Ganglioneuromas in the adrenal medulla of F344 rats

Of 60,048 aging F344 rats, 28 had ganglioneuromas of the adrenal gland. The rats were between 58 and 122 weeks old; 25 of them between 93 and 122 weeks when the tumors were found. The neoplasms were composed of large or small typical ganglion cells with a large pale nucleus and a prominent, often eccentric, nucleolus. The ganglion cells usually were distributed throughout the medullary tumor and surrounded by various numbers of supporting cells, including Schwann's cell types, and capsular or satellite cells. Often there was no clear-cut border between ganglion cells and neoplastic pheochromocytes. Electron microscopy of re-embedded tissues showed neurosecretory granules and cytoplasmic areas with an extensive rough endoplasmic reticulum (Nissl substance) in the ganglion cells and axons. The Schwann's cells were surrounded by a basement membrane and formed long cytoplasmic extensions around the axons. Almost all ganglioneuromas were associated with neoplastic pheochromocytes which usually were located at the edge of the tumors.