BEGINNING OF FUNCTION IN THE THYROID OF THE FETAL RAT

Abstract
Functional ability to store I begins in the 18- to 19-day-old rat embryo. This was shown by treating rats at known stages in pregnancy with radioactive 1. Developing thyroids of the fetuses were examined to determine whether I has been selectively accumulated. The appearance of the ability to store I in embryonic thyroid tissue may be correlated with the first differentiation of follicles with lumina. Follicular differentiation begins in the peripheral portions of the thyroid and gradually extends into the deeper portions.