The central portion of the adrenal gland, the medulla, is composed of a parenchyma of large, pale-staining epithelioid cells called chromaffn cells (medullary cells), connective tissue, numerous sinusoidal blood capillaries, and nerves. The chromaffin cells are, in effect, modified neurons (Folder 21.5). Numerous myelinated, presynaptic sympathetic nerve fibers pass directly to the chromaffin cells of the medulla (see Chapter 12, page 378). 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 postsynaptic 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 blood vessels, and extend outside the gland to the splanchnic nerves innervating abdominal organs.
FIGURE 21.20 • Diagram illustrating the blood supply to the human adrenal gland. The region of the capsule, the zones within the cortex, and the medulla are indicated. (Modified from Warwick R, Williams PL, eds. Gray’s Anatomy, 35th ed. Edinburgh: Churchill Livingstone, 1973.)
Chromaffin cells of the adrenal medulla have a secretory function.
Chromaffn 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. 21.22). The TEM also reveals two populations of chromaffin cells distinguished by the nature of their membrane-bounded vesicles:  One population of cells contains only large dense core vesicles. These cells secrete norepinephrine.  The other population of cells contains vesicles that are smaller, more homogeneous, and less dense. These cells secrete epinephrine.
Exocytosis of the secretory vesicles is triggered by release of acetylcholine from presynaptic sympathetic axons that synapse with each chromaffin cell.
secretes: secretes: secretes: secretes:
FIGURE 21.21 • Diagram illustrating the organization of the cells within the adrenal gland and their relationship to the blood vessels. Refer to Figure 21.20 for identification of the blood vessels. The ultrastructural features of the basic cell types and their secretions are noted. (Modified from Warwick R, Williams PL, eds. Gray’s Anatomy, 35th ed. Edinburgh: Churchill Livingstone, 1973.)