Myosatellite cell
[4] Myosatellite cells are located between the basement membrane and the sarcolemma of muscle fibers,[5] and can lie in grooves either parallel or transversely to the longitudinal axis of the fibre.
Non-proliferative, quiescent myosatellite cells, which adjoin resting skeletal muscles, can be identified by their distinct location between sarcolemma and basal lamina, a high nuclear-to-cytoplasmic volume ratio, few organelles (e.g. ribosomes, endoplasmic reticulum, mitochondria, golgi complexes), small nuclear size, and a large quantity of nuclear heterochromatin relative to myonuclei.
On the other hand, activated satellite cells have an increased number of caveolae, cytoplasmic organelles, and decreased levels of heterochromatin.
[9] Greater activation also results in increased expression of myogenic basic helix-loop-helix transcription factors MyoD, myogenin, and MRF4 – all responsible for the induction of myocyte-specific genes.
These dividing cells are known as the "transit amplifying pool" before undergoing myogenic differentiation to form new (post-mitotic) myotubes.
[1] The process of muscle regeneration involves considerable remodeling of extracellular matrix and, where extensive damage occurs, is incomplete.
[2] It is suggested that exercise triggers the release of signaling molecules including inflammatory substances, cytokines and growth factors from surrounding connective tissues and active skeletal muscles.
[15] Studies have demonstrated that intense exercise generally increases IGF-1 production, though individual responses vary significantly.
[9][19] These results suggest that a light, endurance training regimen may be useful to counteract the age-correlated satellite cell decrease.
Aerobic exercise has been shown to significantly increase granular endoplasmic reticulum, free ribosomes, and mitochondria of the stimulated muscle groups.
[20] Other ultrastructural evidence for activated satellite cells include increased concentration of Golgi apparatus and pinocytotic vesicles.
[21] Satellite cells have a crucial role in muscle regeneration due to their ability to proliferate, differentiate, and self-renew.
The behavior of satellite cells is a highly regulated process to accommodate the balance between dormant and active states.
[22] In times of injury, satellite cells in myofibers receive signals to proliferate from proteins in the crushed skeletal muscle.
Studies found that transplanted satellite cells onto myofibers supported multiple regenerations of new muscle tissue.
The inflammatory response, activation and differentiation of satellite cells, and maturation of the new myofibers are essential for muscle regeneration.
Unfortunately, it seems that transplanted satellite cells have a limited capacity for migration, and are only able to regenerate muscle in the region of the delivery site.
The advantage of using these cell types for therapy in muscle diseases is that they can be systemically delivered, autonomously migrating to the site of injury.
Particularly successful recently has been the delivery of mesoangioblast cells into the Golden Retriever dog model of Duchenne muscular dystrophy, which effectively cured the disease.
Recently, it has been reported that Pax7 expressing cells contribute to dermal wound repair by adopting a fibrotic phenotype through a Wnt/β-catenin mediated process.
Whilst together PAX3 and PAX7 currently form the definitive satellite markers, Pax genes are notoriously poor transcriptional activators.
Increased levels of myostatin up-regulate a cyclin-dependent kinase inhibitor called p21 and thereby inhibit the differentiation of satellite cells.
