Fig. 6. Exogenous BMP4 induces premature differentiation of bone, whereas Noggin treatment delays differentiation of bone. (A) HH23 Quail mandibles treated with BMP4 were cultured for 24 hours and processed for whole-mount in situ hybridization. Control contralateral sides were treated with BSA. (B) Msx1 is expressed medially. Upon treatment with BMP4, Msx1 expression expands laterally. (C) Twist1 is expressed medially but BMP4 treatment causes a downregulation. (D) To evaluate the extent of BMP4-induced Runx2, mandibles were treated at HH21, which is a stage that lacks endogenous Runx2 expression. Runx2 is slightly upregulated following BMP4 treatment. (E) Quail explants were harvested at HH23, treated with BMP4, and cultured for 5 days. (F) Section through a quail mandible treated unilaterally with BMP4 at HH23, cultured for 6 days, and stained with Osteoid (dark blue bone within dashed box). (G) Pixels comprising bone (red) were quantified to determine matrix volume (MV). (H) Pixels within a given condensation (red line) were quantified to determine condensation volume (CV). (I) Following 24 hours of culture, Noggin treatments inhibit Msx1 expression in explants. (J) In control quail mandibles, Noggin delays bone formation prior to 48 hours of culture, but afterwards bone can form (arrow). (K,L) In quck chimeras, this period of responsiveness occurs before 24 hours and bone (arrow) forms thereafter due to the presence of the faster-developing donor quail cells (Q¢PN-positive). Tx, treatment.