Ganglioneuromas of the thyroid gland in a colony of Sprague-Dawley rats.

Ganglioneuromas of the thyroid gland of Sprague-Dawley rats were found in 7.2% of 698 rats used in two, 2-year oncogenicity bioassays. The incidence of the tumor was unrelated to treatment or sex. Of the 52 ganglioneuromas of the thyroid gland identified in 50 animals, 40 (80%) had coincident C cell proliferations in the same or contralateral lobe. Ganglioneuromas were contiguous or commingled with C cell proliferations in 63.5% of the cases. The ganglioneuromas consisted of large ganglion cells and, in a few cases, cells thought to be less differentiated neuronal precursors, in a matrix of neurites and Schwann cells. They grew, infiltrating and expanding in the thyroid parenchyma, and did not metastasize. Immunohistochemical staining for calcitonin, S-100 protein, and neurofilaments, as well as electron microscopy, were used to further characterize the tumors. The close association of ganglioneuromas with C cell tumors supports the theory of neural crest origin of C cells and provides a parallel to the association of ganglioneuromas and pheochromocytomas in the adrenal medulla. This is the first report of ganglioneuromas occurring in the thyroid gland or occurring as a common entity in any species.