Mesenchymal stem cells (MSCs), also termed mesenchymal stromal cells, are multipotent adult stem cells characterized by plastic adherence in standard culture conditions, trilineage differentiation potential into osteoblasts, chondrocytes, and adipocytes, and expression of specific surface markers including CD73, CD90, and CD105 while lacking CD45, CD34, CD14, CD11b, CD19, CD79a, and HLA-DR.[1] These cells possess self-renewal capacity and can be isolated from diverse sources such as bone marrow, adipose tissue, umbilical cord, and dental pulp, with bone marrow historically serving as the primary source due to its high yield of adherent progenitors.[2] MSCs exhibit immunomodulatory properties, including suppression of T-cell activation and promotion of anti-inflammatory cytokine production, which underpin their exploration in treating conditions like graft-versus-host disease and autoimmune disorders.[3] In regenerative medicine, MSCs have shown promise for tissue repair in orthopedic applications such as osteoarthritis and cartilage defects, though clinical translation faces hurdles including heterogeneous cell populations, variable potency across donors, and reliance on paracrine mechanisms rather than direct differentiation, with some trials reporting limited long-term engraftment and efficacy.[4][5] Despite low observed tumorigenic risk in aggregated studies, standardization of isolation and expansion protocols remains critical to mitigate risks of chromosomal instability during ex vivo culture.[6]
Definition and Characteristics
Core Definition and Identifying Criteria
Mesenchymal stem cells (MSCs), designated as multipotent mesenchymal stromal cells by the International Society for Cell & Gene Therapy (ISCT), constitute a subset of adult stromal cells derived from mesodermal tissues, exhibiting self-renewal potential and differentiation capacity into mesenchyme-derived lineages such as osteoblasts, adipocytes, and chondrocytes. First isolated from bone marrow by Alexander Friedenstein in the 1960s as plastic-adherent, colony-forming units-fibroblast (CFU-F) capable of osteogenic differentiation in vivo, the nomenclature "mesenchymal stem cells" was introduced by Arnold Caplan in 1991 to emphasize their regenerative role in mesenchymal tissues.[7][8][9]The ISCT's 2006 position statement delineates minimal experimental criteria for MSC identification: adherence to plastic surfaces under standard culture conditions using basal media supplemented with fetal bovine serum; expression of CD73, CD90, and CD105 on at least 95% of cells, coupled with absence (less than 2%) of CD45, CD34, CD14 or CD11b, CD79a or CD19, and HLA-DR markers assessed via flow cytometry; and demonstrated trilineage differentiation potential in vitro, evidenced by alizarin red staining for osteogenesis, oil red O for adipogenesis, and alcian blue or toluidine blue for chondrogenesis.70881-7/fulltext)[10][1]These criteria, grounded in empirical assays rather than presumptive stemness, facilitate standardization across studies, though subsequent analyses highlight limitations, such as insufficient emphasis on functional potency or tissue-specific variations, prompting calls for expanded metrics including low tissue factor expression to mitigate thrombogenic risks in therapeutic contexts.[11][12]
Nomenclature and Classification Debates
The term "mesenchymal stem cell" (MSC) was coined by Arnold Caplan in 1991 to describe adherent, multipotent progenitor cells isolated from bone marrow and periosteum capable of differentiating into skeletal tissues such as bone, cartilage, and fat, emphasizing their role in tissue regeneration.[13][8] Prior to this, such cells were referred to as "marrow stromal cells" or "fibroblast colony-forming units," reflecting their supportive role in the hematopoietic microenvironment rather than explicit stem cell properties.[14][15]Debates over classification intensified as evidence accumulated that MSCs exhibit limited self-renewal in vivo and primarily exert effects through paracrine signaling rather than direct lineage replacement, challenging their status as true stem cells.[16][17] True stem cells typically demonstrate asymmetric cell division (ACD) to maintain a stem pool while producing differentiated progeny, but MSCs undergo predominantly symmetric divisions, aligning them more closely with progenitor or stromal populations.[16] In 2017, Caplan himself proposed renaming MSCs as "medicinal signaling cells" (MedSCs) to highlight their trophic, immunomodulatory, and anti-inflammatory secretome over stem-like differentiation potential, arguing the original term misled focus toward engraftment and tissue replacement unsupported by clinical data.[18][19]The International Society for Cellular Therapy (ISCT) addressed nomenclature ambiguity in 2006 by establishing minimal criteria for MSCs—plastic adherence, specific surface marker expression (e.g., CD105+, CD73+, CD90+; negative for hematopoietic markers), and trilineage differentiation in vitro—but recommended "multipotent mesenchymal stromal cell" for cells meeting these without proven stemness, distinguishing them from bona fide stem cells.[15][20] Heterogeneity across sources (e.g., bone marrow versus adipose) further complicates classification, as subpopulations vary in potency and marker profiles, leading some researchers to advocate for source-specific subtypes rather than a unified "MSC" category.[21][22] Despite these critiques, "MSC" persists in literature due to its entrenched use, though proposals like "perivascular cells" or "regulatory mesenchymal cells" have emerged to capture their pericytes-like vascular niche roles and broader regulatory functions.[23][24] Ongoing debates underscore that while MSCs meet operational definitions for therapeutic applications, their classification as stem cells lacks rigorous empirical support from in vivo lineage tracing or long-term repopulation assays.[16][25]
