Scleraxis

The scleraxis protein is a member of the basic helix-loop-helix (bHLH) superfamily of transcription factors. In humans, scleraxis is encoded by the SCX gene.

Function

It is thought that early scleraxis-expressing progenitor cells lead to the eventual formation of tendon tissue and other muscle attachments. Scleraxis is involved in mesoderm formation and is expressed in the syndetome (a collection of embryonic tissue that develops into tendon and blood vessels) of developing somites (primitive segments or compartments of embryos).

Inducing scleraxis expression

The syndetome location within the somite is determined by FGF secreted from the center of the myotome (a collection of embryonic tissue that develops into skeletal muscle)- the FGF then induces the adjacent anterior and posterior sclerotome (a collection of embryonic tissue that develops into the axial skeleton) to adopt a tendon cell fate. This ultimately places future scleraxis-expressing cells between the two tissue types they will ultimately join.

Scleraxis expression will be seen throughout the entire sclerotome (rather than just the sclerotome directly anterior and posterior to the myotome) with an overexpression of FGF8, demonstrating that all sclerotome cells are capable of expressing scleraxis in response to FGF signaling. While the FGF interaction has been shown to be necessary for scleraxis expression, it is still unclear as to whether the FGF signaling pathway directly induces the syndetome to secrete scleraxis, or indirectly through a secondary signaling pathway. Most likely, the syndetomal cells, through careful reading of the FGF concentration (coming from the myotome), can precisely determine their location and begin expressing scleraxis. Much of embryonic development follows this model of inducing specific cell fates through the reading of surrounding signaling molecule concentration gradients.