regenerative medicine is a new field of medicine that combines information from several 
 medical areas with the aim to recover the organ function lost due to congenital defects , 
 damage , disease , or age1,2,3,4,5,6,7,8 .
the four main approaches of regenerative medicine include cell 
 therapy , gene therapy , transplantation , and tissue engineering6 , 9,10,11 .
previous results from 
 experimental and clinical studies have confirmed the efficacy and safety of these 
 procedures6 , 7 , 9 , 11,12,13 . at the same time
, they can contribute to the maintenance of 
 homeostasis , with stem cells sustaining long - term tissue regeneration when a biological 
 system has depleted its own self - repair mechanisms and reserves14 , 15 .
in addition , recent research 
 suggests that self - repair mechanisms exist in all tissues of the body14 , 16 .
mesenchymal stem cells ( mscs ) are progenitor cells that have self - renewal and multi - lineage 
 differentiation capabilities along with immunomodulatory activities17 , regenerating all of the cell types in the tissue where 
 they are located18 .
mscs reside in a 
 specialized physical location known as a niche , which constitutes a three - dimensional 
 microenvironment containing , in addition to the mscs , neighboring differentiated cell types 
 and the extracellular matrix19 .
mscs may 
 express greater plasticity than traditionally attributed , since they can cross lineage 
 barriers and be reprogrammed , adopting the functional phenotypes and expression profiles of 
 cells from other tissues .
previously it was thought that mscs are not limited in their application , and that they 
 represent a therapeutic fortune .
however , several researchers , including us , have recently 
 demonstrated that this notion is incorrect and that there are certain limitations in the use 
 of mscs23,24,25 .
the aim of this review 
 is to discuss the possibilities , limitations , and future clinical applications of mscs 
 ( table 1table 1.characteristics of the articles included in this reviewmscs in regenerative rehabilitationmain resultsfirst author , journal , and year of publication ( chronology)ref.mscs in the rehabilitation of damaged 
 musclesrejuvenation of the aged skeletal muscle , 
 increased muscle healing after severe injury , slowed muscle tissue degeneration , 
 without significant complications.mcbride ta et al .
j phys ther sci , 2015.46)mscs in stroke rehabilitation formation of synapses and axons and 
 improvement in electrophysiological parameters , clinical improvement on determined 
 by the stroke impact scale and action research arm test , without significant 
 complications.kondziolka d et al .
j phys ther sci , 2015.53)mscs in the rehabilitation of damaged 
 peripheral nervesimproved axonal organization and increased 
 myelin thickness , better functional recovery and improvement in nerve 
 regeneration , without significant complications.salomone r , et al .
j plast reconstr aesthet surg , 2015.58)mscs in the rehabilitation of damaged 
 cartilagerepair of damaged cartilage , cartilage 
 healing , efficient recovery of function , without significant complications.cao l , et al .
the definition of regenerative rehabilitation can be found on the american physical therapy 
 association website :  regenerative rehabilitation is the integration of principles and 
 approaches from rehabilitation and regenerative medicine with the ultimate goal of 
 developing innovative and effective methods that promote the restoration of function through 
 tissue regeneration and repair.26 in order to become a functionally vital organ , a regenerated organ must adapt to the 
 existing homeostasis of the entire body .
transplanted organs undergo a number of adaptive 
 processes to resume their potential27 , 28 .
rehabilitation is crucial to the success 
 of regenerative medicine , and regenerative medicine and rehabilitation can contribute in 
 many ways in patient treatment and care plans .
the integration of rehabilitation into 
 regenerative medicine is necessary to achieve an optimal end point .
regardless of the cause of injury , muscle injury recovery consists of degeneration , 
 inflammation , regeneration , and fibrosis29 , 30 .
chronic and severe muscle damage usually 
 only partially heals and scar tissue forms or fibrosis results , while acute and minor 
 injuries heal well in most cases . if a scar lingers , total regeneration of the muscle is 
 impossible . an alternative approach has been introduced by regenerative medicine , which 
 includes the treatment of skeletal muscle injuries by promoting myofiber regeneration and 
 inhibiting the formation of scar tissue . a factor contributing to the formation of scar 
 tissue is transforming growth factor beta 1 , tgf-131 , 32 ; hence , administration of 
 tgf-1-specific inhibitors has been suggested as an anti - fibrogenic approach33,34,35 . in animal models ,
tgf-1 antagonists have 
 decreased fibrosis remarkably , while concomitantly improving myofiber regeneration33 , 34 .
