activin one of two gonadal glycoproteins related to transforming growth factor-b (the other is inhibin); present in two forms in human gonads, it exists as a dimer of inhibin bA or bB chains, linked by disulfide bonds. Activin A is a dimer of bA chains; activin AB is a dimer of bA and bB chains. A potent selective stimulator of FSH secretion by the anterior pituitary gland, not via GnRH receptors, it modulates induction of hemoglobin accumulation and proliferation of erythroid progenitor cells in human bone marrow culture, and is important in embryonic axial development.
