Myosatellite cells or satellite cells are small multipotent cells with virtually no cytoplasm found in mature muscle. Satellite cells are precursors to skeletal muscle cells, able to give rise to satellite cells or differentiated skeletal muscle cells. They have the potential to provide additional myonuclei to their parent muscle fiber, or return to a quiescent state. More specifically, upon activation, satellite cells can re-enter the cell cycle to proliferate and differentiate into myoblasts.
Myosatellite cells are located between the basement membrane and the sarcolemma of muscle fibers, and can lie in grooves either parallel or transversely to the longitudinal axis of the fibre. Their distribution across the fibre can vary significantly. 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. Satellite cells are able to differentiate and fuse to augment existing muscle fibers and to form new fibers. These cells represent the oldest known adult stem cell niche, and are involved in the normal growth of muscle, as well as regeneration following injury or disease.
In undamaged muscle, the majority of satellite cells are quiescent; they neither differentiate nor undergo cell division. In response to mechanical strain, satellite cells become activated. Activated satellite cells initially proliferate as skeletal myoblasts before undergoing myogenic differentiation.
Satellite cells express a number of distinctive genetic markers. Current thinking is that most satellite cells express PAX7 and PAX3. Satellite cells in the head musculature have a unique developmental program, and are Pax3-negative. Moreover, both quiescent and activated human satellite cells can be identified by the membrane-bound neural cell adhesion molecule (N-CAM/CD56/Leu-19), a cell-surface glycoprotein. Myocyte nuclear factor (MNF), and c-met proto-oncogene (receptor for hepatocyte growth factor (HGF)) are less commonly used markers.
CD34 and Myf5 markers specifically define the majority of quiescent satellite cells. Activated satellite cells prove problematic to identify, especially as their markers change with the degree of activation; for example, greater activation results in the progressive loss of Pax7 expression as they enter the proliferative stage. However, Pax7 is expressed prominently after satellite cell differentiation. 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. HGF testing is also used to identify active satellite cells. Activated satellite cells also begin expressing muscle-specific filament proteins such as desmin as they differentiate.
The field of satellite cell biology suffers from the same technical difficulties as other stem cell fields. Studies rely almost exclusively on Flow cytometry and fluorescence activated cell sorting (FACS) analysis, which gives no information about cell lineage or behaviour. As such, the satellite cell niche is relatively ill-defined and it is likely that it consists of multiple sub-populations.
When muscle cells undergo injury, quiescent satellite cells are released from beneath the basement membrane. They become activated and re-enter the cell cycle. These dividing cells are known as the "transit amplifying pool" before undergoing myogenic differentiation to form new (post-mitotic) myotubes. There is also evidence suggesting that these cells are capable of fusing with existing myofibers to facilitate growth and repair.
The process of muscle regeneration involves considerable remodeling of extracellular matrix and, where extensive damage occurs, is incomplete. Fibroblasts within the muscle deposit scar tissue, which can impair muscle function, and is a significant part of the pathology of muscular dystrophies.
Satellite cells proliferate following muscle trauma and form new myofibers through a process similar to fetal muscle development. After several cell divisions, the satellite cells begin to fuse with the damaged myotubes and undergo further differentiations and maturation, with peripheral nuclei as in hallmark. One of the first roles described for IGF-1 was its involvement in the proliferation and differentiation of satellite cells. In addition, IGF-1 expression in skeletal muscle extends the capacity to activate satellite cell proliferation (Charkravarthy, et al., 2000), increasing and prolonging the beneficial effects to the aging muscle. 
Aged muscle cells in mammalian hearts constantly regenerate, albeit at a very low rate. Within 18 months around five percent of the heart muscle cells regenerated themselves originating from Sca-1 stem cells
Satellite cell activation is measured by the extent of proliferation and differentiation. Typically, satellite cell content is expressed per muscle fiber or as a percentage of total nuclear content, the sum of satellite cell nuclei and myonuclei. While the adaptive response to exercise largely varies on an individual basis on factors such as genetics, age, diet, acclimatization to exercise, and exercise volume, human studies have demonstrated general trends.
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. Notably, HGF, a cytokine, is transferred from the extracellular matrix into muscles through the nitric-oxide dependent pathway. It is thought that HGF activates satellite cells, while insulin-like growth factor-I (IGF-1) and fibroblast growth factor (FGF) enhance satellite cell proliferation rate following activation. Studies have demonstrated that intense exercise generally increases IGF-1 production, though individual responses vary significantly. More specifically, IGF-1 exists in two isoforms: mechano growth factor (MGF) and IGF-IEa. While the former induces activation and proliferation, the latter causes differentiation of proliferating satellite cells.
Human studies have shown that both high resistance training and endurance training have yielded an increased number of satellite cells. These results suggest that a light, endurance training regimen may be useful to counteract the age-correlated satellite cell decrease. In high-resistance training, activation and proliferation of satellite cells are evidenced by increased cyclin D1 mRNA, and p21 mRNA levels. This is consistent with the fact that cyclin D1 and p21 upregulation correlates to division and differentiation of cells.
Satellite cell activation has also been demonstrated on an ultrastructural level following exercise. Aerobic exercise has been shown to significantly increase granular endoplasmic reticulum, free ribosomes, and mitochondria of the stimulated muscle groups. Additionally, satellite cells have been shown to fuse with muscle fibers, developing new muscle fibers. Other ultrastructural evidence for activated satellite cells include increased concentration of Golgi apparatus and pinocytotic vesicles.
Upon minimal stimulation, satellite cells in vitro or in vivo will undergo a myogenic differentiation program.
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. As such, systemic treatments or even the treatment of an entire muscle in this way is not possible. However, other cells in the body such as pericytes and hematopoietic stem cells have all been shown to be able to contribute to muscle repair in a similar manner to the endogenous satellite cell. 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. However, the sample size used was relatively small and the study has since been criticized for a lack of appropriate controls for the use of immunosuppressive drugs.Recently, it has been reported that Pax7 expressing cells contribute to dermal wound repair by adopting a fibrotic phenotype through a Wnt/-catenin mediated process.
Little is known of the regulation of satellite cells. Whilst together PAX3 and PAX7 currently form the definitive satellite markers, Pax genes are notoriously poor transcriptional activators. The dynamics of activation and quiesence and the induction of the myogenic program through the myogenic regulatory factors, Myf5, MyoD, myogenin, and MRF4 remains to be determined.
There is some research indicating that satellite cells are negatively regulated by a protein called myostatin. Increased levels of myostatin up-regulate a cyclin-dependent kinase inhibitor called p21 and thereby inhibit the differentiation of satellite cells.
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