Fig. 7. Scheme of interactions between embryonic origin and Hox status during adult bone regeneration in mouse. (A,B) In a homotopic tibial or mandibular graft, both embryonic origin and Hox code are equal (+), which leads to osteogenic differentiation of osteoprogenitor cells. (C) When mandibular periosteum is transplanted into a tibial injury, the embryonic origin of the graft and recipient are different (-). Hox expression, however, is the same in graft and recipient because the grafted cells, which are initially Hox-negative, turn on Hox expression when placed into the Hox-positive environment. (D) In the mandibular heterotopic graft, the embryonic origin of graft and recipient are different, and the Hox status of the graft and host remain unequal, which results in chondrogenic differentiation of the osteoprogenitor cells. mn, mandible; tib, tibia.