Piriform sinus fistula and the ultimobranchial body

Piriform sinus fistulae are an underlying abnormality common in patients with acute suppurative thyroiditis. The fistulae arise from the hypopharynx, and end in or adjacent to the thyroid lobe. These congenital fistulae seem to be remnants of one of the pharyngeal pouches in embryonic development, but their exact origin is still controversial. Resected specimens of the thyroid glands and fistulae from 15 patients were examined immunohistochemically with rabbit antisera to human calcitonin and thyroglobulin. The fistulae were lined by squamous, columnar or ciliated epithelium, and sometimes formed branches in the thyroid lobe. Near the branches solid cell nests existed. Mucous glands, follicular structures and thymic tissue were found in the fistula. The follicular structures stained for thyroglobulin. Immunostaining for calcitonin revealed aggregates of many C cells in the thyroid near the fistula. A few calcitonin-positive cells were also found in the fistula. These findings, along with the anatomical relation of the fistulae to major structures of the neck, strongly suggest that the fistulae are remnants related to the ultimobranchial body, and that the fistulae trace the migration route of the ultimobranchial body to the thyroid gland.