FIG. 4.

Endochondral bone formation from mESCs in vitro. mESCs were differentiated with 1,25-OH₂ vitamin D₃ toward osteoblasts, which appear black in phase contrast microscopy. Chondrogenic cultures were treated with BMP-2 from day 3 of differentiation onward. Interestingly, chondrogenic cultures can undergo mineralization when additionally triggered with the osteogenic inducer 1,25-OH₂ vitamin D₃. However, a window of opportunity seems to exist that falls between day 15 and day 20 of differentiation.⁹²