Fig. 8. Gdf5 expression in the joint space (arrows) of heterozygous (A,C) and homozygous (B,D) ColII-FGFR31^ach line 2 mice at E13.5. (A,B) Hematoxylin and Eosin stained knee joint tissue. Inserts in (A,B) are the dark-field images showing transgene (hGH) expression. (C,D) Dark-field images showing Gdf5 expression. (C) In heterozygous tissue, Gdf5 is robustly expressed in the joint space and is complementary to the transgene expression domain. (D) In homozygous tissue, Gdf5 expression is dramatically reduced, even though ColII expression identifies the presumptive joint space. This indicates that loss of Gdf5 expression precedes complete joint fusion.