Fig. 2. Mesenchyme determines the timing of intramembranous ossification. (A) In quck mandibles, quail mesenchyme maintains its faster timetable for bone formation within the slower environment of duck hosts, based on histological detection of matrix (arrow) using Osteoid stain (blue). (B) Staining for bone is coincident only with Q¢PN positive cells (i.e. quail-derived black cells, arrow) on the donor side. (C) Reciprocal transplants that generate duail result in abundant quail host-derived bone in chimeric mandibles (arrow). (D) On the duck donor-derived side, bone has yet to form at 6 days of culture (dashed outline). Note the sporadic Q¢PN-positive angioblasts and endothelial cells, which are derived from quail host mesoderm, among duck donor mesenchyme (Q¢PN-negative). (E,F) After 8 days of culture, which is equivalent to the time required for bone to form in control duck, duck-derived mesenchyme (i.e. Q¢PN-negative) stains positively (dashed outline). (G,H) Whole-mount in situ hybridization reveals that Runx2 and Msx1 are upregulated on the donor-derived side.