The origin, molecular regulation and therapeutic potential of myogenic stem cell populations

21 Pages·2009·0.56 MB·English

Anatomy
Journal of
J.Anat.(2009)215,pp477–497 doi:10.1111/j.1469-7580.2009.01138.x
REVIEW
The origin, molecular regulation and therapeutic
potential of myogenic stem cell populations
A. Otto, H. Collins-Hooper and K. Patel
SchoolofBiologicalSciences,HopkinsBuilding,UniversityofReading,WhiteknightsCampus,Reading,Berkshire,UK
Abstract
Satellite cells, originating in the embryonic dermamyotome, reside beneath the myofibre of mature adult skele-
tal muscle and constitute the tissue-specific stem cell population. Recent advances following the identification
of markers for these cells (including Pax7, Myf5, c-Met and CD34) (CD, cluster of differentiation; c-Met, mesen-
chymal epithelial transition factor) have led to a greater understanding of the role played by satellite cells in
the regeneration of new skeletal muscle during growth and following injury. In response to muscle damage,
satellite cells harbour the ability both to form myogenic precursors and to self-renew to repopulate the stem
cell niche following myofibre damage. More recently, other stem cell populations including bone marrow stem
cells, skeletal muscle side population cells and mesoangioblasts have also been shown to have myogenic poten-
tial in culture, and to be able to form skeletal muscle myofibres in vivo and engraft into the satellite cell niche.
These cell types, along with satellite cells, have shown potential when used as a therapy for skeletal muscle
wasting disorders where the intrinsic stem cell population is genetically unable to repair non-functioning mus-
cle tissue. Accurate understanding of the mechanisms controlling satellite cell lineage progression and self-
renewal as well as the recruitment of other stem cell types towards the myogenic lineage is crucial if we are to
exploit the power of these cells in combating myopathic conditions. Here we highlight the origin, molecular
regulation and therapeutic potential of allthe major cell types capableof undergoing myogenic differentiation
anddiscuss theirpotential therapeutic application.
Keywords bonemarrow; mesoangioblast; pericyte;satellite cell;sidepopulation; skeletal muscle; therapy.
tissue-specificmarkersforthesecellspreventedanyfurther
Introduction
investigation. More recently, the identification of markers
Skeletalmuscleisahighlydynamictissuethatiscapableof of satellite cells such as the transcription factors Pax7 and
regenerationfollowingexercise-,immobilization-orchemi- Myf5 along with the cell surface markers M-cadherin and
cally-induced damage. This regenerative capacity is due to CD34inmurinemusclehaveenabledinvestigatorstoshow
the presence of a tissue-specific population of myogenic thatthesatellitecellisamajorsourceofskeletalmyoblasts
stem cells termed satellite cells. The satellite cell was first thatre-populatedamagedandgrowingpost-natalskeletal
describedbyMauro(1961)andisso-calledduetoitsperiph- muscleaswellasformingnewmyofibres(Beauchampetal.
eral location on the skeletal muscle myofibre where it lies 2000;Sealeetal.2000;Collinsetal.2005,seeTable1).
between the sarcolemma of the myofibre cell and its sur- However, a number of studies have identified other cell
roundingbasallamina(Fig.1).Initialstudiesfirstsuggested types that show the capability of forming differentiated
that the satellite cell may be capable of producing new skeletal muscle tissue in vivo, and more importantly, some
myoblastsfromwhichgrowingordamagedmyofibrescould have been proposed to populate the satellite cell niche,
form.However,lackofrobustexperimentalproceduresand thus forming new muscle stem cells. Differing stem cell
types have been shown to be able to undergo these pro-
cessesincludingskeletalmusclesidepopulationcells,bone
Correspondence
marrow-derived haematopoietic lineages, mesoangioblast
Prof.KetanPatel,SchoolBiologicalSciences,HopkinsBuilding,Uni-
andpericyteendothelialprecursorcellsofbloodvesselwalls
versityofReading,Whiteknights,Reading,Berkshire,RG66UB,UK.
T:[phone];F:[phone];E:ketan.patel@ and,quiterecently,brownfatprecursorcells(Gussonietal.
reading.ac.uk 1999; Sampaolesi et al. 2006; Dellavalle et al. 2007; Seale
etal.2008;Fig.2).Manyofthesecelltypesareabletocarry
Acceptedforpublication20July2009
Articlepublishedonline24August2009 outsatellitecellnicheengraftmentandmyofibreformation
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Journalcompilationªª2009AnatomicalSocietyofGreatBritainandIreland
478 Myogenicstemcellpopulations,A.Ottoetal.
Fig.1 Satellitecelllocationwithinskeletal
musclestructure.Individualmusclebellies,
surroundedbyanepimysium,arecomposed
ofbundlesofmyofibres,eachseparatedby
theperimysiumandwellsuppliedwithblood
vessels.Fibrebundlesareeachsurroundedby
afascicleandinfiltratedwithalargecapillary
networkcontainedwithintheconnective
tissuelayer.Individualmyofibresyncytiums,
surroundedbyanendomysiumorbasal
lamina,arecomposedofmultiplemyonuclei,
mitochondriaandmyofibrilsthatcontainthe
contractilemyofilamentproteins.Satellitecells
(green)liebetweenthesyncytiumandthe
basallaminaofthemyofibre.
Fig.2 Schematicdepictingthemajorcelltypesofthebodyabletoengraftintoregeneratingskeletalmuscle.
following systemic delivery and thus show potential for cell biology and some of the key signalling pathways that
therapeutic use. However, the efficacy of many of these areinvolvedintheactivation,proliferationanddifferentia-
cells to incorporate into damaged muscle has yet to equal tion of these cells into muscle. Subsequently, we focus on
that of the intrinsic satellite cell population. In this review theothermajorcelllineagesthatarealsocapableofform-
wefirstdiscussthekeyfeaturesofskeletalmusclesatellite ing skeletal muscle tissue in vivo and discuss their origin,
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Journalcompilationªª2009AnatomicalSocietyofGreatBritainandIreland
Myogenicstemcellpopulations,A.Ottoetal. 479
properties, tissue developmental repertoire and potential etal.2005).Mostimportantly,thepairedboxtranscription
benefitsfortherapeuticuse. factorPax7isexpressedinthenucleusofallquiescentadult
satellite cells and is necessary for their maintenance as a
viable population during adulthood (Seale et al. 2000;
Satellite cells
Oustaninaetal.2004,seeTable1).