the regeneration of myofibers is mostly accomplished by muscle mscs , 
 or satellite cells ( fig
these cells are concentrated in the myofiber periphery and are activated in 
 response to muscle injury36,37,38 .
the age - related 
 dysfunction of these muscle stem cells in the elderly leads to an impaired healing response 
 following skeletal muscle injury .
factors circulating in the aged microenvironment force the 
 differentiation of muscle stem cells from a myogenic to a fibrogenic lineage39 , resulting in enhanced scar tissue 
 formation . further compromising regeneration ,
the profibrogenic switch is accompanied by 
 decreased proliferative capacity of aged muscle stem cells .
therefore , it is logical to 
 propose transplanting young muscle stem cells in order to enhance the regenerative potential 
 of aged skeletal muscles .
the transplantation of even 
 embryonic stem cells into an aged environment results in a rapid decline in their 
 regenerative potential40 .
rejuvenation of 
 aged skeletal muscles would help transplanted stem cells in the treatment of skeletal muscle 
 injuries . according to a rodent study , running on a treadmill after stem cell 
 transplantation into severely contused muscle increases the myogenic contribution of the 
 donor cells41 .
thus , muscles capable of producing a 
 contraction may serve as a powerful tool in rejuvenating the regenerative potential of aged 
 muscle .
there is plenty of evidence showing that even exercise programs initiated late in 
 life may enhance the ability of muscle healing after severe injury , while concomitantly 
 slowing down tissue degeneration42,43,44,45,46 .
mesenchymal stem cells : possibilities and limitations in their application in addition to application to the recovery of injured muscles , mscs can be applied in 
 rehabilitation after cerebrovascular insult ( fig . 
 1 ) .
sixteen years ago kondziolka et al . published results of the first clinical 
 study of cell therapy for stroke47 . 

twelve patients , 6 months to 6  years after cerebral infarction onset , were treated by 
 transplantation of immortalized nt2n cells into the subcortical grey matter .
these cells 
 were treated with retinoic acid , which led to differentiation into functional neurons .
of 
 note , after transplantation in animal models of stroke , the nt2n cells formed synapses and 
 axons and led to improvements in electrophysiological parameters . however , three patients 
 died before the end of the study .
the reasons for death were recurrent stroke , myocardial 
 infarct ( mi ) , and pneumonia .
in contrast , no complications48 were observed in the following study that included 14 patients . in 
 these patients ,
clinical improvement was determined by the stroke impact scale and action 
 research arm test . however , the drawback of this study was the small number of patients .
in 
 addition , one of the treated patients died 27 months after cell transplantation from mi .
the 
 autopsy showed that the injected cells survived for at least 2  years after therapy49 . in the past few years
, several clinical 
 studies of cellular therapy50,51,52,53 have resulted in many publications on the topic . in these 
 and similar studies , mscs were usually applied intravenously from 59 days to 312 months 
 after stroke .
the transplantations were conducted safely , without significant complications . 
 in spite of this , definitive conclusions from these studies can not be made since the studies 
 included small numbers of patients .
direct treatment of damaged peripheral nerves with mscs has also been demonstrated to be 
 effective .
numerous encouraging animal studies have shown that stem or progenitor cell 
 treatments can rescue some degree of neurological function after injury54,55,56 ( fig .
the results showed that the msc - treated nerve showed improvements in terms of 
 axonal organization and myelin thickness when compared to nerves that had only been 
 sutured57 .
in another study , mscs were 
 injected subepineurally one week after sciatic nerve injury in rabbits .
nerves that were 
 grafted with mscs showed better functional recovery and improved nerve regeneration58 .
although the exact mechanism by which mscs repair damaged cartilage is still not known , the 
 ability of these cells to induce proliferation and differentiation of resident progenitor 
 cells or their innate ability to differentiate into chondrocytes may contribute to the 
 repair of damaged cartilage ( fig .
transduced mscs expressing il-1ra or 
 antagonists of tnf may both enhance the reparative process through their inherent 
 chondrogenic potential and retard the degradative process in cartilage lesions60 .
advances in the fabrication of 
 biodegradable scaffolds that serve as beds for msc implantation will hopefully lead to 
 better biocompatibility and host tissue integration .
in addition , minimal toxicity has been 
 observed in studies using animal models and genetically manipulated stem cells transduced 
 with retroviral and adenoviral vectors61 . 

