Atypical thyroid follicles arising from an ultimobranchial-like cyst in a postnatal canine thyroid gland.

While investigating the intrinsic vasculature of the canine thyroid gland, atypical thyroid follicles were observed histologically, arising from a postnatal thyroid cyst of unknown origin, in stained sections of a male, 1-year-old, mesaticephalic dog. The animal was anesthetized, bled, killed by injection of Toxital directly into the heart, and perfused with 10% buffered neutral formalin solution. The thyroid tissue was fixed, embedded in paraffin, sectioned serially, and mounted on glass slides. Concordant with descriptions in the literature, this structure appeared to be a postnatal ultimobranchial-like cyst with direct connections to both ultimobranchial and mixed follicles. Seemingly, this observation has never been reported previously in the canine and would lend support to the hypothesis that ultimobranchial-like cysts may give rise to these atypical follicles. However, the authors are not totally convinced that all such thyroid cysts are solely ultimobranchial in orgin. A review of the literature supporting this contention is discussed.