Morphological and Phenotypic Markers
Mesenchymal stem cells (MSCs) display a fibroblast-like morphology in standard in vitro culture conditions, characterized by spindle-shaped or elongated cells with a flattened, adherent structure.[26] These cells exhibit plastic adherence, forming colonies of rapidly proliferating, small round cells that transition into larger, flattened forms over passages.[26] Morphological heterogeneity exists, including small rapidly self-renewing cells, intermediate spindle-shaped cells, and large flattened cells, which correlate with varying proliferative and differentiative potentials.[26] However, this morphology overlaps significantly with dermal fibroblasts, complicating distinction based solely on visual traits.[27]Phenotypically, MSCs are identified by specific surface marker expression as outlined by the International Society for Cell Therapy (ISCT) minimal criteria established in 2006.[28] Positive markers include CD73, CD90, and CD105, with at least 95% of cells expressing these in flow cytometry analysis.[28] Negative markers encompass hematopoietic indicators such as CD45, CD34, CD14 (or CD11b), CD79a (or CD19), and HLA-DR, with less than 2% expression required.[28] These criteria emphasize multipotency alongside trilineage differentiation potential into osteoblasts, adipocytes, and chondroblasts, though marker profiles alone do not uniquely define MSCs due to shared expression with fibroblasts and other stromal cells.[27][29]Variations in marker expression occur across tissue sources and culture conditions, with bone marrow-derived MSCs typically showing higher homogeneity in CD105 and CD73 compared to adipose-derived counterparts.[30] Functional assays remain essential, as phenotypic markers lack absolute specificity; for instance, fibroblasts can mimic MSC profiles under similar expansion protocols.[31][27] Recent studies confirm that while ISCT criteria guide identification, advanced techniques like single-cell RNA sequencing reveal subclonal diversity beyond traditional markers.[32]
Tissue Sources and Isolation Methods
Bone Marrow as Primary Source
Bone marrow-derived mesenchymal stem cells (BM-MSCs) represent the original and most extensively studied source of mesenchymal stem cells, first identified by Alexander Friedenstein in the 1960s through experiments demonstrating the presence of non-hematopoietic, adherent cells in rodent bone marrow that formed fibroblast-like colonies and differentiated into osteogenic and chondrogenic lineages.[33] These cells, initially termed colony-forming unit-fibroblasts (CFU-F), were reported in human bone marrow in 1970, comprising a rare population of approximately 0.001% to 0.01% of total nucleated cells.[34] BM-MSCs' historical precedence and robust characterization have established bone marrow as the reference standard for MSC isolation and potency assessment across tissue sources.[7]Isolation of BM-MSCs begins with aspiration of 10 to 50 mL of bone marrow, typically from the posterior superior iliac crest under local anesthesia, yielding mononuclear cells via density gradient centrifugation using media like Ficoll-Paque.[35] The mononuclear fraction is then plated in culture flasks with alpha-minimum essential medium supplemented with fetal bovine serum, where MSCs selectively adhere to plastic surfaces within hours, allowing removal of non-adherent hematopoietic cells through medium changes.[36] Primary cultures reach 70-80% confluence in 14-21 days, with cells exhibiting spindle-shaped morphology and expressing surface markers such as CD73, CD90, and CD105 while lacking CD45 and CD34.[37]Despite their low frequency, BM-MSCs demonstrate superior osteogenic and chondrogenic differentiation potential compared to MSCs from other adult tissues, attributed to the bone marrow niche's enrichment for skeletal progenitors.[38] However, procurement involves invasive procedures with risks like pain and infection, and yields decline with donor age, prompting exploration of alternative sources while BM-MSCs remain pivotal in clinical trials for their established safety profile from over 900 registered studies as of 2020.[39][40]
Adipose and Other Adult Tissues
Adipose-derived mesenchymal stem cells (AD-MSCs), also known as adipose stromal/stem cells, are harvested primarily from subcutaneous white adipose tissue obtained via liposuction or surgical excision.[41]Isolation typically involves enzymatic digestion with collagenase to separate the stromal vascular fraction, followed by centrifugation and culture expansion, though mechanical methods without enzymes have been explored for reduced processing time and potential preservation of cell viability.[42] One gram of adipose tissue yields approximately 5,000 AD-MSCs, significantly higher than the 100–1,000 cells per milliliter of bone marrow aspirate, enabling scalable production for therapeutic applications.[43] Compared to bone marrow-derived MSCs (BM-MSCs), AD-MSCs exhibit greater proliferative capacity and colony-forming efficiency, with no significant decline in quality correlated to donor age, unlike BM-MSCs which show age-related reductions in yield and potency.