Skeletal muscleconsists predominantly of muscle fibers surrounded by a connective tissue layer.This tissue has a great plasticity, and the knowledge of the morphological characteristics,myogenesis, and growth dynamics is important to the understanding of its physiologyand to the farm animal selection for meat production. Most skeletal musclesare derived from mesodermal precursor cells originated from the somites. Duringembryonic development, specification of mesodermal precursor cells to the myogeniclineage is regulated by positive and negative signals from surrounding tissues.Specification to the myogenic lineage requires the up-regulation of four muscle-specifictranscriptional factors (MyoD, Myf5, myogenin, and MRF4), called myogenic regulatoryfactors (MRFs), which have a well-defined role in skeletal muscle developmentand differentiation. The MRFs belong to the basic helix-loop-helix (bHLH) superfamily of transcription factors, that forms heterodimers and bind to the E-box(CANNTG) DNA sequence motif found in the promoters of many muscle specific genes.Specification to the myogenic lineage requires the up-regulation of the MRFsMyoD and Myf5. Proliferative MyoD and/or Myf5 positive myogenic cells are termedmyoblasts. Proliferating myoblasts withdraw from the cell cycle to become terminallydifferentiated myocytes that express the "late" MRFs, myogenin and MRF4. Specializedpopulations of myogenic stem cells, termed satellite cells, arise during thelate stages of embryogenesis are highly active during the postnatal muscle growth,which may occur by muscle fiber hyperplasia or hypertrophy. Quiescent satellitecells do not express detectable levels of MRF; however, during muscle growth,in response to several growth factors, activated satellite cells proliferateand express the MRFs in a similar manner to muscle precursor cells during skeletalmuscle development. Muscle growth mechanisms are controlled by the temporallyexpression of muscle growth related genes induced by the MRFs.
