Ontogenesis of thyroid hormone receptor in foetal lambs

Affinity and concentration of T3 receptor sites have been measured in nuclear extracts from the brain, lung, and liver of foetal lamb tissues at 50, 82 and 100 days of gestational age. Control experiments indicated that the concentration of sites was similar when nuclear extracts or purified nuclei were used, and that maximal binding capacity was obtained after 2 h of incubation at 22 degrees C. The pattern of receptor binding affinity when different thyroid hormone analogs were used in competition assays with [125I]T3 was T3 greater than 3,5,3'-triiodothyroacetic acid (Triac) greater than T4 in the lung and brain. In the liver, Triac had the same affinity as T3. The sedimentation coefficient of the receptor was 3.6 S in lung. There were minor changes of receptor affinity in the brain, but not in the lung or liver, during development with the highest value at 82 days. Receptor concentration increased twice from 50 to 82 days. Since in the brain this is the period of neuroblast proliferation, the results suggest that thyroid hormone is required for proper foetal lamb development and, in particular, for neuroblast proliferation and/or differentiation.