An electron microscopic study on follicular formation and TSH sensitivity of the fetal rat thyroid gland in organ culture.

Thyroids of 15-day rat fetuses prior to culture had no follicles. After 2 days in culture, regardless of added TSH in the medium, thyroids had small, colloid-storing follicles. In the follicular cells, the rER and Golgi apparatus were somewhat well developed as compared with the thyroids prior to culture. Thyroids of 17-day fetuses prior to culture were first inlaid with follicles. With TSH, 2-day cultures of 17-day thyroids showed a more marked dilation of rER and a further development of Golgi apparatus than those without TSH. These results indicate electron microscopically that TSH has no effect on the first formation of follicles as well as on the initial development of follicular cells, but that TSH can promote the further development of follicular cells themselves of once-developed follicles.