Congenital anomalies in the thyroid gland of Wistar rats: Ultimobranchial cyst and ectopic thymus

Ultimobranchial cyst (three) and ectopic thymus (eight) were recorded in the thyroid glands of over 2000 Wistar rats necropsied during different repeated dose toxicity studies. The age of these rats ranged between three to six months. No lesion of pathological significance was recorded grossly. Microscopically, the ultimobranchial cyst was lined by flattened squamous epithelium and the lumen was filled up with a mixture of keratin and tissue debris. Ectopic thymus was located either within the parenchyma of thyroid gland or was lying in the adjacent tissue. Histological features compared well with normal histology of thymus. Both cortical and medullary components were well discernible. The histogenesis is discussed and considered to be of congenital origin.