Ontogenic changes in multiplication-stimulating activity binding to tissues and serum somatomedin-like receptor activity in the ovine fetus

Fetal serum somatomedin-like receptor activity and the specific binding of Multiplication-stimulating activity to preparations of a variety of fetal and maternal tissues were measured in sheep throughout the latter half of gestation. Serum activity and the specific binding to fetal liver and lung preparations increased as gestation progressed, while the binding to fetal heart and placenta, and maternal kidney decreased. Most tissues exhibited greater specific binding of Multiplication-stimulating activity than insulin at term. At all ages fetal liver had the highest and fetal brain exhibited the lowest binding of Multiplication-stimulating activity. The results indicate that a factor which is similar to Multiplication-stimulating activity is present in ovine serum and that ovine tissues possess receptors for this factor. The ontogenic changes observed suggest that this factor is involved in the regulation of growth of the fetus.