Figure 2017

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.

Cells of the Adrenal Medulla

Chromaffin cells located in the adrenal medulla are innervated by preganglionic sympathetic neurons

The central portion of the adrenal gland, the medulla, is composed of a parenchyma of large, pale-staining epithelioid cells called chromaffin cells (medullary cells); connective tissue; numerous sinusoidal blood capillaries; and nerves. The chromaffin cells are, in effect, modified neurons (see Box 20.4). Numerous myelinated, preganglionic sympathetic nerve fibers pass directly to the chromaffin cells of the medulla. When nerve impulses carried by the sympathetic fibers reach the catecholamine-secreting chromaffin cells, they release their secretory products. Therefore, chromaffin cells are considered the equivalent of postganglionic neurons. However, they lack axonal processes. Experimental studies reveal that when chromaffin cells are grown in culture, they extend axon-like processes. However, axonal growth can be inhibited by glucocorticoids—hormones secreted by the adrenal cortex. Thus, the hormones of the adrenal cortex exert control over the morphology of the chromaffin cells and prevent them from forming neural processes. Chromaffin cells therefore more closely resemble typical endocrine cells, in that their secretory product enters the bloodstream via the fenestrated capillaries.

Ganglion cells are also present in the medulla. Their axons extend peripherally to the parenchyma of the adrenal cortex to modulate its secretory activity and innervate zona fasciculata zona glomerulosa medulla

aorta sympathetic , ganglion (from / neural crest)

cortical primordium of fetal cortex (from intermediate mesoderm)

urogenital ridge dorsal mesentery aorta

chromaffin cells of future medulla

fetal cortex cells aorta dorsal mesentery fetal cortex cells fetal cortex permanent medulla sympathetic , ganglion (from / neural crest)

cortical primordium of fetal cortex (from intermediate mesoderm)

urogenital ridge zona fasciculata zona glomerulosa aorta chromaffin cells of future medulla

TABLE 20.9. Hormones of the Adrenal Glands

Hormone

Adrenal cortex

Mineralocorticoids (95% of mineralocorti-coid activity in aldosterone)

Glucocorticoids (corti-costerone, and Cortisol; 95% of glucocorticoid activity in Cortisol)

Gonadocorticoids (de-hydroepiandrosterone [DHEA] Is a major sex steroid produced in both men and women)

Adrenal medulla

Norepinephrine and epinephrine (in human, 80% epinephrine)

Composition

Steroid hormones (cholesterol derivatives)

Steroid hormones (cholesterol derivatives)

Steroid hormones (cholesterol derivatives)

Catecholamines (amino acid derivatives)

Source

Parenchymal cells of the zona glomerulosa

Parenchymal cells of the zona fascicu-lata (and to a lesser extent of the zona reticularis)

Parenchymal cells of the zona reticularis (and to a lesser extent of the zona fasciculata)

Chromaffin cells

Major Functions

Aid in controlling electrolyte homeostasis (act on distal tubule of kidney to increase sodium reabsorption and decrease potassium reabsorption); function in maintaining the osmotic balance in the urine and in preventing serum acidosis

Promote normal metabolism, particularly carbohydrate metabolism (increase rate of amino acid transport to liver, promote removal of protein from skeletal muscle and its transport to the 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

Induce weak masculinizing effect; at normal serum levels usually their function is insignificant

Sympathomimetic (produce effects similar to those induced by the sympathetic division of the autonomic nervous system)": 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

"The catecholamines influence the activity of glandular epithelium, cardiac muscle, and smooth muscle located in the walls of blood vessels and viscera.

blood vessels, and extend outside the gland to the splanchnic nerves innervating abdominal organs.

Chromaffin cells of the adrenal medulla have a secretory function

Chromaffin cells are organized in ovoid clusters and short interconnecting cords. The blood capillaries are arranged in intimate relation to the parenchyma. They originate either from the cortical capillaries or, as branches, from the cortical arterioles.

Ultrastructurally, the chromaffin cells are characterized by numerous secretory vesicles with diameters of 100 to 300 nm, profiles of rER, and a well-developed Golgi apparatus. The secretory material in the vesicles can be stained specifically to demonstrate histochemically that the catecholamines epinephrine and norepinephrine secreted by the chromaffin cells are produced by different cell types (Fig. 20.20). The TEM also reveals two populations of

0 0

Post a comment