Fig. 6. Lack of orthotopic bone formation at E18.5. In situ hybridization using ³⁵S-labeled riboprobes against Ap (A1,A2), Bsp (B1,B2), Osx (C1,C2) and Oc (D1,D2), on longitudinal sections through the humerus from E18.5 wild type (A1-D1) and DKO (A2-D2) embryos. Shown are the distal halves of the humerus. Green arrows indicate either signals in the perichondrium (A1-D1) or the corresponding regions missing the signals (A2-D2). Black double-headed arrows (A2-D2) indicate the widened hypertrophy zone. Green asterisks (A2-D2) indicate the ectopic bone. BC, bone collar; PH, prehypertrophic zone; H, hypertrophic zone; PS, primary spongiosa.