Skeletalmusclestemcells,namedsatellitecellsduetotheir
peripheral, sub-laminar position, were first discovered
Pax genes and the satellite cell lineage
through electron microscopic analysis of skeletal muscle
myofibres (Mauro, 1961; Fig. 1). Even in this initial study, The ‘Paired Box’, or Pax, family of genes function as tran-
Mauro hypothesized that the satellite cell population may scription factors and play essential roles during embryonic
beinvolvedinregenerationandgrowthofitssurrounding development(Boppetal.1986;Treismanetal.1991;Man-
tissue. Satellite cells were shown to function as myoblasts sourietal.1999).TherearenineknownPaxgenes(Pax1-9)
by Reznik (1969), who was the first to label them as stem inmammalsthataredividedintofoursubgroupsbasedon
cells. This role was supported by Moss & Leblond (1970, genomic structure, sequence similarity and function (Wal-
1971), who demonstrated through radioactive thymidine theretal.1991).Pax3andPax7arehomologousgenesthat
incorporationexperimentsthatsatellitecellsbecamemitoti- makeuponeofthefoursubgroups.Despitetheirstructural
cally active and contribute nuclei to both damaged and homology, Pax3 and Pax7 have differing functions during
growing myofibres. In a landmark study, Snow (1977b) embryological development. In the mouse and chick, Pax3
demonstratedthatsatellitecellsandnotmyofibrescontrib- and Pax7 are expressed throughout the embryonic dorsal
ute nuclei to regenerating skeletal muscle. Following the neural tube and in the neural crest cells (Goulding et al.
invitrocultureofindividuallydissectedmyofibres,satellite 1991;Mansourietal.1996;Lacostaetal.2005).Micehomo-
cells survived the dispersal process and were stimulated to zygousforthePax3spontaneousmutant‘splotch’diedur-
divide, forming clones, whereas the fibres themselves ing gestation and harbour numerous neural crest
degenerated,showingthesatellitecellasarobustpopula- deficiencies,particularlyinthecaudalregionoftheembryo
tionabletosurvivemyofibredegenerationandproliferate (Auerbach, 1954). Conversely, Pax7-null mice exhibit ante-
(Bischoff,1975). rior (cephalic) neural crest defects and most die within
Themechanismofsatellitecellactivationremainedunex- 3weeks of birth; however, some survive until adulthood
ploreduntilBischoff(1986)showedsatellitecellsonsingle (Mansourietal.1996;Oustaninaetal.2004).
isolatedmyofibreswerestimulatedtodivideinresponseto OnemajorroleforPax3andPax7genesintheembryois
a mitogen originating from crushed skeletal muscle, sug- inthemyogenicpathway,andparticularlythespecification
gestingexternalfactorsactuponthequiescentsatellitecell. ofmyogenicstemcells.Intheembryo,Pax3isexpressedin
Furtherstudiesonsatellitecellresponsestoknownexternal thedevelopingpresomiticmesoderm;however,itbecomes
factors ascertained that transforming growth factor beta localizedtothedermamyotomeasthesomitedifferentiates
(TGF-b) inhibited their differentiation through the myo- (Gouldingetal.1991),andisstronglyexpressedintheepax-
geniclineage(Allen&Boxhorn,1987).Moreover,fibroblast ial and hypaxial lips (Goulding et al. 1994). Pax3 mutant
growthfactor(FGF)wasshowntoincreasesatellitecellpro- miceshowsomiteformationdefectsandanabsenceofthe
liferation and perturb differentiation whereas insulin-like hypaxial dermamyotome, leading to trunk muscle defects
growthfactor-1(IGF-1)wasseentoslightlyincreaseprolif- (Tremblayetal.1998;Schubertetal.2001).Moreover,Pax3
erationandincreasedifferentiationofthelineage(Allen& is essential for expression of the C-Met tyrosine kinase
Boxhorn,1989).Bischoff(1990)showedthatsatellitecellsin receptor in the hypaxial somite, necessary for myoblast
contactwithmyofibresinculturehaveareducedmitogenic migration(Bladtetal.1995;Epsteinetal.1996),andsubse-
response compared to isolated cells, suggesting the myofi- quentlyPax3-nullmiceshownomigrationofmuscleprecur-
bre regulates the initial satellite cell activation response, sors from the hypaxial dermamyotome into the limbs
providing the first evidence that the niche environment is (Gouldingetal.1994).Thereisnorescueofhypaxialmyo-
crucialincontrollingthebehaviourofthesatellitecell. genic cell migration when Pax3 is replaced by Pax7 in the
Thenatureofsatellitecellresearchchangeddramatically mouse,showingapparentspecificrolesforthetwogenes,
following the discovery of specific satellite cell molecular andtheymayhavedivergedfollowingtheduplicationofa
markers enabling their identification. Satellite cells have commonPax3⁄7precursorgeneinresponsetotherequire-
beenshowntoexpressthecelladhesionproteinM-cadherin, mentsofappendagemuscleformation(Relaixetal.2004).
along with other markers including the myogenic regula- Tajbakhsh et al. (1997) demonstrated that Pax3 isinvolved
toryfactorMyf5,thetyrosinereceptorkinaseC-Met,CD34 in controlling the entry of precursor cells to the myogenic
and,morerecently,thecaveolae-formingproteinCaveolin- lineage through acting upstream of the myogenic regula-
1,andthecellsurfaceproteinsSyndecan-3and4(Irintchev tory factor MyoD in the mouse embryo. Further investiga-
et al. 1994; Allen et al. 1995; Cornelison & Wold, 1997; tionsanalysingtheMyf5promotersequencerevealedthat
Beauchamp et al. 2000; Cornelison et al. 2001; Volonte Pax3 binds to a 145-base pair element that is essential for
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Journalcompilationªª2009AnatomicalSocietyofGreatBritainandIreland
480 Myogenicstemcellpopulations,A.Ottoetal.
Myf5 transcription in the hypaxial somite, the hypoglossal necessityforPax7inmusclegrowthandrepair.Pax3+satel-
cordandthelimbs(Bajardetal.2006). litecellswerepresentinthePax7-nullmicebutinterestingly
Pax7-nullmiceshowednoskeletalmuscledevelopmental were not detected in the hind limbs. Intriguingly, they
defects, suggesting a redundancy through Pax3 which is reported a Pax3+ side population of non-sub-laminar cells
sufficienttomaintainmyogenesisintheembryo(Mansouri andsuggestedadistinctroleforPax3inthisseparatepopu-
et al. 1996). However, Pax7 is expressed in the developing lation(seesidepopulationcells).
