SKELETAL-MUSCLE FIBER TYPES - DETECTION METHODS AND EMBRYONIC DETERMINANTS

Muscle fibres used to be simply classified as either type I, IIa or II b. Advances in molecular and histological techniques have, however, le ad to the realisation that the phenotypes of muscles are more varied t han this. An additional fibre type (IIX/IID) has been discovered, fibr es with intermediate fibre types have been described and there is accu mulating evidence that the fibres types described from the study of li mb muscles are not necessarily applicable to other skeletal muscles, s uch as the jaw and extra-ocular muscles. Further to this has been the discovery that diversity occurs at all stages of muscle development. T here are subpopulations of myoblasts and myotubes as well as various t ypes of muscle fibres. The relationships between the different stages of development is still under study. However, it is clear that each st age of muscle development is influenced to a certain degree by prior e vents. Consequently, the characteristics of mature fibres reflect both their developmental origins and influences from the adult environment , such as their patterns of muscle activation.