The Development of Thyroid-Stimulating Hormone-Releasing Factor Activity in the Neonatal Rabbit

Abstract

Nine-day chick embryos received grafts of hypothalamus, adenohypophysis, thyroid, and fat (controls) from 0-, 7-, 14-and 21-day-old rabbits to the chorio-allantois. In addition, aqueous rabbit adenohypophysial and hypothalamic extracts were given to 1-day-old chicks. Animals injected with thyrotrophin and saline served as controls. On the 5th day of incubation of treated embryos, or 24 h after the administration of the extracts, the chicks were killed and their thyroids studied histologically.The hypothalamic grafts or extracts activated the chick thyroid gland and a similar trend was found in adenohypophysial or thyroid-stimulating hormone-treated chicks. The degree of activation of the embryo chick thyroid gland was related to the age of the donor rabbit. In general, thyroidstimulating hormone-releasing factor activity in hypothalamic heterografts or extracts appeared first in 14-day-old rabbits and was correlated with increasing thyrotrophic potency of the rabbit adenohypophysis. a comparison was made between the newborn pig and rabbit, as representatives of poikilothermic and homeothermic animals at birth, with respect to the hypothalamopituitary-thyroid system (Slebodzinski & Srebro, 1968). It was shown that in the