dermamyotome, suggesting that a function for Pax7 exists
during myogenesis (Jostes et al. 1990). Seale et al. (2000)
Embryonic origins of satellite cells
failed to detect satellite cells in Pax7-null mice, suggesting
anessentialroleforPax7inthespecificationofthesatellite Through classical embryological experimentation using the
cell lineage in vertebrates. However, subsequent studies chickasamodelsystemithasbeenknownformanyyears
showedthepresenceofsatellitecells(althoughinfrequent) that the vertebrate skeletal muscle of the trunk and limbs
in Pax7-null animals, suggesting Pax7 is essential for the originatesinthesomites(reviewedbyScaal&Christ,2004).
renewal and propagation of the satellite cell population Only recently, however, has the origin of skeletal muscle
throughoutembryogenesisandadulthood(Oustaninaetal. satellite cells been distinguished. Using Pax3 and Pax7 as
2004). Pax7 has been shown to act as a cell survival signal satellitecellmarkers,recentstudiesusinggreenfluorescent
for satellite cells through the prevention of the expression protein (GFP) lineage tracing and chick-quail transplanta-
of the myogenic regulatory factors MyoD and myogenin tion techniques have identified a population of myogenic
(Olguin & Olwin, 2004; Relaix et al. 2006). Furthermore, precursor cells that originate in the central portion of the
myogenin has been shown to down-regulate Pax7 expres- dermamyotome,persistinaproliferating,undifferentiated
sion, suggesting that a reciprocal inhibition may exist progenitor state as the dermamyotome breaks down and
between Pax7 and the myogenic regulatory factors, and become interspersed within the differentiated myogenic
that Pax7 must be down-regulated to allow myogenic dif- cells of the myotome, where they continue to be a source
ferentiationtoproceed(Olguinetal.2007).However,Zam- forembryonicandfetalmuscle(Grosetal.2005).Moreover,
mit et al. (2006) showed that Pax7 over-expression is it has been shown that these cells remain as progenitors
permissive for MyoD and myogenin expression in satellite throughout late fetal development, and subsequently
cellcultures,althoughmyogeninonsetwasdelayed.More- becomeenvelopedbeneaththebasallaminaofdeveloping
over, it has been shown that Pax7 over-expression in the myofibres where they take up the position of quiescent
Pax7-null murine C2C12 myogenic cell line up-regulated satellitecellsinlatefetalandpost-nataltrunkmuscle,dem-
MyoD expression but perturbed differentiation (Zammit onstrating asomitic origin for trunk satellite cells in verte-
etal.2006).TogetherthesedatasuggestaroleforPax7in brates(Grosetal.2005;Kassar-Duchossoyetal.2005;Relaix
maintainingsatellitecellproliferationbutnotinducingqui- etal.2005).
escence. Thepresenceof satellitecells in Pax7-null animals Traditionaldogmastatesthatembryonicandfetalmuscle
would suggest that an amount of redundancy exists develops through two successive stages, primary and sec-
betweenPax3andPax7inthespecificationofsatellitecells. ondary myogenesis, and that these processes are carried
Indeed, Pax3 is expressed in a subset of the adult satellite outbyprimaryandsecondarymyoblasts,respectively(Feld-
cell lineage, which includes cells from the diaphragm and man&Stockdale,1992).Priortosatellitecelllineagetracing
50%offorelimbmuscles.Moreover,Pax3hasbeenusedas experiments,itwasthoughtthatsatellitecellsemergedasa
amarkertoisolatepurepopulationsofadultsatellitecells finalpopulationofmyoblastsduringlatefetaldevelopment
from the diaphragm of Pax3GFP⁄+ mice (Conboy & Rando, (Feldman & Stockdale, 1992). However, as discussed in the
2002;Buckinghametal.2003;Montarrasetal.2005;Relaix previous section, it has been demonstrated that satellite
et al. 2006). However, the majority of hind limb satellite cells emerge from a population of undifferentiated stem
cellsarenegativeforPax3andhindlimbmusculatureshows cellsthatoriginateintheembryonicdermamyotome(Gros
a down-regulation of Pax3 expression in certain muscles etal.2005;Kassar-Duchossoyetal.2005;Relaixetal.2005).
during fetal development (Otto et al. 2006; Relaix et al. Moreover, the relationship between the satellite cell line-
2006). Moreover, in Pax7-null mice, satellite cells are lost age and the primary and secondary myoblasts has been
progressively during adulthood in Pax3-expressing and - established, in that both primary and secondary myogenic
non-expressing muscle through apoptosis showing that regulatoryfactor-positive(MRF+)myoblastpopulationsarise
Pax3 cannot substitute for the anti-apoptotic role of Pax7 from the pool of Pax3+⁄Pax7+ myogenic precursor cells
invivo(Relaixetal.2006).C57⁄BL6Pax7-nullmicedieat2– throughout development (Relaix et al. 2005) and neither
3weeks of age, however, through back crossing onto the primary or secondary myoblast populations go on to form
129⁄Sv⁄Jgeneticbackground,Kuangetal.reportedviable satellite cells in the adult. However, distinctions between
Pax7-null mice (Mansouri et al. 1996; Seale et al. 2000; the pool of Pax3+⁄Pax7+ precursor cells that exist during
Kuang et al. 2006). These mice showed extensive muscle embryonic and fetal development have been uncovered.
