Analysis of the tendon cell fate using Scleraxis, a specific marker for tendons and ligaments

Little is known about the genesis and patterning of tendons and other conne ctive tissues, mostly owing to the absence of early markers. We have found that Scleraxis, a bHLH transcription factor, is a highly specific marker fo r all the connective tissues that mediate attachment of muscle to bone in c hick and mouse, including the limb tendons, and show that early scleraxis e xpression marks the progenitor cell populations for these tissues. In the e arly limb bud, the tendon progenitor population is found in the superficial proximomedial mesenchyme. Using the scleraxis gene as a marker we show tha t these progenitors are induced by ectodermal signals and restricted by bon e morphogenetic protein (BMP) signaling within the mesenchyme. Application of Noggin protein antagonizes this endogenous BMT activity and induces ecto pic scleraxis expression. However, the presence of excess tendon progenitor s does not lead to the production of additional or longer tendons, indicati ng that additional signals are required for the final formation of a tendon . Finally, we show that the endogenous expression of noggin within the cond ensing digit cartilage contributes to the induction of distal tendons.