Activin, a peptide growth factor, is a member of the transforming growth factor-β (TGF-β) superfamily. It was originally isolated from follicle fluid as a gonadal hormone that stimulates follicle-stimulating hormone (FSH) secretion and is identical to EDF, the erythroid differentiation factor (which stimulates erythroleukemia cells to differentiate into hemoglobin-producing cells) (1 ). It also appears to be related, if not identical, to the so-called “vegetalizing factor”described originally by Tiedemann and colleagues, which can induce amphibian embryonic tissue rudiments to display various differentiation patterns (reviewed inref. 2). In addition, it appears to be identical to the XTC factor isolated by Smith et al. (3 ) from transformedXenopusfibroblasts. Thus, the identification of activin as a potential morphogen in amphibian embryos (4 ) solved several mysteries surrounding the puzzle regarding the molecular nature of various hitherto ill-characterized “inducing substances.“