wasting and severely perturbed regeneration, showing a Uncoupling of embryonic and fetal myogenesis using
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Myogenicstemcellpopulations,A.Ottoetal. 481
MyoD:Myf5 double mutant mice lines that only form are negative in Pax3cre mice. Importantly, they show that
embryonic but not fetal muscle and Mrf4nlacZ mutants to satellite cells from non-migratory head muscles originate
traceMRF4+cellshaveshownthatembryonicbutnotfetal fromMesP1+cellsandnotfromPax3+cells,demonstrating
precursor cells (from the Pax3+⁄Pax7+ pool) also express thatheadmusclesatellitecellsaregeneticallydistinctinori-
MRF4 and are therefore genetically distinct populations gin and lineage from those of the trunk and limb muscles
(Kassar-Duchossoy et al. 2005). Fetal Pax7+Myf5) and (Harel et al. 2009). Furthermore, using Myf5Cre, Islet1Cre,
Pax7+⁄Myf5+ precursor cells have both been found in nor- Nkx2.5Cre,MesP1CreandPax3Cremouselines(Nkx2.5,NK2
mal fetal muscle, suggesting that the two populations are transcriptionfactor-related,locus5)theydemonstratethat
in the same lineage, as observed in the adult (Kassar- separatemusclesoftheheadshowdistinctlineagesofmyo-
Duchossoy et al. 2005; Kuang et al. 2007). Moreover, in genic cells: eye muscles (Myf5+, MesP1+); mastication mus-
Pax3sp⁄sp:Myf5nlacZ⁄nlacZ mutants that show no embryonic cles(Myf5+,MesP1+,Isl1+,Nkx2.5+)(Isl1,islet1);andtongue
orfetalmyogenesis(andthereforearedevoidofembryonic muscles(Myf5+,MesP1+,Pax3+).Moreover,theythendem-
and fetal myoblasts), fetal Pax7+⁄Myf5+ precursor cells are onstrate that cells from the Isl1+ lineage form the satellite
also observed and are therefore a distinct population of cells found only in themastication muscles, not in the eye
cells from the fetal myoblasts (Kassar-Duchossoy et al. or tongue muscles. Additionally, using VE-CadCre mouse
2005). These elegant genetic studies have clearly revealed lines(markingcellsofendothelialorigin),theyruleoutthe
the lineage progression of myogenic cells from Pax3+⁄ possibility that these Isl1+ satellite cells are endothelial in
Pax7+⁄MRF) progenitors to primary and secondary myo- origin.Thus,thesedatashowthatsatellitecellsofthedif-
blasts. fering head muscles are developmentally linked to their
Furtherstudieshavealsotracedtheoriginsoflimbmus- embryonic muscle origins (Harel et al. 2009). Despite these
clesatellitecellsbacktotheembryonicdermamyotome.We differing genetic origins, when a single myofibre harbour-
have previously shown using the chick model that Pax3+ ingIsl1+satellitecellsfromtheheadwastransplantedinto
and Pax7+ myogenic stem cells migrate from the hypaxial aninjuredtibialisanterior(TA)muscleofthelimb,itreadily
somite into the limb bud mesenchyme (Otto et al. 2006). formednascentmyofibresshowingnormalmyogenicdiffer-
Moreover, it has been demonstrated that Pax3+ migrating entiation, suggesting the regeneration capacity of somite-
myoblastsundergoasimilarmigrationprocessintothelimb and cranial paraxial mesoderm-derived satellite cells of
bud mesenchyme in the mouse, following which they differinggeneticlineagesissimilar(Hareletal.2009).
up-regulate Pax7 expression and remain MRF-negative
(Kassar-Duchossoy et al. 2005). Finally, retroviral labelling
Adult satellite cells as the main source for
techniques and chick-quail transplant experiments in the
skeletal muscle
avian model, along with somite-specific Pax3 cre mouse
lines have convincingly demonstrated that all limb muscle Satellitecellsareabletoregeneratedamagedskeletalmus-
satellitecellsoriginatefromPax3+cellsofthehypaxialder- cle in vivo (Moss & Leblond, 1971; Snow, 1977a) and have
mamyotome(Schiendaetal.2006). beenthetargetsforcellgraftingtherapy(reviewedbyMor-
Themajorityofmusclesintheheadarisefromthecranial gan & Partridge, 2003). Attempts to graft FACS-isolated
paraxial head mesoderm, and not from the somites (see Pax3GFP⁄+ satellite cells directly into mdx TA muscle gave
Tzahor, 2009). It has been well documented that genetic varied levels of cell incorporation (Montarras et al. 2005).
differencesareapparentbetweentheprogrammesofskele- For example, the expansion of the same FACS-isolated
talmuscledevelopmentintheheadvs.thetrunkandlimbs Pax3GFP⁄+ satellite cell pool in vitro prior to engraftment
of vertebrates. For example, mutant mice lines that show reducedtheincorporationpotentialofthecellswithfewer
nodevelopmentoftrunkandlimbmusclesarestillableto dystrophin+myofibresdetected(Montarrasetal.2005).Fur-
form embryonic and fetal muscles of the head (Rudnicki thermore, in separate studies, a side population of stem
et al. 1993; Tajbakhsh et al. 1997), suggesting distinct lin- cellsisolatedfromskeletalmuscle,bonemarrowstemcells,
eagesofmyoblastsareresponsibleforbothtrunkandhead and vasculature progenitors (mesoangioblasts and peri-
muscletissueformation.Headmusclesalsoharboursatellite cytes)hasbeenshowntocontributetonewlyformedmyo-
cells; however, until recently their embryonic origins were fibres and enter the satellite cell niche with varying
unknown. Harel et al. (2009) have elegantly demonstrated efficacieswithinregeneratingskeletalmuscle(Gussonietal.
throughretrovirallabellingandchick-quailtransplantation 1999;Asakuraetal.2002;LaBarge&Blau,2002;Polesskaya
experimentsthatalongwithheadmusculature,thesatellite etal.2003;Sampaolesietal.2006;Dellavalleetal.2007;see
cellsfromtheheadmusclesalsooriginateinthecranialpar- followingsectionsandFig.2).Followingthesestudies, the
axial mesoderm. Furthermore, through the use of Pax3Cre roleforthesatellitecellasthetruemyogenicstemcellwas
and MesP1Cre (MesP1, mesoderm posterior 1) mouse lines questioned.However,ithassubsequentlybeenshownthat
tomarktrunkandheadmuscles,respectively,theydemon- satellite cells are the definitive muscle-regenerating cell
stratethatthesatellitecellsfromthetrunkarederivedfrom population, as when just one single intact myofibre taken
Pax3+cells,whereasthesatellitecellsfromtheheadmuscles from 3F-nLacZ-2E mice [in which b-galactosidase (b-gal)
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482 Myogenicstemcellpopulations,A.Ottoetal.
reportsexpressionofthemyosinlightchain3Fgeneinthe bresandalsorepopulatethestemcellnichethroughaself-
nuclei of fast myofibers] containing approximately seven renewal mechanism. These processes require one of two
satellitecells wasengraftedintoirradiated mdxdystrophic possible mechanisms to occur. Either a subpopulation of
muscle (b-gal)⁄dystrophin)), thousands of b-gal+⁄dystro- the satellite cell progeny must stop itself from progressing
phin+ myonuclei on hundreds of myofibres within the downthemyogeniclineageandrevertbacktoaquiescent
engrafted muscle were generated, highlighting the huge state, or a subpopulation of the satellite cells must divide
regenerative potential of a minute population of satellite asymmetrically,thusformingbothdifferentiatingandstem
cells (Collins et al. 2005). Moreover, when the sameproce- cell progeny (Fig. 3). Both scenarios are not necessarily
durewascarriedoutwithasingleMyf5nLacZ⁄+myofibrein mutually exclusive and there is supporting evidence that
whichMyf5+satellitecellsarealsob-gal+,thepopulationof bothmodelsmayoccurinvivo(Zammitetal.2004;Kuang
satellite cells formed following the regenerative process etal.2007).
