Development of thyroid hormone dependence for growth in the rat: a study involving transplanted fetal, neonatal, and juvenile tissues.

The role of thyroid hormones (TH) in the growth of rat tissue of different ages was investigated in a transplant system. Paws from 14- to 15-day-old fetal rats or pieces of xiphoid cartilage from 18- to 20-day fetuses and 2-, 9-, or 30-day-old rats were transplanted under the kidney capsule of control or thyroidectomized (TX) syngeneic adult female hosts and grown there for 11 days. Paw transplants grew equally well, and tissue differentiation was similar in control and TX hosts. The degree of ossification was measured by a histometric procedure and was found to be equivalent in paws that developed in situ until 4 days postpartum and in paw transplants of equivalent age grown for 11 days on the kidneys of euthyroid or TX hosts. These results show that growth and differentiation of the skeletal and other tissues of the paw are not dependent on TH during the transplant period, which encompassed the equivalent of the late fetal and first few days of neonatal life. Transplanted xiphoid cartilage was used to analyze growth and the development of TH dependence of a differentiated tissue which is relatively uniform in animals of different ages. In control and TX hosts, the growth capacity of transplanted xiphoids declined progressively with increasing donor age. The tissue also showed an age-related increase in dependence on TH for growth. The xiphoid transplants from the fetal rats grew equally well in the euthyroid and hypothyroid hosts, but those from 30-day-old rats showed a near-total dependence on TH for growth. Overall, the results indicate that rat supportive tissues begin to develop a dependence on TH for growth during the second week postpartum.