Electron microscopical and immunohistochemical study on parafollicular cell complex with reference to parafollicular cell as a paraneuron.

In mammalian species, parafollicular cells were usually disseminated among the follicles of the thyroid gland; but they occasionally formed fairly large clusters around the parathyroid IV and thymus IV. These cell clusters were called parafollicular cell complexes and were regarded as the remnant of the embryonic ultimobranchial body. The parafollicular cell complex contained follicular cells in addition to parafollicular cells. Colloid-containing follicles also occurred. In order to assess the developmental and functional relationship between follicular cells and parafollicular cells, the parafollicular cell complex in the dog was investigated by means of immunohistochemical method and electron microscopy. Follicular cells in the parafollicular cell complex showed various ultrastructures representing every developmental stage. The cells forming solid clusters without follicles were indistinguishable from the precursor cells of the parafollicular cells. Parafollicular cells occasionally formed narrow colloid-containing follicles. The results of the immunoperoxidase study indicated that the follicular cells in the parafollicular cell complex elaborated 19s thyroglobulin and stored it as colloid droplets in the follicular lumen. The antigenicity of the colloid droplets in the parafollicular cell comples was much stronger than that of the thyroid follicles. Based on these results, the origin of the parafollicular cells was discussed with special reference to the paraneuron concept of FUJITA (1976). The theory that both follicular and parafollicular cells were of endodermal origin seemed valid.