was shown to have expanded 10-fold, demonstrating the Followingactivation,themajorityofsatellitecellsswitch
cellsharbouragreatabilitytoself-renew.Furthermore,the ontheexpressionofthemyogenicregulatoryfactorMyoD,
engrafted stem cell population was stable and over time andondifferentiationtheyexpressmyogenin(Koishietal.
producedincreasedamountsofmyofibres,albeitatmuscle- 1995;Cornelison&Wold,1997;Zammitetal.2004).Inakey
specific varying rates, suggesting long-term engraftment study, Zammit et al. (2004) showed that a subset of acti-
potential of satellite cells in this manner unlike that seen vated satellite cells (i.e. MyoD+) switches off that marker
for transplanted myoblasts (see satellite cells in skeletal whileretainingPax7expression.Thesecellsreturntoaqui-
muscle disease therapy). Critically, the progeny from one escent state, suggesting that satellite cells replenish the
engrafted myofibre were able to re-enter the myogenic niche environment through reversal of the myogenic line-
programme following further rounds of muscle injury, age programme. However, recently satellite cells have also
showing that the satellite cell pool not only efficiently been shown to divide asymmetrically giving differential
replenishes damaged muscle but also repopulates its own daughter cells (Conboy & Rando, 2002; Shinin et al. 2006;
stemcellpool,whichisthencapableofre-activationonfur- Kuang et al. 2007). Initial studies suggested that Notch-1
ther injury to carry out tissue repair (Collins et al. 2005). signallingwascrucialinmaintainingstemcellstatuswithin
Finally,thisstudyhighlightedthefactthatsingle,mechani- the satellite cell population with the antagonist to Notch,
cally isolated satellite cells, separated from the myofibre, Numb, becoming segregated asymmetrically, inducing a
are equally able to engraft successfully into the host and
form new myofibres following intramuscular injection as
are satellite cells within their niche on the myofibre; just
150singlesatellitecellsincorporatedinto10%ofthenewly A
formed muscles fibres. However, satellite cells isolated
through enzymatic digestion were 1000-fold less efficient
atformingnewmyofibresfollowinginjury(Sherwoodetal.
2004a;Collinsetal.2005).Thisprovidesgoodevidencethat
themethodofsatellitecellisolationiscrucialinthesuccess B C
of the engrafted cells to form new tissue. Collins et al.
(2005)istheonlystudythusfartoshowacellpopulation,
thesatellitecells,iscapableofnotjustformingnewmyofi-
bres,orevennewsatellitecells,butisaself-renewingpopu-
lation that is able to continually propagate new skeletal
muscle tissue. This study highlights the fact that if a cell
exists within the satellite cell niche, it is not necessarily a
prerequisiteforfurthermyogenicstemcellactivityandthat
the satellite cell is approximately 1000-fold more efficient
atformingskeletalmusclewhendeliveredinanunchanged
state from that which occurs in the normal niche environ-
ment.
Fig.3 Diagramshowingtheproposedmethodsofsatellitecell
division.(A)Symmetricsatellitecelldivisionformingtwoequivalent
Satellite cell lineage progression and self-
daughtercells,bothexpressingMyf5.(B)Asymmetricdivision
renewal controlledbythenicheenvironmentwiththetruestemcellbeing
juxtaposedagainstthebasallamina.(C)Asymmetricdivisionbrought
As a tissue-specific stem cell population, the satellite cell
aboutthroughmorphologicalchangesinthedifferentiatingprogeny,
must carry out two roles: it must provide differentiating thusinducingthesecellstoinheritincreasedlevelsofcertain
progenythatwillfusewithbothnewanddamagedmyofi- differentiation-inducingproteins.
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Myogenicstemcellpopulations,A.Ottoetal. 483
heterogeneous population of progeny cells, with those However, asymmetric cell division can be achieved inde-
expressing Numb undergoing MRF-induced differentiation pendentlyofnuclearposition.Recently,ithasbeendemon-
(Conboy&Rando,2002).Shininetal.(2006)havesuggested stratedthattheubiquitinligasetripartitemotifprotein32
that satellite cells are capable of asymmetric division (TRIM32)playsakeyroleintheasymmetricdivisionwithin
whereby one of the cells retains the maternal ‘immortal’ neuronal stem cells of the developing murine cortex
DNA strand – as proposed by Cairns (1975). The immortal throughregulatingthelevelsofcMycwithinthecellpopu-
strand hypothesis states that the true stem cell, when lation (Schwamborn et al. 2009). We have subsequently
undergoing division, retains the template strand of DNA, shown that TRIM32 is also necessary and sufficient to
while the newly synthesized strand, which would contain induce differentiation in the satellite cell lineage, where
any mismatchedbase pairs, would beinherited by thecell it acts through a similar pathway within the cell (J.C.
destined for differentiation and therefore mitotic silence, Schwamborn,A.Otto,K.PatelandJ.Knoblich,unpublished
thus reducing the likelihood for a build-up of potentially data).Incorticalembryonicneuronalstemcells,asymmetric
oncogenic mutations in the stem cell population (Cairns, segregation of TRIM32 is not brought about through the
1975).However,ithasbeendemonstratedthatheterogene- alignment of the mitotic spindle, unlike the proposed
ityexistswithinthesatellitecellpopulationwithonlyasub- modelforsatellitecells.Instead,themorphologyofthecells
set of cells which can undergo asymmetric divisions and differsasthecelldivides,withoneprospectivedaughtercell
retainimmortalDNAstrandsinthismanner,suggestingnot harbouring the morphology similar to a differentiating
all Pax7+ quiescent satellite cells on adult myofibres have neuronandtheothermaintainingstemcell-likecharacteris-
equivalent lineage status (Kuang et al. 2006; Shinin et al. tics(Fig.3C).Almostalways,theTRIM32proteinsegregates
2006). Moreover, Beauchamp et al. (2000) showed that into the cell which resembles a differentiating neuron,
when identified through Myf5 expression, fewer satellite regardless of cell alignment (Schwamborn et al. 2009).
