Stemcellsareundifferentiatedcellsofmetazoansthatarecharacterizedbytheirabilitytobothself-renewthroughsymmetricaldivisionanddifferentiateintospecializedcelltypesthroughasymmetricaldivision.Stemcellscanbeclassifiedaccordingtotheirdifferentiationpotential:pluripotentstemcells,suchasembryonicstemcells,canpresumablygiverisetoanycelltype,whereasmulti-potentstemcellshavelimiteddifferentiationpotentialandunipotentstemcellscandifferentiateonlyalongonelineage.Intissues,stemcellsfunctiontomaintainhomeostasis.Theyareoftenquiescentandretaintheirprogenitorpropertiesbyself-renewing.Activation(forexample,byextrinsiccellularfactorsordamagedresidentcells)inducesstemcellstoproliferateanddifferentiatetoregeneratethedamagedtissue.Inadditiontoregenerat-ingtissueinresponsetonormalwearandtear,traumaordisease,residentstemcellsarenowalsounderstoodtoactivelycommunicatewiththetissuemicroenvironment,especiallywithinflammatorycomponents.Mesenchymalstemcells(MSCs)areamongthemostwidelystudiedmultipotentstemcells.Unlikemanyotherstemcellsthataretissue-specific,MSCshavebeenidentifiedinvarioustissues.Their‘stemness’isexemplifiedbytheirabilitytodifferentiateintoosteo-blasts,chondrocytesandadipocytes1.However,MSCsarealsooftenreferredtoas‘mesenchymalstromalcells’owingtotheirabilitytosustainthehomeostasisofthetissuemicroenvironmentbysupportingthefunctionsofparenchymalcells,suchashaematopoieticstemcells(HSCs)2,3.Duetovariationsinexperimentalprotocolsandcellsourcesbetweenstudies,noconsensusexistsregardingthenomenclatureofthesecells.InthisReview,weuse‘MSCs’torefertothestemandstromalfunctionsofthesecells.OwingtothelackofspecificmarkerstostudyendogenousMSCs,mostofourknowledgeofthebiologicalpropertiesofMSCshasbeenobtainedfromthestudyofinvitroexpandedandexogenouslyadministeredMSCs.ExogenouslyadministeredMSCshavebeenfoundtoaccumulateatdamagedtissues4,wheretheypromotetissueregenerationthroughcellreplacementorbyempoweringtheregenerativeImmunoregulatorymechanismsofmesenchymalstemandstromalcellsininflammator...
