(A–E) Control femurs at 1 wk (A) and 3 wks (D) of age. Boxed areas in (A) are enlarged in (B) and (C). Boxed area in (D) is enlarged in (E). Blue arrows in (C) point to osteoblast cells lining the surface of cortical bone. Pink arrows in (G) show similar cells to those in (C) in Bmp2C/C; Bmp4C/C; Prx1::cre femurs. Red star in (A) and (D) marks the secondary ossification center. Pale blue–stained tissue in (E), marked by red arrows, is trabecular bone. bm, marks the bone marrow cavity in (D).