cells were seen on extensor digitorum longus (EDL) myofi- Therefore, we propose that a similar model could also be
brescomparedtothenumberof‘negative’satellitecellson involved in the satellite cell lineage whereby altered mor-
myofibres marked with myosin light chain-specific phologyofonedaughtercellwithinthenicheenvironment
3F-nLacZ-2Etransgene(markingallmyonucleiofthefibre), causes the segregation of certain proteins such as TRIM32
suggestingthatamajority,butnotall,ofthetotalsatellite to accumulate and subsequently causes only one of the
cellpoolexpressesMyf5.Usingcreloxmouselinesexpress- daughtercellstoundergodifferentiation,perhapsthrough
ing yellow fluorescent protein (YFP) under the Myf5 pro- Myf5initiation,leavingthesecondtoremainasatruestem
moter,Kuang etal.(2007)wereabletomarkall cellsthat cell (Fig. 3C). In support of this model we have previously
had once expressed Myf5 and found that approximately observed asymmetric segregation of TRIM32 within the
10%ofsatellitecellshadneverexpressedthegene(Kuang satellitecellalongwiththeproteinNumbshowingthat,on
etal.2007).Moreover,oninitialdivision,thissubsetofcells asymmetricdivision,TRIM32andNumbmaysegregateinto
underwent an asymmetric division, predominantly in the thedifferentiatingcellprogeny(J.C.Schwamborn,A.Otto,
vertical plane to the myofibre, which resulted in one K.PatelandJ.Knoblich,unpublisheddata).
daughter cell becoming YFP+ and thus switching on Myf5,
while the other daughter remained Myf5) (Kuang et al.
Regulation of satellite cell proliferation
2007;seeFig.3B).Importantly,cellsdividinginthehorizon-
tal plane of the myofibre predominantly underwent sym- Recently,wehavedemonstratedthatcanonicalWntsignal-
metric divisions (Fig. 3A). Therefore it seems that the ling, acting through the nuclear localization of b-catenin,
directionofcellseparationandthealignmentofthemitotic playsaroleintheearlyproliferativeresponseofthesatel-
spindlemayplayaroleindecidingwhetherthesatellitecell lite cell population to muscle damage (Otto et al. 2008).
formstwo‘differentiating’daughtercells,orasymmetrically Recentworkhasalsoshownnuclearexpressionofb-catenin
formingtwodifferentcelltypes.Itwasshownthatthecell duringthisphasebuthasrelatedtheroleoftheWntpath-
thatremainsincontactwiththebasallaminaofthemyofi- way to the maintenance of the self-renewal programme
brewillretainitsstemcellstatusfollowingasymmetricdivi- withinthesatellitecells(Perez-Ruizetal.2008).Othershave
sion, whereasthe cell that loses contact initiates Myf5 shownthattheWntpathwayalsoisinvolvedinthediffer-
expressionandwillsubsequentlydifferentiate(Fig.3B).This entiation of myoblasts, and that increased Wnt signalling
highlightstheroleofthenicheenvironmentincontrolling followingmuscledamageleadstoinitialacceleratedmyofi-
satellitecellproliferation,andisinkeepingwiththeorigi- bre formation followed by an exhaustion of the myoblast
nal idea from Bischoff (1990) showing that satellite cells pool and overall a perturbed regenerative response (Brack
keptincontactwiththemyofibrearelessabletoprolifer- etal.2008).Wesuggestthattheseseeminglyincompatible
ate. The population of Myf5) satellite cells has been pro- resultsmaybeduetoboththelevelsandtimingoftheWnt
posed as the true stem cells and are shown to repopulate signal during the muscle regenerative response. It is well
thesatellitecellnicheatafarhigherratethanMyf5+cells established that canonical Wnts act to up-regulate the
wheninjectedinvivo(Kuangetal.2007). levels of the myogenic regulatory factor MyoD during
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484 Myogenicstemcellpopulations,A.Ottoetal.
embryonic skeletal muscle formation (Munsterberg et al. are expressed at sufficient levels directly following muscle
1995; Stern et al. 1995; Wagner et al. 2000). Recently, we injury to control satellite cell proliferation rates remains
haveshownthatintheabsenceofMyoDthereisasignifi- unclear.Muchofthefocusofrecentresearchintosatellite
cant delay in the proliferative response of satellite cells cell activation and proliferation has focused on theeffects
immediately following myofibre isolation, along with a of nitric oxide (NO) in inducing hepatocyte growth factor
reductioninthelevelsofnuclearb-catenin(R.Macharia,A. (HGF) and insulin-like growth factor-1 (IGF-1) in mediating
OttoandK.Patel,unpublisheddata).Moreover,ithasalso this response (Allen et al. 1995; Tatsumi et al. 1998; Shee-
been shown that the delay in regeneration due to the han et al. 2000). Both endogenous and neuronal nitric
absenceofMyoDisduetobothperturbationsinthelevels oxide synthase (NOS) proteins are expressed in skeletal
of cell proliferation as well as differentiation (Megeney muscle (Nakane et al. 1993; Kapur et al. 1997) and induce
etal.1996).Thereisstrongmolecularevidencesuggesting the activation of matrix metalloproteinases, causing the
that the levels of active MyoD protein within the satellite breakdownoftheextra-cellularmatrix(ECM)thatcontains
cellarecrucialinregulatingtherateofcellproliferationor bound HGF. Following the breakdown of the ECM, the
differentiation, thus explaining the dual role for MyoD. HGF is released and acts upon its target receptor c-Met,
Prior studies have shown that MyoD transcription within robustlyexpressedonthesatellitecellmembrane(Tatsumi
myoblasts in vitro and in vivo is positively regulated by et al. 2006; Yamada et al. 2008, 2006; see Fig. 4C). It is
serum response factor (SRF) that binds to the regulatory knownthatthec-MetreceptorcanactthroughAkt,inhibit
serumresponseelement(SRE)upstreamoftheMyoDgene GSK-3andstimulatethenuclearaccumulationofb-catenin
(Gauthier-Rouviereetal.1996;L’Honoreetal.2003).More (Monga et al. 2002; Apte et al. 2006). Indeed, our experi-
recently, L’Honore et al. reported that theSRE isa hybrid- ments suggest that ectopic HGF augments the early accu-
binding element regulated through the binding both SRF mulationofb-cateninwithinthesatellitecellnucleusprior
(weakly) and myocyte enhancer factor 2 (MEF2) (strongly). toinitialdivision(A.Otto,andK.Patel,unpublisheddata).
