Tendons are dense, fibrous tissues connecting muscle to bone, and their primary function is to transmit muscle forces to the appropriate skeletal elements, thereby enabling movement. In the limb, flexion and extension of the hand (autopod) and wrist are controlled by long tendons that insert into muscles in the arm (zeugopod) [1]. Although tendons are critically important in mediating joint movement, the cellular and molecular events underlying tendon formation remain largely unknown. Using the transcription factor Scleraxis (Sex), which labels all tendon progenitors, we previously showed that in the mouse limb bud, Sex-expressing tendon progenitors are first induced in the mesenchyme underneath the ectoderm at E10.5; at E12.5, progenitors are loosely organized between the cartilage condensations and developing muscles, condensing to form distinct tendons by E13.5 [2]. By El4.5, limb tendon patterning is largely complete, with continued elongation and deposition of matrix from this stage onward.
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