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EARLY efforts to purify a bone-derived growth factor from rat bone culture medium revealed its biological and biochemical similarity to transforming growth factor-β type 1 (TGF-β1) extracted from human blood platelets. Cartilage inducing factors (CIF-A and CIF-B) from bovine bone matrix were then identified as TGF-β isoforms, and it soon became evident that bone cells synthesize TGF-βs and that bone matrix was a major repository of these factors in the organism. The TGF-β supergene family comprises at least five closely related proteins termed TGF-β1 through TGF-β5 in addition to a growing number of more distant but highly homologous growth regulators. Several of the closely related TGF-β isoforms occur in many animals, and appear to control development, growth, and differentiation in humans as well as a variety of other species. More importantly from the skeletal tissue perspective, some of the less closely related gene family members, activin-A and the bone morphogenetic proteins (BMPs), are also expressed by bone cells, stored in bone matrix, and increase bone cell activity in vivo and in vitro.
Horowitz et al. (Tue,) studied this question.