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Myosatellite cells or satellite cells are small mononuclear 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.〔 ==Genetic markers of satellite cells== 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Myosatellite cell」の詳細全文を読む スポンサード リンク
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