Fig. 4. Scleraxis is expressed in putative limb tendon progenitors. Scleraxis expression in developing wings and legs was analyzed by whole-mount and section in situ hybridization. All limbs are shown in dorsal view. (A,B) Scleraxis is first detected in stage 21 leg buds in a superficial proximomedial domain and expression in this domain is enhanced by stage 23. (C) In situ hybridization to sections of a leg bud at stage 23 illustrates that the expression is superficial in both the dorsal and ventral mesenchyme. (D) This expression domain overlaps partially with the domain of migrating myoblasts, detected in an adjacent section by expression of Pax3. By stage 25, the scleraxis-expressing cells coalesce to form more discrete, limb-specific patterns in the leg (E) and wing (H). By stage 27, the dynamic expression of scleraxis continues to change in both leg (F) and wing (I). The first fibrous tendon elements can be seen in the proximal leg bud (F), concurrent with the onset of scleraxis expression in the forming autopod. The expression is elaborated by stage 29 to include in both leg (G) and wing (J) much longer tendon fibers and the phalangeal tendon blastemas. (K-M) To determine the spatial relationship between the scleraxis-expressing cells and the related tissues alternating transverse limb bud sections were hybridized with probes to scleraxis, a muscle marker, MyoD, and an early cartilage marker, autotaxin. In stage 27 leg buds, the domain of scleraxis-expressing cells (M) is still largely overlapping with that of the now differentiating MyoD-expressing cells (L). However, neither overlaps with the differentiating cartilage elements in the deeper limb mesenchyme (K).