C-cell differentiation in the wall of an aberrant ultimobranchial sinus in the thyroid gland of an old rat.

The ultimobranchial sinus is an exceptional finding for representing the first spontaneous abnormality in the development of UBB reported in rats, and the opportunity to observe sustained C-cell differentiation from stem cells in an old rat. These findings are consistent with a common origin of both C cells and follicular cells from UBB.