The rat adrenal medulla.

Adult adrenal medullary cells, in many strains of rats, develop diffuse and nodular hyperplasia and neoplasia under a variety of conditions. Both endogenous and exogenous factors affect the development of these proliferative changes. The former include the animals' strain, age, and sex. The latter include drugs and other environmental agents, diet, and perhaps stress. Adrenal medullary neoplasms which arise under diverse circumstances often closely resemble each other both morphologically and functionally, and exhibit characteristics of immature chromaffin cells. Recent data indicate that normal, mature-appearing epinephrine- and norepinephrine-type chromaffin cells are able to divide, and suggest that signals which regulate chromaffin cell function also regulate cell proliferation. Prolongation of these signals or superimposed abnormalities might initiate pathological proliferative states. It remains to be determined whether the mechanisms which promote or prevent cell proliferation in the adult adrenal are related to those involved in normal development.