Fig. 6. Type II collagen expression and cell morphology in the knee joint of homozygous ColII-FGFR31^ach mice at E13.5 and E14.5. (A) At E13.5, the presumptive joint position (indicated by an arrow) is well defined by a collagen II-negative zone in wild-type tissue. (B) The presumptive joint space in tissue from homozygous ColII-FGFR31^ach transgenic mice has diffuse type II collagen expression, indicating the initiation of chondrification of the joint interzone. (C,D) Cell morphology in the developing joint. (C) Wild-type E13.5 joint tissue showing elongated mesenchymal cell shape. (D) Homozygous ColII-FGFR31^ach line 2 transgenic tissue showing a rounded cell morphology. The presumptive joint space is located between the arrows. (E) The type II collagen-free region (arrow) in the developing knee joint in the wild-type animal is well established by E14.5. (F) The knee joint (arrow) is completely replaced by type II collagen-expressing cells in homozygous ColII-FGFR31^ach line 2 tissue.