The cysts in C cell complexes of dog thyroids studied by immunoperoxidase staining and autoradiography.

The C cell complexes in the dog thyroid are the remnants of ultimobranchial bodies which retain fetal characteristics, and contain cyst structures showing various sizes, shapes and liminal contents. This study deals with these cysts with regard to their morphology, immunoperoxidase reactions to anti-calcitonin, anti-19S thyroglobulin and anti-C-thyroglobulin antisera, and ability to incorporate radioiodine. Small cysts lined by densely packed small cells are devoid of a stainable substance in their lumina and show no reaction to any of the antisera. They are regarded as immature structures. The large cysts, covered mainly by a single layer of cuboidal cells, store varying amounts of secretory products in their lumina. The secretory products showing colloid-like, flocculent, or granular features are intensely stained with the anti-19S thyroglobulin and anti-C-thyroglobulin antisera. However, they scarcely accumulate silver grains after injection of Na125I. C cells are distributed in the cyst epithelium but the cystic contents reveal no immunoreaction for calcitonin. In conclusion, the present study indicates that the cyst structures in C cell complexes synthesize and store a thyroglobulin-like glycoprotein but are not directly involved in thyroid hormone synthesis.