[44][45]AD-MSCs demonstrate robust multipotency, particularly enhanced adipogenic differentiation with increased lipid vesicle formation and expression of adipocyte markers, alongside comparable osteogenic and chondrogenic potential to BM-MSCs.[46] Their harvest is less invasive, avoiding the pain and morbidity of bone marrowaspiration, and leverages abundant, discarded lipoaspirate as a byproduct of cosmetic procedures, reducing ethical and logistical barriers.[41] Empirical data from comparative studies indicate AD-MSCs support superior hematopoiesis and angiogenesis in preclinical models, though clinical translation requires addressing variability in tissue processing and donor factors like body mass index, which influence cell yield and growth.[47][48]Beyond adipose tissue, MSCs are isolated from other adult sources including dental pulp, periodontal ligament, gingival tissue, and skin dermis. Dental pulp stem cells (DPSCs), derived from the pulp of permanent or deciduous teeth via enzymatic digestion or explant culture, exhibit high self-renewal and trilineage differentiation, with preclinical evidence of efficacy in regenerating periodontal ligaments, alveolar bone, and ischemic tissues.[49][50] DPSCs and gingival-derived MSCs demonstrate immunomodulatory effects via adenosinergic pathways, potentially more efficient than BM-MSCs in suppressing inflammation, as shown in vitro and in animal models of wound healing and autoimmunity.[51] Yields from dental tissues are lower than adipose (typically 10^4–10^5 cells per tooth), but their neural crest origin confers unique neuroregenerative potential, evidenced by differentiation into neuron-like cells and support for peripheral nerve repair.[52][53]Skin-derived precursor cells and dermal fibroblasts with MSC-like properties are obtained through biopsy and culture, offering accessibility for autologous use but with lower yields and more heterogeneous populations requiring stringent marker-based purification (e.g., CD34+/CD90+).[54] Peripheral blood contains circulating MSCs at very low frequencies (1–10 per 10^6 mononuclear cells), necessitating mobilization protocols like G-CSF administration for enrichment, though this source remains suboptimal due to inconsistent potency and scalability compared to solid tissues.[54] These alternative adult sources provide tissue-specific advantages, such as oral MSCs' trophic effects in maxillofacial regeneration, but face challenges including donor site morbidity and variability in differentiation efficiency across studies.[55] Overall, while adipose remains the most practical non-marrow adult source for high-volume applications, empirical comparisons highlight the need for source-specific optimization to maximize therapeutic outcomes.[56]
Perinatal and Alternative Sources
Perinatal tissues, such as the umbilical cord, placenta, and amniotic fluid, serve as rich, non-invasive sources of mesenchymal stem cells (MSCs) obtainable post-delivery from otherwise discarded materials, circumventing ethical issues tied to embryonic derivations. These MSCs exhibit enhanced proliferative rates, lower senescence, and hypoimmunogenic profiles relative to adult tissue counterparts, attributed to their intermediate developmental stage between embryonic and adult cells.[57][58]Umbilical cord MSCs (UC-MSCs) are predominantly isolated from Wharton's jelly, the gelatinous matrix enveloping umbilical vessels, via methods including mechanical mincing followed by explant culture or enzymatic digestion with collagenase type I and hyaluronidase, yielding 10^6 to 10^8 cells per cord with high colony-forming unit-fibroblast (CFU-F) efficiency exceeding 1%. UC-MSCs demonstrate population doubling times of 24-48 hours and over 30 doublings before senescence, surpassing bone marrow MSCs in expansion potential.[59][57][60]Placental-derived MSCs encompass subtypes from the amniotic membrane (AM-MSCs), chorionic villi (CV-MSCs), and decidua, isolated through differential enzymatic treatments like trypsin-EDTA for amniotic layers or collagenase/dispase for chorionic components, often combined with Percoll density gradient separation to achieve purity above 90%. These cells maintain tri-lineage differentiation (adipogenic, osteogenic, chondrogenic) and secrete higher levels of trophic factors such as hepatocyte growth factor compared to adult MSCs.[61][57]Amniotic fluid MSCs (AF-MSCs), harvestable via amniocentesis (typically 15-20 weeks gestation) or post-partum, are selectively expanded in mesenchymal media after initial mononuclear cell isolation by centrifugation, originating from fetal compartments including neural crest and yielding cells with broad multipotency, including neurogenic and hepatic lineages. AF-MSCs proliferate faster than adult MSCs, with yields of up to 10^5 cells per ml of fluid and minimal ethical barriers due to non-controversial access.[62][63][64]Alternative sources beyond adult and perinatal tissues include dental pulp stem cells (DPSCs) from permanent or deciduous teeth, isolated by enzymatic dissociation (e.g., collagenase/dispase) from pulp tissue post-extraction, providing 10^4-10^5 cells per tooth with pronounced odontogenic and neurogenic differentiation but limited scalability due to donor age dependency. Synovial fluid and membrane MSCs, extracted via arthroscopic lavage or biopsy digestion, offer joint-specific tropism for osteoarthritis applications, though isolation yields remain low at 1-5% of total nucleated cells. Induced pluripotent stem cell (iPSC)-derived MSCs, generated by reprogramming somatic cells and directing toward mesenchymal fate via small molecules like CHIR99021, represent a scalable, patient-specific alternative but require rigorous validation for tumorigenic risks absent in primary sources.[65][66][67]
