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'''Myosatellite cells''', also known as '''satellite cells''' or '''muscle stem cells''', are small [[multipotent]] cells with very little [[cytoplasm]] found in mature [[muscle]].<ref name="ReferenceBirbrair2015">{{cite journal |title=Pericytes are Essential for Skeletal Muscle Formation |year=2015 |last1=Birbrair |first1=A. |last2=Delbono |first2=O. |journal=Stem Cell Reviews and Reports|volume=11 |issue=4 |pages=547–548 | pmid = 25896402 | doi = 10.1007/s12015-015-9588-6 }}</ref> Satellite cells are precursors to [[skeletal muscle]] cells, able to give rise to satellite cells or differentiated skeletal muscle cells.<ref name="rep1">{{cite journal | vauthors = Kadi F, Charifi N, Denis C, Lexell J, Andersen JL, Schjerling P, Olsen S, Kjaer M | title = The behaviour of satellite cells in response to exercise: what have we learned from human studies? | journal = Pflügers Arch. | volume = 451 | issue = 2 | pages = 319–27 | date = November 2005 | pmid = 16091958 | doi = 10.1007/s00424-005-1406-6 }}</ref> They have the potential to provide additional [[myonuclei]] to their parent muscle fiber, or return to a [[wikt:quiescence|quiescent]] state.<ref name="kadi2">{{cite journal | vauthors = Kadi F, Schjerling P, Andersen LL, Charifi N, Madsen JL, Christensen LR, Andersen JL | title = The effects of heavy resistance training and detraining on satellite cells in human skeletal muscles | journal = J. Physiol. (Lond.) | volume = 558 | issue = Pt 3 | pages = 1005–12 | date = August 2004 | pmid = 15218062 | pmc = 1665027 | doi = 10.1113/jphysiol.2004.065904 }}</ref> More specifically, upon activation, satellite cells can re-enter the cell cycle to proliferate and differentiate into [[myoblast]]s.<ref name="pmid21798086">{{cite journal | vauthors = Siegel AL, Kuhlmann PK, Cornelison DD | title = Muscle satellite cell proliferation and association: new insights from myofiber time-lapse imaging | journal = Skeletal Muscle | volume = 1 | issue = 1 | pages = 7 | date = February 2011 | pmid = 21798086 | pmc = 3157006 | doi = 10.1186/2044-5040-1-7 }}</ref>
Myosatellite cells are located between the [[basement membrane]] and the [[sarcolemma]] of muscle fibers,<ref>{{cite journal|last1=Zammit|first1=PS|last2=Partridge|first2=TA|last3=Yablonka-Reuveni|first3=Z|title=The skeletal muscle satellite cell: the stem cell that came in from the cold.|journal=Journal of Histochemistry and Cytochemistry|date=November 2006|volume=54|issue=11|pages=1177–91|pmid=16899758|doi=10.1369/jhc.6r6995.2006|doi-access=free}}</ref> 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.<ref name="rep1" /> 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 [[myoblast]]s before undergoing myogenic [[Cellular differentiation|differentiation]].<ref name="ReferenceBirbrair2015"/>
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==Structure==
===Genetic markers===
Satellite cells express a number of distinctive [[genetic markers]]. Current thinking is that most satellite cells express [[PAX7]] and [[PAX3]].<ref name="pmid15843801">{{cite journal | vauthors = Relaix F, Rocancourt D, Mansouri A, Buckingham M | title = A Pax3/Pax7-dependent population of skeletal muscle progenitor cells | journal = Nature | volume = 435 | issue = 7044 | pages = 948–53 | date = June 2005 | pmid = 15843801 | doi = 10.1038/nature03594 | hdl = 11858/00-001M-0000-0012-E8E0-9 | hdl-access = free }}</ref> Satellite cells in the head musculature have a unique developmental program,<ref name="pmid19531353">{{cite journal | vauthors = Harel I, Nathan E, Tirosh-Finkel L, Zigdon H, Guimarães-Camboa N, Evans SM, Tzahor E | title = Distinct origins and genetic programs of head muscle satellite cells | journal = Dev. Cell | volume = 16 | issue = 6 | pages = 822–32 | date = June 2009 | pmid = 19531353 | pmc = 3684422 | doi = 10.1016/j.devcel.2009.05.007 }}</ref> 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 ([[Hepatocyte growth factor|HGF]])) are less commonly used markers.<ref name="rep1" />
[[CD34]] and [[Myf5]] markers specifically define the majority of quiescent satellite cells.<ref>{{cite journal | last1 = Beauchamp | first1 = JR | last2 = Heslop | first2 = L | last3 = Yu | first3 = DS | last4 = Tajbakhsh | first4 = S | last5 = Kelly | first5 = RG | last6 = Wernig | first6 = A | last7 = Buckingham | first7 = ME | last8 = Partridge | first8 = TA | last9 = Zammit | first9 = PS | year = 2000 | title = Expression of CD34 and Myf5 defines the majority of quiescent adult skeletal muscle satellite cells | journal = J Cell Biol | volume = 151 | issue = 6| pages = 1221–34 | doi=10.1083/jcb.151.6.1221 | pmid=11121437 | pmc=2190588}}</ref> 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.<ref name="crameri">{{cite journal | last1 = Crameri | first1 = R | last2 = Aagaard | first2 = P | last3 = Qvortrup | first3 = K | last4 = Kjaer | first4 = M | year = 2004 | title = N-CAM and Pax7 immunoreactive cells are expressed differently in the human vastus lateralis after a single bout of exhaustive eccentric exercise | url = | journal = J Physiol | volume = 565 | issue = | page = 165 }}</ref> 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.<ref>{{cite journal| doi = 10.1002/stem.1248 | pmid=23034923 | title=CCAAT/Enhancer Binding Protein Beta is Expressed in Satellite Cells and Controls Myogenesis | journal=Stem Cells | date=2012 | volume=30 | issue=12 | pages=2619–2630 | first=François | last=Marchildon}}</ref> HGF testing is also used to identify active satellite cells.<ref name="rep1" /> Activated satellite cells also begin expressing muscle-specific filament proteins such as [[desmin]] as they differentiate.
