Somatomedin and Thyroid Hormones in the Developing Chick Embryo

Abstract

Abstract

Embryonic chick cartilage is used to detect somatomedin activity, however, the presence of somatomedin in the developing embryo has not been reported. We have shown that thyroid hormones stimulate somatomedin activity in the rat. This study compares changes in thyroid hormones and somatomedin in the developing chick embryo. Somatomedin activity, thyroxine (T₄), and triiodothyronine (T₃) were determined in sera obtained from embryonic chicks on Days 10 through 19 of incubation and from young adult chickens. Somatomedin was determined by bioassay which measures the sulfate incorporation into pelvic cartilage of 12-day-old chick embryos. T₄ and T₃ were determined by radioimmunoassay. Somatomedin activity was not detected in 10- to 12-day-old embryos, was measurable at 13 days, and increased to a peak value at 15 days (potency of 0.93 compared to normal rat serum value of 1.0). No somatomedin was detected in the serum immediately prior to hatching or in young chicken serum. T, was not detected until Day 15 (0.42 μg T₄/100 ml) and increased to 1.39 μg T₄/100 ml on Day 19. T₃ was present in the serum throughout the study, with an increase occurring on Day 14 (51 ng T₃/100 ml). The appearance of somatomedin coincides with collagen formation and ossification in the chick embryo skeleton. The changes in thyroid hormone concentrations apparently do not influence somatomedin in the chick embryo as has been demonstrated in the rat.