Differentiation, growth and development of muscular tissue

Abstract

Muscular tissue is comprised of multinucleate, cylindrical cells called muscle fibers. They are grouped into fasciculi enclosed by connective tissue. Several fiber types can be distinguished by differences in their physiological behaviour, and more precisely by their metabolic and contractile characteristics. Muscle fibers may have either a glycolytic or an oxydative energy metabolism. Some fibers are able to contract slowly, whereas others are recruited for rapid and short contraction. Muscle cells originate in the embryo in the mesodermic region. Myogenic cells multiply themselves in the so called proliferation cycle. They leave this cycle and enter the differentiation pathway where they are transformed into myoblasts. The fusion of myoblasts produces myotubes which give, finally, mature muscle fibers. Differentiation is under the control of nerves and hormones. The differentiation of muscular tissue in cattle takes place during foetal life. In the 2-3 months old foetus, muscular tissue is characterised by the presence of a population of myotubes, by a rapid increase of nuclei number and also in fiber number. During months 3 to 5, the myotubes are transformed into muscular fibers. Fast and slow fiber types become distinguishable at this stage of development. The proliferation of nuclei is still rapid, but the increase in the number of muscle fibers becomes slower. From months 5 to 9, differentiation of fibers continues, and metabolic fiber types become recognisable. The fiber number reaches a plateau, whereas the nuclei number still increases, due to multiplication and fusion of satellite cells. Foetal life is characterised by the differentiation of fibers and the increase of fiber number. Postnatal life is characterised by the maturation of fibers, and by the increase of their diameter until mature size.