Fig. 4. Wnt9a and Wnt4 act redundantly in maintaining joint integrity. (A) Van Kossa/Alcian Blue/Eosin-stained sections through the ankle (a,d), knee (b,e) and foot region (c,f,g) of wild-type (=Wnt9a^+/-; Wnt4^+/-) and Wnt9a^-/-;Wnt4^-/- newborn littermates. (a) Ankle joint. (b) Knee joint. (c) Foot region. (d,e) Synovial chondroid metaplasia in the ankle joint (d, arrow) and in the joint capsule of the knee (e, arrow) of Wnt9a^-/-;Wnt4^-/- mutants. (f) Fusion between the intermediate cuneiform and navicular tarsal elements (arrow). (g) Fusion between the calcaneus and cuboid tarsal elements (arrow) and synovial chondroid metaplasia in the joint capsule ligament of digit I (asterisk). (B) Sections through the wrist regions of newborns, E13.5 and E15.5 embryos. (a,e) Van Kossa/Alcian Blue/Eosin staining, showing normal arrangement of carpal elements in wild type (a) and fusion of the three carpal elements 2, c and 3 in Wnt9a^-/-;Wnt4^-/- mutants (e). (b,f) Gdf5 staining on wrist sections. The three carpal elements 2, c and 3 are separated in wild type (b) and Wnt9a^-/-;Wnt4^-/- mutants at E13.5 (f). (c,g) Gdf5 staining on wrist sections. The three carpal elements are separated in wild type (d) but are fused in the Wnt9a^-/-;Wnt4^-/- mutants (g). (d,h) Col2a1 staining. The carpal elements in wild type (d) but are fused in Wnt9a^-/-;Wnt4^-/- mutants (h).