FIGURE 21.24 • Electron micrograph of cells in the zona fasciculata. The boundary between adjacent cells of the cord is indicated by the arrowheads. Lipid droplets (L) are numerous (the lipid has been partially extracted). 15,000. Inset. A higher magnification of an area in the cell at the top of the micrograph reveals the extensive sER that is characteristic of steroid-secreting cells. Portions of the Golgi apparatus are also evident. 40,000 eosinophilic cell mass that will become the functional fetal terms of body weight and is only slightly smaller than the cortex or zone (see Fig. 21.19b). Later, a second wave of adjacent kidney. At term, the adrenal glands are equivalent cells proliferates from the mesenchyme and surrounds the in size and weight to those of the adult and produce 100 to primary cell mass (see Fig. 21.19c). By the fourth fetal 200 mg of steroid compounds per day, about twice that of month, the adrenal gland reaches its maximum mass in the adult adrenals.
 FOLDER 21.6
Functional Considerations: Biosynthesis of Adrenal Hormones
Cholesterol is the basic precursor of several steroid hor-mones, namely, corticosteroids, sex hormones, bile acids, and vitamin D. About half of the cholesterol in the body comes from the diet and the other half derives from de novo biosynthesis. Cholesterol synthesis occurs in the cytoplasm and organelles from acetyl-CoA. Biosynthesis in the liver accounts for approximately 10%, and in the in-testines, approximately 15%, of the amount produced each day. In addition, a small portion of cholesterol is synthe-sized by the adrenal cortical cells. Both dietary cholesterol and that synthesized de novo are transported within low-density lipoproteins (LDLs). Cholesterol is stored in lipid droplets within the cytoplasm of adrenal cortical cells in the form of cholesterol esters. Steroid hormones in adrenal glands are synthesized from cholesterol esters by removal of part of the side chain and modifications at specific sites on the remainder of the molecule. The enzymes catalyzing these modifications are located in different zones of the cortex as well as in differ-ent cytoplasmic sites within the cells. For instance, cleav-age of the side chain is catalyzed by P450-linked side chain cleavage enzyme (P450ssc) or desmolase, which is found only in mitochondria of steroid-producing cells. The other enzymes necessary for steroid production are located within sER, cytosol, and mitochondria. Thus, a precursor molecule may move from the sER to a mitochon-drion and back again several times before the definitive molecular structure of a given corticosteroid is obtained. Cholesterol esters removed from cytoplasmic lipid droplets and used in steroid hormone synthesis are quickly replenished from the cholesterol esters contained within LDL carried in the bloodstream. These esters are the pri-mary source of the cholesterol used in corticosteroid syn-thesis. Under conditions of short-term or prolonged ACTH stimulation, the lipid stores in adrenal cortical cells are re-cruited for corticosteroid synthesis.