Fig. 5. The onset of the tail tendon phenotype in Scx^-/- embryos occurs during the transition from progenitors to organized tendons at E13.5. (A-I) In situ hybridization to detect the Scx transcript (A,B) and ScxGFP transcript (F,G) in tails at E12.5 (A,F) and E13.5 (B,G) and ScxGFP in later stages (C-E,H,I). The rostral end of the tail is pointing right in A,B,F,G and pointing up in C,D,E. (C,E,H,I) Yellow arrowheads show the annulus fibrosus, pink arrowheads show tail tendons and white arrows show the insertion of individual tendons. (A-D,H) The development of tail tendons in wild-type (WT) embryos: (A,B) Scx in situ hybridization of whole tails. Black arrowheads show the splitting of the syndetome at E12.5; (C,D) detection of expression of the ScxGFP reporter in skinned tails at E18.5 (C) and at P14 (D); (H) cross section of a tail at E15.5. (E,F,G,I) The tail tendon phenotype in Scx^-/- embryos. (F,G) In situ hybridization of tails to detect the transcript of the ScxGFP reporter. (E,I) The later tail phenotype, detected by ScxGFP fluorescence in a whole tail at E18.5 (E) and in a cross section at E15.5 (I). (J-M) Cell proliferation in tail tendons of wild-type (J,K) and Scx^-/- (L,M) embryos at E13.5. BrdU was detected using a DAB stain and the images were overlaid with the ScxGFP signal from an immediate alternate section to highlight the tendons. (K,M) Enlargements of the boxed areas in J and L, respectively. Arrowheads show tendons with BrdU-positive cells.