Biological Mechanisms
Self-Renewal and Multipotent Differentiation
Mesenchymal stem cells (MSCs) possess self-renewal capacity, enabling them to undergo multiple divisions while preserving an undifferentiated state, a property demonstrated through clonal expansion assays where single MSCs generate daughter cells retaining multipotency.[68] This process is regulated by signaling pathways such as Wnt/β-catenin, which promotes proliferation and inhibits differentiation by stabilizing β-catenin and activating target genes like cyclin D1, and TGF-β, which modulates senescence to extend replicative lifespan in primitive MSCs.[69] However, self-renewal is limited compared to embryonic stem cells, with MSCs typically expanding 20-50 population doublings before senescence, influenced by donor age and culture conditions.[70]The multipotent differentiation potential of MSCs is primarily evidenced by their ability to commit to mesodermal lineages under specific in vitro inductive cues, forming osteoblasts (via upregulation of Runx2 and alkaline phosphatase, yielding mineralized nodules), chondrocytes (producing collagen type II and sulfated glycosaminoglycans in pellet cultures), and adipocytes (accumulating lipid vacuoles with PPARγ expression).[71] These outcomes are quantified in studies showing over 80% differentiation efficiency in optimized media, such as dexamethasone-supplemented conditions for osteogenesis.[72] While some reports suggest broader plasticity, including neuroectodermal or endodermal markers, rigorous lineage tracing confirms predominant mesodermal restriction, with in vivo contributions often augmented by paracrine rather than direct transdifferentiation.[73] Heterogeneity among MSC populations, such as bone marrow versus adipose-derived, affects differentiation bias, with adipose MSCs favoring adipogenesis.[74]
Paracrine Signaling and Immunomodulation
Mesenchymal stem cells (MSCs) primarily exert therapeutic effects via paracrine signaling, secreting bioactive molecules that modulate the microenvironment without relying on extensive engraftment or differentiation.[75] This secretome comprises cytokines, growth factors, chemokines, and extracellular vesicles such as exosomes and microvesicles, which are released in response to local inflammatory or hypoxic cues.[76] Unlike direct cellular replacement, paracrine actions promote tissue repair through anti-apoptotic, pro-angiogenic, and anti-fibrotic mechanisms, with empirical studies demonstrating that conditioned media from MSCs recapitulates many in vivo benefits observed with whole-cell infusions.[77]Key components of the MSC secretome include vascular endothelial growth factor (VEGF), hepatocyte growth factor (HGF), fibroblast growth factor (FGF), and insulin-like growth factor-1 (IGF-1), which enhance endothelial cell proliferation and vascularization while inhibiting apoptosis in damaged tissues.[78] Exosomes, nanoscale vesicles enriched in microRNAs (e.g., miR-146a) and proteins, facilitate intercellular communication by transferring cargos that downregulate pro-inflammatory pathways like NF-κB in recipient cells.[79] These factors collectively foster a trophic environment, as evidenced by preclinical models where MSC-derived secretome accelerated wound healing and reduced scar formation in rodentskin injury assays.[80]Immunomodulation by MSCs occurs predominantly through paracrine effectors that sense and respond to immune activation, licensing MSCs to suppress excessive inflammation while preserving basal immunity.[81] In inflammatory milieus, MSCs upregulate indoleamine 2,3-dioxygenase (IDO), which depletes tryptophan to arrest T-cell proliferation, alongside prostaglandin E2 (PGE2) and transforming growth factor-β (TGF-β) that inhibit effector T-helper 1 (Th1) and Th17 cells while expanding regulatory T cells (Tregs).[79] This bidirectional regulation also polarizes macrophages from pro-inflammatory M1 to anti-inflammatory M2 states via IL-10 and CCL18 secretion, and dampens B-cell antibody production through soluble factors like TGF-β.[78]Exosome-mediated immunomodulation further amplifies these effects, with MSC-derived exosomes delivering miRNAs that suppress Th17 differentiation and cytokine storms in models of autoimmune uveitis and sepsis.[79] In vitro and animal studies confirm dose-dependent inhibition of mixed lymphocyte reactions by MSC secretome, reducing IFN-γ and TNF-α while elevating IL-10 levels.[82] Clinical translation, though limited by secretome standardization challenges, shows preliminary efficacy; for instance, phase I/II trials of MSC exosomes in graft-versus-host disease (GVHD) reported reduced acute symptoms via lowered pro-inflammatory cytokines, though larger randomized trials are needed to verify causality over placebo effects.[75] Variability in donor MSCs and licensing conditions underscores the need for rigorous empirical validation, as not all secretome components consistently elicit immunosuppression across contexts.[76]
Additional Properties: Antimicrobial and Trophic Effects
