University:
Western Sydney University
Course:
MEDI 2003 | Clinical Sciences 2
Academic year:

2024
Views:

422

Pages:

3
Author:

Mariam A.

Myogenesis 1) Skeletal muscles derive from somites of the embryo. As development proceeds within the somites, single-celled mono-nucleated muscle fibre precursors appear – the myoblasts (primitive cell type). First step in myogenesis is aggregation of myoblasts into clusters forming small, multinucleated cell known as myotubes (100-300μm long) 2) Further length increase takes place by fusion of more myoblasts onto the myotubes ends ( primary myoblasts). With further development, primary myotubes + some associated, less differentiated cells, separate into clusters that are surrounded by a sheath (the sheath is known as the basement membrane or basal lamina) 3) More myoblasts aggregate beneath the basal lamina using the primary myotubes as scaffold, and subsequently forming into secondary myotubes such that the cell cluster (enclosed in a basal lamina) contains a primary and associated secondary myotubes along with unfused myoblasts now known as satellite cells. 4) Further maturation of myotubes involves filling with contractile proteins (+ forces the nuclei to fibre periphery) Muscle regeneration  The regenerative process largely mimics the developmental process. The major steps include digestion of the damaged cellular components, and activation and proliferation of  satellite cells to form new muscle-building material  fusion of satellite cells to form new myotubes + muscle fibres Steps of muscle regeneration are shown below: 1) A basal lamina surround both the myotube and associated satellite cell. Mitotically quiescent satellite cells in the adult can resume proliferation in response to stress or trauma. The expression of MyoD, a transcription factor, induces the proliferation of satellite cells. The c-Met receptor on the surface of satellite cells has strong binding affinity for the chemotactic agent HGF (hepatocyte growth factor) 2) Daughter cells of the activated satellite cells – myogenic precursor cells – undergo several rounds of cell division. HGF-cMet binding induces the proliferation of satellite cells 3) Myogenic precursor cells – expressing the myoblast-specific MRF Myf5 and MyoD – fuse with existing or new myotubes 4) A population of stem cells in adult skeletal muscle – called side-population cells – have the capacity to differentiate into all major blood cell lineages Muscle properties following regeneration     Variable muscle diameter (greater variability in fibre diameter) Some fibres do not mature fully and retain internal / central nuclei Groups of fibres enclosed in fibrous sheaths Incomplete fusion of myogenic cells leads to branching and "splitting" (these results in decrease in force generation / "force deficit") However,   Contraction times, maximum shortening velocity, fibre length and cross-sectional area eventually return to normal Restoration of the muscle mass, the peak tetanic tension and the muscle endurance may lag behind