Scleraxis

The scleraxis protein is a member of the basic helix-loop-helix (bHLH) superfamily of transcription factors. Currently two genes (SCXA and SCXB respectively) have been identified to code for identical scleraxis proteins.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).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. bHLH transcription factors have been shown to have a wide array of functions in developmental processes. More precisely, they have critical roles in the control of cellular differentiation, proliferation and regulation of oncogenesis. To date, 242 eukaryotic proteins belonging to the HLH superfamily have been reported. They have varied expression patterns in all eukaryotes from yeast to humans.