Mesenchymal stem cells (MSCs) exhibit trophic effects primarily through paracrine secretion of bioactive molecules, including growth factors such as vascular endothelial growth factor (VEGF), hepatocyte growth factor (HGF), fibroblast growth factor-2 (FGF-2), and insulin-like growth factor-1 (IGF-1), which promote angiogenesis, inhibit apoptosis, and stimulate proliferation and migration of endogenous cells in damaged tissues.[83][84] These factors, along with cytokines like interleukin-6 (IL-6) and transforming growth factor-beta (TGF-β), contribute to tissue repair by modulating the extracellular matrix and enhancing survival of neighboring cells without requiring MSC differentiation or engraftment.[85] Extracellular vesicles derived from MSCs further amplify these trophic actions by delivering microRNAs and proteins that support neuroprotection, endothelial cell function, and anti-inflammatory responses in preclinical models of injury.[86][87]In addition to trophic support, MSCs demonstrate antimicrobial properties via both direct and indirect mechanisms. Direct effects involve the secretion of antimicrobial peptides (AMPs) such as LL-37 (cathelicidin), hepcidin, and lipocalin-2, which disrupt bacterial membranes, sequester iron to limit microbial growth, and exhibit activity against Gram-positive and Gram-negative bacteria, as well as some fungi and viruses.[88][89] Indirect antimicrobial actions arise from MSC-induced recruitment and activation of innate immune cells, such as macrophages and neutrophils, alongside modulation of the inflammatory milieu to favor pathogen clearance while minimizing excessive tissue damage.[90] Studies in human and animal models, including those using adipose-derived MSCs, confirm enhanced bacterial killing in vitro and reduced infection burden in vivo, particularly against clinical isolates like Staphylococcus aureus and Escherichia coli.[91][92] These properties are donor- and condition-dependent, with inflammatory priming (e.g., via lipopolysaccharide exposure) upregulating AMP production.[93]
Clinical Applications and Evidence
Established Uses with Empirical Support
Mesenchymal stromal cells (MSCs), particularly bone marrow-derived allogeneic formulations, have demonstrated empirical support in the treatment of steroid-refractory acute graft-versus-host disease (SR-aGVHD) in pediatric patients following hematopoietic stem cell transplantation. On December 18, 2024, the U.S. Food and Drug Administration (FDA) approved Ryoncil (remestemcel-L-rknd), the first MSC-based therapy for this indication, for use in patients aged 2 months to 17 years who have failed systemic corticosteroidtherapy.[94][95] The product consists of culture-expanded MSCs administered intravenously at a dose of 2 million cells per kilogram of body weight, twice weekly for four weeks.[95]Approval was supported by data from the phase 3 GVHD001 trial (NCT02336230), a single-arm study involving 54 pediatric patients with grades B-D SR-aGVHD, which reported a Day 28 overall response rate (ORR) of 70.4%, surpassing the prespecified historical controlthreshold of 45%.[96] This ORR included complete responses in gastrointestinal and liver involvement, with sustained responses observed at Day 100 in subsets of responders.[96] The mechanism underlying efficacy is attributed primarily to MSCs' immunomodulatory effects, including suppression of T-cell proliferation and reduction of pro-inflammatory cytokines, rather than long-term engraftment.[97] Prior phase 3 trials in adults failed to meet endpoints against active controls, highlighting age-specific differences in response, possibly due to variations in immune dynamics or disease severity.[4]No other MSC therapies have achieved regulatory approval with comparable empirical validation in major jurisdictions as of October 2025, though investigational applications in conditions like refractory fistulizing Crohn's disease (e.g., darvadstrocel) previously showed phase 3 efficacy in combined remission rates of approximately 50% at 24 weeks but faced subsequent market withdrawals due to commercial or safety considerations.[98][99] Systematic reviews confirm consistent safety across MSC administrations, with infusional toxicities below 10% and no tumorigenicity signals in long-term follow-up, but underscore that GVHD remains the sole indication with level 1 evidence from regulatory-endorsed trials.[4][100]
Investigational Therapies: Autoimmune and Degenerative Diseases
Mesenchymal stem cells (MSCs) are being investigated for their immunomodulatory effects in autoimmune diseases, where they suppress pro-inflammatory T-cell responses and promote regulatory T-cell expansion, potentially mitigating tissue damage from dysregulated immunity. A 2025 meta-analysis of randomized controlled trials involving MSCs for autoimmune and rheumatic conditions, including rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE), reported significant improvements in disease activity scores and clinical symptoms, with no increased risk of serious adverse events compared to controls.[101] In RA, intravenous or intra-articular MSC infusions have reduced Disease Activity Score 28 (DAS28) by up to 1.5 points at 6-12 months post-treatment in phase II trials, attributed to decreased synovial inflammation and autoantibody levels.[102]For multiple sclerosis (MS), intrathecal or intravenous MSCs have shown preliminary efficacy in stabilizing Expanded Disability Status Scale (EDSS) scores, with one phase II trial reporting halted progression in 55% of relapsing-remitting MS patients over 2 years and reduced gadolinium-enhancing lesions on MRI, alongside favorable safety profiles lacking severe complications.[103] In Crohn's disease, human umbilical cord-derived MSCs achieved endoscopic response rates (≥50% reduction in Simple Endoscopic Score for Crohn's Disease) in 60-70% of refractory patients at week 52 in a 2024 phase III-equivalent study, correlating with clinical remission (Crohn's Disease Activity Index <150) sustained without routine immunosuppression escalation.