Fig. 6. Hox-negative neural crest skeletal progenitor cells assume a Hox-positive status when placed into a Hox-positive environment. (A,B) The adult tibial periosteum expresses Hoxa13 and Hoxa11. (C) At post-surgical day 3, the tibial injury site is occupied by Hoxa11-expressing cells. (D,E) By contrast, the mandibular periosteum is devoid of Hoxa11 expression (D), and injury does not induce Hoxa11 expression (E). (F,G) Hox-positive tibial periosteum maintains its expression when transplanted into a Hox-negative mandibular injury site. (H) GFP antibody staining labels Hox-negative mandibular periosteum in a tibial injury site. (I) Adjacent section shows Hoxa11 expression in the transplanted mandibular periosteum (dashed line). (J,K) Skeletal progenitor cells derived from β-actin GFP tibial periosteum express GFP (J) and Hoxa11 (K). (L,M) When these GFP-positive, Hox-positive cells were co-cultured with Hox-negative neural crest-derived cells (L) the formerly Hox-negative mandibular cells begin to express Hoxa11 (M). bm, bone marrow; ca, cartilage; mn, mandible; po, periosteum; tib, tibia. Scale bar: 50 µm in A,B,D,F,G; 100 µm in E; 200 µm in C,H,I.