thus , future strategies may incorporate the use of mscs as delivery vehicles for 
 chondrogenic and angiogenic growth factors .
traditional rehabilitation training programs focus on the whole body and physiological 
 responses to mechanical loading and/or modalities .
in contrast , regenerative medicine pays 
 more attention to molecular , cellular , and histological aspects of tissue regeneration .
the 
 integration of the two approaches is a great challenge , but would likely have a synergetic 
 impact on tissue regeneration .
the aging process has been shown to alter the immune system , as well as the functionality 
 of stem cells25 .
the proliferative 
 activity of mscs significantly decreases during the aging process , which limits their 
 potential for differentiation62 .
new 
 results have indicated that age - related changes in mscs should be taken into account 
 whenever these cells are considered for therapy ( fig . 
 1 ) .
although mscs from older volunteers had the ability to form colonies , mscs 
 obtained from young donors formed a larger number of colonies with more cells63 .
these studies confirmed previous findings 
 in the literature on the impact of aging on the differentiation and therapeutic potential of 
 mscs .
the effect of msc therapy on the growth or metastasis of 
 existing tumors has not yet been clarified .
it is possible that a small number of stem cells 
 may escape and expand in the graft site and form a tumor64 . as discussed by reya
et al.65 , there is a relationship between stem cells and tumor cells , and 
 normal stem cells could be the targets of transforming mutations .
their 
 research was focused on investigating murine bone marrow - derived mscs and their ability to 
 become spontaneously transformed with accumulated chromosomal abnormalities ( fig .
1 ) , i.e. , structural and numerical aberrations 
 during passage , to form fibrosarcoma in vivo .
the process of transformation 
 was clearly documented , leading to the first report on tumorigenesis of murine bone 
 marrow - derived mscs .
although the authors did not observe spontaneous immortalization during 
 studies on the same type of stem cells derived from humans , this can be explained by the far 
 more complicated mechanisms that keep human cells more stable . in another study ,
the procedure for expanding 
 cell lines was carried out independently in two laboratories , showing rates of 
 transformation of 40% and 50% .
after malignant transformation , the level of telomerase 
 significantly increased , which is common for human mscs68,69,70 . in summary
, the induction of cancer by transplantation and 
 mutational transformation of mscs can not yet be excluded71 .
additional aspects related to the practicality of msc application are their handling and 
 commercialization , which makes it difficult to industrial these processes .
it is very 
 important to ensure pure cell lines , not cross - contaminated ones , before msc therapies can 
 be commercialized .
torsvik et al.72 
 clearly showed , after facing many problems they did not expect , such as cross - contamination , 
 that all cell lines should be verified by dna fingerprinting and the use of electronic 
 databases of authenticated dna profiles .
furthermore , scientific journals , as well as 
 funders of grant proposals , should require such verification of cell lines used in 
 experiments . in conclusion , to fully and reliably investigate the therapeutic potential of mscs , it is 
 crucial to use proper equipment and establish efficient and reproducible processes of 
 isolation , cultivation , and differentiation , so as to obtain a homogenous msc population for 
 use in clinical applications ( fig .
this is one 
 of the biggest challenges for achieving greater successes in this field73 .
 [ purpose ] regenerative medicine and rehabilitation contribute in many ways to a specific 
 plan of care based on a patient s medical status . 
 the intrinsic self - renewing , 
 multipotent , regenerative , and immunosuppressive properties of mesenchymal stem cells 
 offer great promise in the treatment of numerous autoimmune , degenerative , and 
 graft - versus - host diseases , as well as tissue injuries . as such 
 , mesenchymal stem cells 
 represent a therapeutic fortune in regenerative medicine . 
 the aim of this review is to 
 discuss possibilities , limitations , and future clinical applications of mesenchymal stem 
 cells . 
 [ subjects and methods ] the authors have identified and discussed clinically and 
 scientifically relevant articles from pubmed that have met the inclusion criteria . 
 [ results ] direct treatment of muscle injuries , stroke , damaged peripheral nerves , and 
 cartilage with mesenchymal stem cells has been demonstrated to be effective , with 
 synergies seen between cellular and physical therapies . over the past few years 
 , several 
 researchers , including us , have shown that there are certain limitations in the use of 
 mesenchymal stem cells . aging and spontaneous malignant transformation of mesenchymal stem 
 cells significantly affect the functionality of these cells . 
 [ conclusion ] definitive 
 conclusions can not be made by these studies because limited numbers of patients were 
 included . 
 studies clarifying these results are expected in the near future . 
INTRODUCTION
REGENERATIVE REHABILITATIONA NEW FIELD OF MEDICINE
POSSIBILITIES IN THE APPLICATION OF MSCS TO REGENERATIVE REHABILITATION
LIMITATIONS IN THE APPLICATION OF MSCS TO REGENERATIVE REHABILITATION