During the myoblast proliferative phase, SRF regulates Therefore, in vivo, the early satellite cell proliferative
MyoDexpression,causinglowlevelstobuildupinthecell response that involves nuclear b-catenin may be induced
and allowing proliferation to occur (L’Honore et al. 2007). through HGF or IGF-1 signalling and not canonical Wnt
Only prior to differentiation is MEF2 expression initiated (Fig.4C).
and it out-competes SRF for the SRE binding site. MEF2
strongly induces MyoD expression, thus increasing the
Satellite cells in skeletal muscle disease
MyoD:Pax7 ratio and inducing differentiation (L’Honore
therapy
etal.2007).IthasbeenshownthatRhoAactsupstreamof
SRF to regulate MyoD and that the RhoA effector protein Theinjectionofpurepopulationsofinvitroexpanded,iso-
mDiaphanous either acts dependently on SRF or indepen- latedprimarymurinemyoblastsintodamagedregenerating
dently through b-catenin, positively regulating MyoD murine skeletal muscle has had mixed but never striking
expression during C2C12 proliferation. Therefore b-catenin results,withlittleornoincorporationofinjectedcellsinto
may act directly to regulate MyoD in the myoblast, as is thesatellitecellnicheandarapidloss(within2–7days)of
observed in the embryo (Munsterberg et al. 1995; Carnac themajorityofinjectedmyoblasts(Rando&Blau,1994;Fan
et al. 1998; Gopinath et al. 2007). Furthermore, it has also etal.1996;Beauchampetal.1999).Furtherproblemshave
been demonstrated that active cyclin D1 (a Wnt b-catenin arisen through the use of in vitro expanded myoblasts in
target gene) induces the nuclear expression of the cyclin- disease therapy. Through systemic delivery, injected myo-
dependentkinase(Cdk)Cdk4.Cdk4bindstoMyoDdirectly, blastcellsareunabletocrossvasculaturebarriersandthere-
maintaining it in an inactive state, allowing cell cycle pro- fore require direct injection into the muscles of patients
gression until the mitogenic signal is depleted. Following (Skuk et al. 2007). Although intra-muscular migration of
mitogen depletion, Cyclin D1 expression drops and the myoblasts can be carried out easily, the area that the
Cdk4repressionofMyoDactivityislost,allowingMyoDto injected population can expand into is limited, and there-
function along with E-protein family members to induce foremultipleinjectionsarerequired(Hughes&Blau,1990;
myogenic gene transcription and myoblast differentiation Skuketal.2007).Moreover,initialstudiesutilizedimmuno-
(reviewedbyWei&Paterson,2001). suppressive agents such as cyclosporin A, that were, first,
Our data have shown that canonical Wnts are tran- inadequate and resulted in graft rejection, and secondly,
scribedduringtheearlysatellitecellresponsetoinjuryand actedtokilltheinjectedcellsastheyunderwentdifferentia-
that signalling through b-catenin in vitro can induce an tion (Hardiman et al. 1993). The poor efficacy of myoblast
early proliferative response in the satellite cell (Otto et al. cellgraftsmaybecounteredthroughtheinjectionofhigh
2008).However,Bracketal.(2008)havedemonstratedthat numbersofcells(upto100·106)thatindeedresultedina
thelevelsofWntsignallinginvivoslowlybuildupinskele- reasonable level of dystrophin+ reconstituted myofibres in
tal muscle during the regenerative response to injury and Duchenne muscular dystrophy (DMD) patients (Skuk et al.
areinitiallylow.ThereforewhethercanonicalWntproteins 1999,2006).However,advancementsintheefficacyofcell
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Journalcompilationªª2009AnatomicalSocietyofGreatBritainandIreland
Myogenicstemcellpopulations,A.Ottoetal. 485
myofibre,satellitecellsarelessmitogenicwhenculturedin
A
vitro, suggesting that the myofibre or satellite cell niche
may be acting to perturb their division (Bischoff, 1990).
Takentogetherwithpreviouslydiscussedworks,thesedata
suggest that contact with the niche environment is crucial
in suppressing satellite cell division and that exposure to
theexternalenvironmentthroughthelossoftheirprotec-
tive niche causes satellite cells to activate their intrinsically
programmedmyogeniccascade,thuslosingtheirabilityto
B produce new stem cell progeny able to return to a quies-
centstatebeneathnewlyformedmyofibres.Thereforethe
isolationofpure,non-activatedpopulationsofsatellitecells
wouldnegatetheneedforlargenumbersofinjectedcells
on treatment, reducing the invasiveness of the procedure
(Montarras et al. 2005). Although isolated satellite cells
showpoorefficacyofengraftmentfollowingsystemicdeliv-
ery, inducing the need for intra-muscular injection, it has
beensuggestedthatthedirectinjectionofcellsintomuscle
would be a more favourable procedure with no risk of
C
potentialembolismoccurring(Peaultetal.2007).
Bone marrow stem cells and muscle
Theadultbonemarrowcontainsapopulationofhaemato-
poietic stem cells (HSC) that are known to form all the
major constituent cell types of the blood system including
erythrocytes, granulocytes and lymphocytes. Bone marrow
also contains stromal cells [often called mesenchymal stem
cells(MSC)]thatarecapableofformingosteoblasts,adipo-
cytes, chondrocytes and fibroblasts (Prockop, 1997). Jiang
etal.(2002)demonstratedthatCD34),CD44),CD45),c-Kit)
multipotentadultprogenitorcells(MAPCs)frombonemar-
row are able to differentiate into all three germ layers
in vitroandform45%chimaeric micefollowing blastocyst
injection, indicating that bone marrow stem cells (BMSCs)
Fig.4 Satellitecellactivationprocessisregulatedthroughnitricoxide
harbour remarkable multipotent characteristics. Moreover,
(NO)-mediatedhepatocytegrowthfactor(HGF)releasefromtheextra-
theskeletalmusclesidepopulationcellsthatarecapableof
cellularmatrix(ECM).(A)Transversecross-sectionthroughamouse
haematopoietic reconstitution are derived from the bone
extensordigitorumlongusmusclestainedwithantibodiesagainst
laminin(red)markingthebasallaminaofindividualmyofibres. marrow (Kawada & Ogawa, 2001a,b; McKinney-Freeman
QuiescentPax7+(green)satellitecellsareseenbeneaththebasal etal.2002).EarliercellculturestudiesrevealedthatBMSCs
lamina.(B)RecentlyformedsatellitecelldoubletmarkedthroughPax7 can differentiateintomyotubesinvitro,showingtheycan
expression(green)onthesurfaceofamatureadultmyofibre.DAPI potentially form skeletal muscle which was later shown to
(blue)stainsallthenucleipresentinAandB.(C)Schematicshowing
occurundernormalmyogeniccultureconditions(Wakitani
howtheregulationofsatellitecellproliferationisregulatedthrough
et al. 1995; Muguruma et al. 2003). Ferrari et al. (1998)
NOproductionwithinthedamagedmyofibre,causingtheactivation
showedforthefirsttimethatinjectionofwholebonemar-
ofmatrixmetalloproteinases,thebreakdownofthesurroundingECM
andthesubsequentreleaseofboundHGF.HGFinducessatellitecell row directly into injured skeletal muscle resulted in the
progenyproliferation,thusregulatingtheregenerationresponseto incorporation of BMSCs into the newly forming skeletal
injury. muscletissue.However,whencomparedtoskeletalmuscle
satellitecells,BMSCswerelessefficientatmyotubefusion,
engraftmentmustbemadetoprovideasufficientlevelof suggesting they were not necessarily an appropriate cell
symptomalleviation. type for potential therapeutic use. Crucially, however,
Mechanically isolated satellite cells show significant abil- unlikesatellitecells,BMSCsmaintainedtheirstemcellchar-
ity to repopulate the stem cell niche following intra- acteristics when systemically delivered, and more readily
muscular injection (Collins et al. 2005; Montarras et al. pass through vascular walls into target tissues. Bone mar-
2005).Ithasbeendemonstratedthatinthepresenceofthe rowcontainsaspecificpopulationofhaematopoieticstem
