Fig. 3. Jaws^-/- digits develop ectopic joints. (A) Longitudinal cavities lacking Alcian Blue staining form in Jaws^-/- digits (E18.5). Higher-magnification views of digit 3 are shown to the right of each limb, with arrows indicating joint position and orientation. The arrowhead indicates failed knee joint formation proximal to a lack of tibial ossification in the Jaws^-/- hindlimb. (B) Whole-mount in situ hybridization for Gdf5 expression (E14.5). (C) Histological and in situ hybridization analysis showing longitudinal expression of joint interzone markers in individual Jaws^-/- hindlimb digits (E14.5). (D) Exclusion of Bmpr1b RNA and collagen 2 protein expression from ectopic Jaws^-/- joints (E14.5). (E) Ectopic joints express Gdf5 but not markers of hypertrophic chondrocytes (Col10a1), osteoblasts (Runx2), or tendons and ligaments (Scx) (E18.5 forelimb digits). AB/vK, Alcian Blue/von Kossa. (F) Defective cavitation of E18.5 ectopic joint (arrow), evidenced by the absence of CD44 immunostaining (arrowhead, wild-type staining).