Fig. 3. Growth-plate defects in Bmpr1a^CKO and Bmpr1a^CKO;1b^+/- mice. (A,B) Alcian blue-stained sections through E14.5 (A) and E16.5 (B) distal femurs. Restriction in the width of the growth plate at the junction between the resting and proliferative zones is observed in Bmpr1a^CKO and Bmpr1a^CKO;1b^+/- mice compared with wild-type. (C) H/E-stained sections through columnar zones of E16.5 wild-type, Bmpr1a^CKO and Bmpr1a^CKO;1b^+/- littermates reveal a failure to adopt a flattened cell shape and intercalate in mutants. (D) H/E-stained sections through the hypertrophic zones of E16.5 wild-type, Bmpr1a^CKO and Bmpr1a^CKO;1b^+/- littermates reveal the accumulation in mutants of cells with tightly packed nuclei (arrows), a characteristic feature of late hypertrophic chondrocytes. hyp, hypertrophic zone; ph, prehypertrophic zone; prol, proliferative zone; res, resting zone.