Influence of an early grafted adenohypophysis on the thyroid of the chick embryo.

A hypophyseal pars distalis from an 11-day-old chick embryonic donor was implanted in the extraembryonic coelom of a 3-day-old recipient in order to evaluate the reactions of the thyroid of the host embryo and thus the functional capabilities of the grafted pituitary related to the thyroid. The graft did not modify the general growth of the embryo, but strongly influenced thyroidal development. The volume, colloid content, and radioiodide uptake of the thyroid gland were significantly increased, from Day 10 of incubation, compared with controls, as was the development of cytological structures, studied with the electron microscope. However, all these characteristics progressively returned to normal values between Days 14 and 17, in spite of the presence of this additional hypophysis. The grafted hypophysis thus acted autonomously during a first period resulting in increased stimulation of the thyroid. However, from Day 14, the influence of the graft decreased progressively and disappeared after Day 16. This signified the loss of its autonomy, and the mechanism of this loss, particularly the possible role of the hypothalamus, is discussed.