The prehatching development of the thyroid gland of the fathead minnow, Pimephales promelas (Rafinesque).

The prehatching development of the thyroid gland in the fathead minnow, Pimephales promelas, has been studied using histochemical and autoradiographic methods. The thyroid arises as a solid nodular bud from the pharyngeal epithelium at the posterior border of the hyomandibular junction and then grows ventrocaudally into a tractus thyroglossus from which the follicles eventually form. During subsequent development, the follicles exhibit an exponential increase in number, migrate from the subhyoid region, become diffusely distributed throughout the pharynx and at ectopic loci, and show an increase in the size of the follicular lumina with an accumulation of colloid and a decrease in follicular cell heights due to follicular emptying. Autoradiographic studies suggest that thyroid tissue traps and concentrates iodine prior to the appearance of histochemically demonstrable colloid. Histochemical observations suggest that the formation of thyroid follicles is preceded by the secretion of colloid in the intercellular space and into the tractus thyroglossus.