FIGURE21.19 • Development of the adrenal gland. a. In this early stage, the cortex is shown developing from cells of the intermediate mesoderm, and the medulla is shown differentiating from cells in the neural crest and migrating from the neighboring sympathetic ganglion. Note that the gland develops between the root of the dorsal mesentery of the primitive gut and the developing urogenital ridges. b. Mesodermal cells from the fetal cortex surround the cells of the developing medulla. c. At this stage (about 7 months of development), the fetal cortex occupies about 70% of the cortex. The permanent cortex develops outside the fetal cortex. d. The fully developed adrenal cortex is visible at the age of 4 months. The permanent cortex replaces the fetal cortex, which at this age has completely disappeared. Note the fully developed zonation of the permanent cortex.
TABLE Hormones of the Adrenal Glands 21.9 Hormone Composition Source Major Functions Adrenal cortex Mineralocorticoids (95% of mineralocorticoid activity in aldosterone) Steriod hormones (cholesterol derivatives) Parenchymal cells of the zona glomerulosa Aid in controlling electrolyte homeostasis (act on distal tubule of kidney to increase sodium reabsorption and decrease potassium reabsorp-tion); function in maintaining the osmotic  balance in the urine and in preventing serum acidosis Glucocorticoids (cortico -sterone, and cortisol; 95% of glucocorticoid activity in cortisol) Steriod hormones (cholesterol derivatives) Parenchymal cells of the zona fasciculata (and to a lesser extent of the zona reticularis) Promote normal metabolism, particularly carbohy-drate metabolism (increase rate of amino acid transport to live, promote removal of protein from skeletal muscle and its transport to liver, reduce rate of glucose metabolism by cells and stimulate glycogen synthesis by liver, stimulate mobilization of fats from storage deposits for energy use); provide resistance to stress; suppress inflammatory response and some allergic reactions Gonadocorticoids (dehydroepiandrosterone [DHEA] is a major sex steroid produced in both men and women) Steriod hormones (cholesterol derivatives) Parenchymal cells of the zona reticularis (and to a lesser extent of the zona fasciculata) Induce weak masculinizing effect; at normal serum levels usually their function is insignificant Adrenal medulla Norepinephrine and epinephrine (in human, 80% epinephrine) Catecholamines (amino acid derivatives) Chromaffin cells Sympathomimetic (produce effects similar to those induced by the sympathetic division of the autonomic nervous system)a; increase heart rate, increase blood pressure, reduce blood flow to viscera and skin; stimulate conversion of glycogen to glucose; increase sweating; induce dilation of bronchioles; increase rate of respiration; decrease digestion; decrease enzyme production by digestive system glands; decrease urine production a The catecholamines influence the activity of glandular epithelium, cardiac muscle, and smooth muscle located in the walls of blood vessels and viscera.
Lymphatic vessels have also been demonstrated in the parenchyma of the adrenal medulla. Recent physiologic studies indicate an important role of the lymph vessels in distributing high-molecular-weight secretory products of chromaffin cells, such as chromogranin A, into the circulation.
Cells of the Adrenal Medulla
Chromaffin cells located in the adrenal medulla are innervated by presynaptic sympathetic neurons.