Figure 2019

Diagram illustrating the organization of the cells within the adrenal gland and their relationship to the blood vessels. Refer to Figure 20.18 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.)

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 preganglionic sympathetic axons that synapse with each chromaffin cell

Epinephrine and norepinephrine account for less than 20% of the contents of the medullary secretory vesicles. The vesicles also contain large amounts of soluble 48-l<Da proteins, called cbromogranins, that appear to impart the density to the vesicle contents. These proteins, along with ATP and Ca2+, may help to bind the low-molecular-weight catecholamines and are released with the hormones during exocytosis. The catecholamines, synthesized in the cytosol, are transported into the vesicles through the action of a

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