Fig. 1. Wnt5a expression and delayed cell differentiation in the developing cartilage of the Wnt5a mutant. Sections of developing long bones were stained according to the Weigert-Safranin staining procedure. Proliferative chondrocytes are red. Gene expression was detected by in situ hybridization with ³⁵S-labeled radioactive riboprobes on consecutive sections of the developing humerus at 13.5 dpc. (A) Hypertrophic chondrocytes (indicated by arrows) were not detected until 18.5 dpc in the radius (R) and ulna (U) region of the mutant (right). Hu, humerus. (B) Upper panel: osteoblast and blood vessel invasion (arrow) was incomplete in the mutant humerus. The hypertrophic region (double headed arrows) in the mutant (right) was longer than that in the wild-type (left) embryo, although the mutant humerus was much shorter overall. Lower panel: the boxed bone collar regions are shown at higher magnification. The bone matrix indicated by the arrows was reduced in the mutant. P, proliferative chondrocytes; H, hypertrophic chondrocytes. (C) Wnt5a was expressed at the boundary of proliferative and prehypertrophic chondrocytes where Ihh and Cbfa1 were expressed. Wnt5a expression flanked the expression domain of Col10a1. In the perichondrium, Wnt5a was expressed in the outer layer and Cbfa1 was expressed in the inner layer.