ªª2009TheAuthors
Journalcompilationªª2009AnatomicalSocietyofGreatBritainandIreland
486 Myogenicstemcellpopulations,A.Ottoetal.
cells termed the side population, based on their ability to myogenic in culture, showed satellite cell characteristics
exclude Hoechst 33341 dye (Goodell et al. 1996). It was and these cells were able to form GFP+ mature myofibres
shown that following systemic injection of either whole following injection into damaged skeletal muscle (LaBarge
BMSCs or the side population bone marrow fraction into &Blau,2002).
irradiatedmdxmice,dystrophin-expressingmyofibreswere Althoughthelevelofcellengraftmentintoregenerating
observedwithinthemuscle,showingthatBMSCswereable muscle tissue is very low, specific muscles incorporate sys-
to differentiate and fuse with existing myofibres through temically delivered BMSCs at largely varied rates with the
vascular delivery, and thus potentially are a key tool in panniculuscarnosus(PC)muscle,showingthehighestlevels
treating hereditary muscular pathologies (Gussoni et al. of BMSC-containing myofibres at 5% compared to the TA
1999).However,Gussonietal.(1999)reportedthatonlythe muscle(usedforthemajorityofcardiotoxinstudies)display-
skeletal muscle-specific side population of stem cells was ing on average just 0.07% (Brazelton et al. 2003). These
abletorepopulatethesatellitecellniche(avitalfeaturefor resultsmayreflectthedifferingratesofmyofibreturn-over
any long lasting therapy – see side population section in specific muscles, as skeletal muscle stress or injury has
below),whereaswholeBMSCsorbonemarrowsidepopu- beendemonstratedtobeanecessaryprerequisitetoBMSC
lation cells failed to do so. Despite BMSCs showing poten- engraftment (Abedi et al. 2004; Sherwood et al. 2004b;
tial therapeutic value, the actual incorporation rates and Palermo et al. 2005). Conflicting evidence exists as to
number of myofibres expressing markers of injected cells whether BMSCs are capable of engrafting into normal
remain extremely low and therefore below the level of healthyskeletalmusclewithoutirradiation,chronicoracute
patient acceptability in most studies reporting the use of injury responses occurring. However, one study has shown
thesecells. thatBMSCsareabletoengraftsuccessfullyintotheTAmus-
Followingthesekeystudies,manygroupssoughttoelu- clefollowingexercise-inducedmusclestress,suggestingthat
cidatetheexactBMSCpopulationabletorepopulatedam- under normal circumstances this process may occur natu-
aged skeletal muscle and the mechanisms by which this rally(Palermoetal.2005).Dreyfusetal.(2004)alsoshowed
processoccurs.AnumberofdifferingBMSCfractionshave that GFP+ BMSCs formed satellite cells and myofibres in
been isolated and used in transplantation studies. A myo- uninjured but irradiated muscle, and these populations
genic BMSC fraction expressing Pax3 and Myf5 has been expanded with time following injection, suggesting that
reported, but showed no increase in dystrophin myofibre muscle injury is not a prerequisite for myofibre fusion of
formation over that of whole bone marrow following BMSCs. Efficiency or robustness of BMSC skeletal muscle
injection into mdx mice (Corti et al. 2002). Using GFP+ incorporationhasbeenassessedthroughvariousbonemar-
BMSCs to lineage-trace transplanted cells, LaBarge & Blau row (BM) transplantation methods. Irradiation and injury
(2002) showed convincingly for the first time that BMSCs havebeenshowntoincreasecellincorporationintomyofi-
areabletotakeupapositionbeneaththebasallaminaof bre formation. Furthermore, mobilization of cells prior to
murine adult skeletal myofibres and express markers of injection through treatment with granulocyte colony-
satellite cells including Myf5, alpha7-integrin and c-Met. stimulating factor (G-CSF) enhanced myofibre formation,
Interestingly, after injection of BMSCs, GFP+ myofibres anddirectinjectionintomusclegaveengraftmentlevelsof
were only observed following exercise-induced muscle up to 12%, suggesting the limitation in myofibre engraft-
damage. Importantly, BMSC-formed satellite cells were ment could be through loss of cells following systemic
Table1 Summaryofseparatemyogenicstemcellpopulationmarkersandregenerativepotentials
Satellite
Invitromyotube Invivomyofibre cellniche mdxdystrophin
Celltype Cellmarkers formation formation engraftment restoral
Satellitecell Pax7,Myf5,CD34,c-Met, Yes Yes Yes Yes
HSC c-Kit,Sca1,CD45 Yes Yes No Yes
HSCSP c-Kit,Sca1,CD45 Yes Yes Yes Yes
SkeletalmuscleSP Sca1,Syndecan4,Pax3? Yes Yes Yes Yes
Muscle-derivedstemcells CD34,Bcl2 Yes Yes Unknown Yes
Mesoangioblast CD34,c-Kit,Flk1 Yes Yes Unknown Yes*
Pericyte a-SMA Yes Yes Yes Yes
Adiposetissue-derived CD13,CD44,CD73,CD90 Yes Yes Unknown Yes
mesenchymalstemcells
*Mesoangioblastssuccessfullyrestoreda-sarcoglycanexpressionfollowingdeliveryintoa-sarcoglycan)⁄)mice(Sampaolesietal.,
2003).
ªª2009TheAuthors
Journalcompilationªª2009AnatomicalSocietyofGreatBritainandIreland