[104] However, trial heterogeneity in MSC source, dose (typically 1-2 × 10^6 cells/kg), and patient selection limits generalizability, with some studies noting transient benefits waning after 12 months.[105]In degenerative diseases, MSCs are explored for paracrine-mediated neuroprotection and tissue repair, particularly in osteoarthritis (OA) and neurodegenerative disorders like amyotrophic lateral sclerosis (ALS) and Parkinson's disease. Intra-articular bone marrow-derived MSCs (50-100 × 10^6 cells) improved pain scores by 30-40% and function (Western Ontario and McMaster Universities Osteoarthritis Index) at 12 months in multiple phase II trials for knee OA, with MRI evidence of cartilage volume stabilization, though placebo effects and small sample sizes (n=20-60) temper conclusions.[106] For ALS, intrathecal MSCs enhanced somatosensory evoked potentials in 55% of patients and voluntary muscle contraction in 44% in a 2022 study, suggesting trophic support to motor neurons, but overall survival benefits remain unproven in larger cohorts.[107]Parkinson's disease trials using stereotactic or intravenous MSCs have demonstrated modest dopamine neuron preservation via anti-apoptotic factors in preclinical models translated to phase I/II, with one 2025 review noting improved Unified Parkinson's Disease Rating Scale motor scores by 10-15% at 6 months, yet lacking disease-modifying evidence due to short follow-up and variable engraftment.[108] Across these applications, MSCs exhibit consistent safety with infusion-related events in <10% of cases, but efficacy hinges on optimizing cell potency and delivery, as meta-analyses highlight inconsistent outcomes from donor variability and licensing inefficiencies.[70] Ongoing phase III trials emphasize the need for standardized potency assays to discern true regenerative signals from symptomatic relief.[4]
Regenerative and Other Experimental Indications
Mesenchymal stem cells (MSCs) have been investigated for regenerative applications targeting tissue repair in conditions such as osteoarthritis (OA), where intra-articular injections aim to promote cartilage regeneration and reduce inflammation. Clinical trials, including phase II randomized studies, have demonstrated that allogeneic adipose-derived MSCs improve knee pain and function in unoperated OA patients, with evidence of cartilage volume preservation via MRI assessments up to 24 months post-injection.[109] However, while short-term symptomatic relief is consistent, histological confirmation of robust cartilage regeneration remains limited, with benefits often attributed to paracrine effects rather than direct differentiation.[110]In cardiac repair following myocardial infarction (MI), MSCs delivered via intracoronary or intramyocardial routes have shown potential to enhance left ventricular ejection fraction and reduce adverse remodeling in preclinical and early-phase human trials. A 2025 meta-analysis of randomized controlled trials reported significant improvements in cardiac function metrics, such as ejection fraction increases of 3-5%, alongside lower major adverse cardiac event rates, though long-term survival benefits require further validation in larger cohorts.[111] Delivery route influences efficacy, with intramyocardial injection yielding superior retention compared to intravenous administration in animal models of chronic ischemic cardiomyopathy.[112]For spinal cord injury (SCI), experimental MSC therapies focus on neuroprotection, angiogenesis, and axonal remyelination through intrathecal or intravenous administration. Systematic reviews of clinical trials indicate safety and modest locomotor improvements, such as enhanced ASIA scores in traumatic SCI patients treated within 6 months of injury, potentially via immunomodulation and trophic factor secretion rather than neuronal replacement.[113] A 2024 phase I/II trial using adipose-derived MSCs intrathecally reported no serious adverse events and preliminary gains in sensory-motor function, underscoring the need for phase III studies to assess durability.[114]Other experimental indications include bone non-union fractures and muscular dystrophy, where MSCs scaffolded with biomaterials promote osteogenesis and muscle regeneration. Trials for critical-sized cranial defects and Duchenne muscular dystrophy have shown radiological healing and functional gains, respectively, but with variable engraftment rates below 5% in human tissues.[115] In wound healing, topical or systemic MSC application accelerates closure in chronic ulcers by enhancing epithelialization and vascularization, as evidenced by phase II data from diabetic foot trials.[116] Overall, while MSCs exhibit low immunogenicity and feasibility across these indications, inconsistent regenerative outcomes highlight challenges in cell homing, survival, and scalable manufacturing for clinical translation.[117]
Observed Risks, Side Effects, and Limitations
