Osteogenic culture or switch prevents endochondral ossification but addition of β-glycerophosphate does not. Representative hematoxilin-eosin stained slides of implanted pellets in immune deficient mice for 8 weeks. Primed chondrogenically bone, cartilage and marrow stroma are visible (Ai). For the switch 1 condition the chondrogenic medium was replaced during the last 7 days for osteogenic medium which resulted in cartilage-like tissue in the inside and undefined tissue on the outside (Bi). For the switch 2 condition β-glycerophosphate was added during the last 7 days of culture and bone, cartilage and marrow stroma are observed (Ci). When the chondrogenic primed pellets were implanted for 14 weeks only bone and marrow stroma were visible. For quantitative analysis all pictures were pseudo colored, red (bone), blue (marrow stroma) green (cartilage), undefined tissue (yellow) (Aii, Bii, Cii, Dii). Figure 2, E and F show Safranin O staining of in vitro chondrogenically cultured pellets retrieved after 8 week in vivo. Weakly positive staining demonstrates the presence of glycosaminoglycans within a cartilage matrix being degraded to make way for bone and marrow formation which surrounds the remnants of the cartilage matrix.