Fig. 3. Mesenchyme sets the timing of tissue interactions required for osteogenesis. (A) HH23 quail mandibles cultured for 6 days are positive for Osteoid (blue). (B,C) By contrast, quail mesenchyme harvested without epithelium at HH25 and HH26, and cultured for up to 8 days forms cartilage but not bone. (D) By HH27, mesenchyme cultured without epithelium makes bone (arrow). (E,F) Mandibles harvested from quail at HH25 and cultured with and without epithelium for 24 hours express Col2a1 in Meckel's cartilage as shown in lateral view. (G) HH25 mesenchyme cultured with epithelium for 24 hours expresses Runx2 (arrow). (H) When mandibles are cultured without epithelium, Runx2 expression is significantly downregulated (asterisk). (I,J) Quck mandibles harvested at HH25 and cultured for 6 days without epithelium form bone (arrow) due to faster developing quail donor mesenchyme (arrow).