Mesenchymal stem cell (MSC) therapies have demonstrated a generally favorable short-term safety profile in controlled clinical trials, with meta-analyses of over 1,000 patients across various indications reporting no significant increase in serious adverse events compared to controls, though transient infusion-related reactions such as fever, chills, and headache occur in up to 20-30% of cases.[118][119] However, thromboembolism and pulmonary embolism have been documented as more specific risks, particularly with intravenous administration, linked to cell aggregation and embolization in lung vasculature, with incidence rates ranging from 1-5% in phase I/II trials.[119] Fibrosis at injection sites or ectopic tissue formation represents another observed complication, potentially arising from uncontrolled differentiation or paracrine signaling, as reported in preclinical models and early human studies.[119][120]Tumorigenicity remains a theoretical yet empirically low-risk concern, with no confirmed cases of de novo tumor formation directly attributed to MSCs in rigorous trials up to 2024, though MSCs may inadvertently promote angiogenesis in preexisting malignancies via secreted factors like VEGF, prompting exclusion criteria in cancer patients.[121][122] Unregulated or unapproved MSC interventions, often marketed directly-to-consumer, have led to severe outcomes including bacterial infections, blindness from ocular injections, and at least five reported deaths from complications like sepsis or tumor progression as of 2021, underscoring risks from inadequate manufacturing and lack of oversight rather than inherent cellular properties.[123][124]Key limitations include MSC heterogeneity, where donor age, tissue source, and culture expansion introduce variability in potency and safety, contributing to inconsistent therapeutic outcomes and potential off-target effects like unintended immunomodulation.[125] Poor engraftment and retention—often below 5% at target sites—limit efficacy, as MSCs predominantly exert paracrine effects before rapid clearance, necessitating repeated dosing that escalates costs and cumulative risks.[126][125] Regulatory challenges persist, with the FDA approving only hematopoietic stem cells for specific indications as of 2025, while MSC products face hurdles in demonstrating durable benefits beyond placebo in large randomized controlled trials for most applications.[127] Long-term data gaps, spanning beyond 2-5 years, hinder assessment of delayed risks like chromosomal instability from prolonged ex vivo expansion.[120] Despite these, intravenous MSC delivery shows no excess mortality in meta-analyses of diverse conditions, though minor events like transient constipation or fatigue warrant monitoring.[128][118]
Research Progress and Innovations
Preclinical Models and Mechanistic Insights
Preclinical investigations of mesenchymal stem cells (MSCs), often termed mesenchymal stromal cells in recent literature, have predominantly employed rodent models of inflammatory, ischemic, and degenerative conditions to probe therapeutic mechanisms. These studies reveal that MSCs rarely achieve significant long-term engraftment or differentiation into host tissue lineages, challenging early assumptions of robust multipotency; instead, transient presence suffices for benefits via secreted factors. A meta-analysis of over 30 ischemic stroke models in rodents demonstrated improved sensorimotor function and reduced infarct volume, with efficacy linked to administration route—direct intracerebral or intra-arterial delivery outperforming intravenous due to higher homing to lesion sites.[129] Similar patterns emerge in myocardial infarction (MI) rodent models, where bone marrow-derived MSCs administered intravenously post-infarct reduced scar size by up to 30% and preserved ejection fraction, primarily through paracrine release of vascular endothelial growth factor (VEGF) and hepatocyte growth factor (HGF) that stimulate endogenous repair without substantial cardiomyocyte transdifferentiation.[70]Immunomodulatory mechanisms dominate findings from autoimmune disease models, such as collagen-induced arthritis in mice and experimental autoimmune encephalomyelitis (EAE) for multiple sclerosis. In arthritis models, systemic MSC infusion halted joint destruction in 70-80% of studies, correlating with suppressed T-helper 17 cell proliferation and elevated regulatory T cells via prostaglandin E2 (PGE2) and indoleamine 2,3-dioxygenase (IDO) secretion, rather than direct cartilage regeneration.[130] EAE models further showed MSCs attenuating central nervous system inflammation by polarizing macrophages toward an M2 anti-inflammatory phenotype and inhibiting dendritic cell maturation, with effects persisting beyond cell survival (typically <1% engraftment at 4 weeks). These insights underscore dose- and timing-dependency, with preconditioning (e.g., hypoxia or interferon-gamma priming) enhancing factor secretion and outcomes in 2023-2025 studies.[131]In vitro co-culture assays complement animal data, isolating MSC secretome effects: conditioned media from MSCs replicated neuroprotective outcomes in stroke-simulated neuronal cultures by downregulating pro-apoptotic caspases and upregulating anti-oxidant enzymes like superoxide dismutase. For degenerative models like osteoarthritis, explant studies indicate MSCs mitigate matrix metalloproteinase activity via exosomes carrying microRNAs (e.g., miR-140), preserving extracellular matrix integrity without cellular fusion. However, variability across models highlights limitations, including strain-specific responses in mice versus rats and inconsistent scalability to larger animals, informing translational gaps noted in 2024 reviews. Controversial preclinical reports of pro-tumorigenic polarization (MSC2 phenotype) in glioma co-injection models urge caution, as tumor microenvironment cues can shift MSCs toward growth promotion via IL-6 signaling, though anti-tumor MSC1 shifts predominate in non-malignant inflammation.[132][133]
Clinical Trial Landscape: Outcomes and Challenges