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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<ref>{{cite journal |vauthors=Seale P, Polesskaya A, Rudnicki MA |title=Adult stem cell specification by Wnt signaling in muscle regeneration |journal=Cell Cycle |volume=2 |issue=5 |pages=418–9 |year=2003 |pmid=12963830 |doi= 10.4161/cc.2.5.498 |url=http://www.landesbioscience.com/journals/cc/abstract.php?id=498|doi-access=free }}</ref> and form new myofibers through a process similar to fetal muscle development.<ref name=Parker03>{{cite journal |vauthors=Parker MH, Seale P, Rudnicki MA |title=Looking back to the embryo: defining transcriptional networks in adult myogenesis |journal=Nat. Rev. Genet. |volume=4 |issue=7 |pages=497–507 |date=July 2003 |pmid=12838342 |doi=10.1038/nrg1109 }}</ref> 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.<ref name=Parker03/> 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.
<ref>{{cite journal |vauthors=Mourkioti F, Rosenthal N |title=IGF-1, inflammation and stem cells: interactions during muscle regeneration |journal=Trends Immunol. |volume=26 |issue=10 |pages=535–42 |date=October 2005 |pmid=16109502 |doi=10.1016/j.it.2005.08.002 |url= }}</ref>
<ref>{{cite journal |vauthors=Hawke TJ, Garry DJ |title=Myogenic satellite cells: physiology to molecular biology |journal=J. Appl. Physiol. |volume=91 |issue=2 |pages=534–51 |date=August 2001 |pmid=11457764 |doi=10.1152/jappl.2001.91.2.534}}</ref>
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Human studies have shown that both high resistance training and endurance training have yielded an increased number of satellite cells.<ref name="crameri" /><ref name="pmid12811778">{{cite journal | vauthors = Charifi N, Kadi F, Féasson L, Denis C | title = Effects of endurance training on satellite cell frequency in skeletal muscle of old men | journal = Muscle Nerve | volume = 28 | issue = 1 | pages = 87–92 | date = July 2003 | pmid = 12811778 | doi = 10.1002/mus.10394 }}</ref> These results suggest that a light, endurance training regimen may be useful to counteract the age-correlated satellite cell decrease.<ref name="rep1" /> 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.<ref name="kadi2" />
Satellite cell activation has also been demonstrated on an ultrastructural level following exercise. [[Aerobic 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.<ref>{{cite journal | last1 = Appell | first1 = HJ | last2 = Forsberg | first2 = S | last3 = Hollmann | first3 = W | year = 1988 | title = Satellite cell activation in human skeletal muscle after training: evidence for muscle fiber neoformation | url = | journal = Int J Sports Med | volume = 9 | issue = 4| pages = 297–99 | doi=10.1055/s-2007-1025026| pmid = 3182162 }}</ref> Other ultrastructural evidence for activated satellite cells include increased concentration of Golgi apparatus and pinocytotic vesicles.<ref name="pmid11382785">{{cite journal | vauthors = Roth SM, Martel GF, Ivey FM, Lemmer JT, Tracy BL, Metter EJ, Hurley BF, Rogers MA | title = Skeletal muscle satellite cell characteristics in young and older men and women after heavy resistance strength training | journal = J. Gerontol. A Biol. Sci. Med. Sci. | volume = 56 | issue = 6 | pages = B240–7 | date = June 2001 | pmid = 11382785 | doi = 10.1093/gerona/56.6.B240 | doi-access = free }}</ref>
==Research==
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