BEGINNING OF FUNCTION IN THE THYROID OF THE FETAL RAT

Abstract

THE DETERMINATION of the time of beginning of function in embryonic endocrine glands has long interested embryologists and physiologists, especially since the data accumulated so far attest to the fact that hormones are imporatnt morphogenetic agents in developing organisms as well as in adults. The spectacular participa-tion of the thyroid in the metamorphosis of larval amphibia is an example of this. The postponement of cachectic symptoms in thyroidectomiz;ed pregnant mammals until delivery of full term fetuses provides the hint that fetal thyroids may contribute sig-nificantly to the physiologic economy of the mother. In the present experiments radio-active iodine1 was injected into rats in a known stage of pregnancy, and the developing thyroids of the fetuses were examined by the radio-autograph method (1, 2) to determine the presence of accumulated iodine and its distribution. The iodine was given subcutaneously in a single injection of 1.0 cc. of physiologic saline solution containing 35 microcuries of radio activity. The mass of iodine was negligible, being less than 0.01 mg. (as Nal). The radio-activity in the preparation used was due almost exclusively to I131 an isotope of iodine having a halflife-of 8 days.