Degree of osteogenesis in cultures treated with chondroinducing medium (BMP, supplement combination G), osteoinducing medium (VD3 as described in ref. 18) and control ES medium (ctrl). Osteoblast phenotype and mineralization could be increased in chondrocyte cultures by adding VD3 to chondroinducing supplements at day 20 (BMP+VD3). (A) Extent of mineralization as measured by Ca2+ content. **P < 0.01; ***P = 0.001. (B) RT-PCR for chondrocyte-specific and osteoblast-specific marker genes as well as Collagen X as a marker for the hypertrophic state of chondrocytes. (C) Morphology as seen in phase contrast. Black appearing cells were identified as being mineralized osteocalcin expressing osteoblasts previously . No such cells can be seen in control (ctrl) and chondrocyte (BMP) but in osteoblast (VD3) and rescued osteoblast cultures (BMP+VD3).