation of injury is the mobilization of glucose, a readily usable source of energy for running away or healing injury. The adaptation of animals to stressful environments is often accompanied by enlargement of the adrenal cortex, such as in domestic chickens raised in crowded conditions and wild animals living in relatively high density.