GDF2 and osteoporosis: One strategy to nullify SDA-Col I-induced adipogenesis and concomitantly promote (trans)differentiation of preadipocytes already present in bone, particular in osteoporosis treatment strategies, might be to combine SDA-Col I with retinoic acid (RA), since various forms of RA have been shown to repress adipogenic differentiation and cooperate with BMP2 and BMP9 in osteogenic (trans)differentiation in preadipocytes [44, 45].