Mesenchyme serves as the source for the formation of different varieties of muscle tissue. Mesen-chymal cells differentiate into primordial muscle cells called myoblasts. These are spindle-shaped cells containing a single, ovoid nucleus. Differentiation of mesenchyme into myoblasts (myogenesis) is induced by the muscle-specific gene called
Myo-D which is a member of the family of myogenic regulatory factors (MRFs).