Fig. 4. The earliest tendon phenotype in Scx^-/- embryos is detected at E13.5, concurrently with the overt formation of tendons. (A) Schematic representation of the major flexor tendons of the forelimb: FDP tendons (green), the FDS (red), and the lumbrical muscles and tendons (blue). Section levels are marked by black lines: (1) digit level, (2) metacarpal-phalangeal joint, (3) mid metacarpal, (4) proximal metacarpal. (B-M) Arrowhead assignments are: yellow, flexor digitorum profoundus (FDP); red, flexor digitorium superficialis (FDS); blue, lumbrical tendons; pink, extensor digitorium communis (EDC); white, flexor vinculum (B,E) or the palmar metacarpal ligament (J). (B-G) Sections through the autopod of wild-type (WT) and Scx^-/- embryos at E18.5 highlighting the flexor and extensor tendons. The sections were taken at levels 1 (B,E), 2 (C,F) and 3 (D,G). (H,K) Scx in situ hybridization in limbs at E12.5 (H) and E14.0 (K), showing that, in wild-type mice, overt tendon formation occurs between E12.5 and E14.0. (I,L) ScxGFP detected by in situ hybridization in wild-type (I) and Scx^-/- (L) forelimbs at E13.5. The EDC phenotype is first detected at E13.5 in mutant mice. (J,M) Sections through wild-type (J) and Scx^-/- (M) forelimbs at E13.5 at a proximal metacarpal level (level 4 in A). The onset of the FDP phenotype occurs at E13.5. (N-Q) Cell proliferation in tendon cells of wild-type (N,O) and Scx^-/- (P,Q) embryos at E14.5 in sections directly proximal to the wrist. 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. (O,Q) Enlargements of the boxed areas in N and P, respectively. Arrowheads indicate tendons with BrdU-positive cells. (R-U) TUNEL staining in sections at digit level (level 1 in A) of wild-type (R,S) and Scx^-/- (T,U) embryos at E14.5. (S,U) Enlargements of the boxed areas in R and T, respectively.