The Adrenal Cortex Has Three Zones: the Zona GlomeruIosafWhich Secretes Mineralocorticoids, and the Zona Fasciculata and the Zona Reticularis, Which Secrete Glucocorticoids and Sex Steroids
The adrenal cortex is organized into three zones in mammals (see Figure 34-7). The outer zone, the zona glomerulosa, is relatively narrow, and its cells are organized in a whorl-type arrangement.
The middle zone, the zona fasciculata, is relatively wide, and its cells are organized in columns. In the cow and sheep, the zona fasciculata is further divided into inner and outer layers. The inner zone of the adrenal cortex, the zona reticularis, which is adjacent to the adrenal medulla, is intermediate in size, and cells are more randomly organized.All the cells of the adrenal cortex have intracellular features characteristic of steroid hormone synthesis: an abundance of lipid droplets (containing cholesterol esters), mitochondria, and smooth endoplasmic reticulum. Human adrenal glands have an additional zone, the fetal zone, that is present during fetal life and for the first year of life. The fetal zone participates with the placenta in the production of estrogen during gestation. Immature mice and rabbits have an inner X zone that becomes the zona reticularis at puberty.
'Γhe adrenal cortex produces two major types of steroid hormones: the mineralocorticoids and the glucocorticoids. These hormones have distinctly different functions. The mineralocorticoids, produced by the zona glomerulosa, play an important role in electrolyte balance and therefore are important in the regulation of blood pressure (see later discussion). The major mineralocorticoid is aldosterone. The glucocorticoids, produced by the zona fasciculata (which accounts for the majority of glucocorticoid production) and zona reticularis, are important in the regulation of all aspects of metabolism, either directly or through an interaction with other hormones. The major glucocorticoid is cortisol.
FIGURE 34-8 Mechanism of action of corticotropin (adrenocorticotropic hormone, ACTH} on adrenocortical Steroidogenesis-The numbers indicate the processes stimulated (indicated by plus signs) by corticotropin as follows: 1t stimulation of the uptake of low-density lipoproteins (LDL)t which are further processed to free cholesterol;
2, stimulation of the hydrolysis of stored cholesterol esters to generate free cholesterol;
3, stimulation of the transport of cholesterol into mitochondria, where cleavage of the cholesterol side chain occurs; and 4t promotion of the binding of cholesterol to the enzyme. ACt Adenyl cyclase; ATPt adenosine triphosphate; cAMP, cyclic adenosine monophosphate; Rt receptor. (From Hedge GA, Colby HD, Goodman RL: Clinical endocrine physiology, Philadelphia, 1987, Saunders.)