As of 2025, over 1,000 clinical trials involving mesenchymal stromal cells (MSCs) have been registered on platforms like ClinicalTrials.gov, predominantly in phases I and II, with applications spanning graft-versus-host disease (GVHD), autoimmune disorders, neurological conditions, and acute respiratory distress. These trials have consistently affirmed the safety profile of MSCs, with meta-analyses reporting low rates of severe adverse events such as infusion-related reactions or ectopic tissue formation across intravenous, intra-articular, and other delivery routes, even at doses exceeding 10^6 cells/kg. However, efficacy remains inconsistent, as evidenced by multiple phase III failures where MSCs failed to outperform standard care in primary endpoints like overall survival or symptom remission.[4][128][134]Promising outcomes have emerged in niche indications. In steroid-refractory acute GVHD, MSC infusions have shown response rates of 60-70% in pediatric cohorts, leading to conditional approvals in Japan and Canada, though U.S. FDA reviews, such as for remestemcel-L in 2020 and 2024, cited insufficient efficacy separation from controls in adult subgroups. Systematic reviews of MSC therapy for severe COVID-19, pooling data from 2020-2023 trials, indicate reduced mortality (odds ratio 0.32) and improved oxygenation, attributed to paracrine anti-inflammatory effects rather than direct tissue regeneration. Similarly, in acute-on-chronic liver failure, a 2025 meta-analysis of randomized trials reported enhanced short-term survival (up to 90 days) with umbilical cord-derived MSCs, with hazard ratios favoring treatment groups. These successes often correlate with early-phase, smaller-scale studies using fresh or minimally expanded cells from potent sources like bone marrow.[135][136][101]Persistent challenges undermine broader translation. MSC heterogeneity—arising from variable donor age, tissue origin (e.g., bone marrow vs. adipose), expansion protocols, and potency assays—leads to unpredictable therapeutic effects, as cells from the same source can differ in immunomodulatory cytokine secretion by factors of 10-fold. Poor pharmacokinetics, including rapid clearance (half-life <24 hours post-infusion) and limited homing to inflamed sites due to downregulated chemokine receptors, results in minimal engraftment (<1% retention at target tissues), favoring transient paracrine signaling over durable repair. Trial designs exacerbate issues: lack of standardized potency biomarkers, underpowered phase III studies, and placebo effects in subjective endpoints like pain scores have contributed to failures, as seen in cardiovascular trials where MSCs improved left ventricular function in phase II but not in larger cohorts. Regulatory hurdles, including batch-to-batch variability and the need for real-time viability monitoring, further delay commercialization, with only a handful of products like Prochymal achieving market entry amid ongoing scrutiny.[137][138][126]
Challenge
Description
Impact on Trials
Cell Variability
Differences in source, culture conditions, and donor factors affect potency.
Inconsistent dosing and outcomes across studies.[139]
Engraftment/Homing
Low retention and targeting efficiency post-administration.
Reliance on short-term effects, failure in chronic diseases.[140]
Standardization
Absence of uniform manufacturing and release criteria.
Regulatory rejections and reproducibility issues.[141]
Efficacy Endpoints
Difficulty measuring paracrine vs. regenerative contributions.
Phase III misses despite phase II signals.[142]
Advances in Engineering and Derivatives (e.g., Exosomes)
Genetic engineering of mesenchymal stem cells (MSCs) has advanced their therapeutic potential by enhancing survival, homing, and secretory functions. Techniques such as lentiviral transduction and CRISPR/Cas9 editing enable overexpression of genes like Akt and Bcl-2, which improve cell viability under hypoxic conditions and post-transplantation engraftment in ischemic tissues.[143] For instance, MSCs modified to express CXCR4 exhibit increased migration to tumor sites or inflamed areas via SDF-1 gradients, as demonstrated in preclinical models of myocardial infarction and cancer.[144] Non-viral methods, including electroporation and nanoparticle delivery, have been optimized for safer, transient modifications, reducing risks of insertional mutagenesis while achieving up to 50% transfection efficiency in human bone marrow-derived MSCs.[145]Surface engineering and biomaterial integration further refine MSC functionality. Coating MSCs with antibodies or peptides targets specific tissues, such as brain endothelium in stroke models, enhancing delivery efficiency by 2-3 fold compared to unmodified cells.[146] Preconditioning with hypoxia or cytokines, combined with genetic tweaks, boosts paracrine factor secretion, including VEGF and HGF, supporting angiogenesis in diabetic wounds.[147] These strategies have progressed to phase I/II trials for conditions like graft-versus-host disease, where engineered MSCs show sustained immunomodulation without long-term persistence.[4]MSC-derived exosomes represent a cell-free derivative with advantages in stability and reduced immunogenicity. Isolated via ultracentrifugation, these extracellular vesicles carry miRNAs, proteins, and lipids mediating anti-inflammatory and regenerative effects, mimicking MSC paracrine signaling.[148] Engineering approaches include parental MSC modification to enrich exosome cargo; for example, MSCs overexpressing miR-126-loaded exosomes accelerate endothelial repair in atherosclerosis models.[149] Direct loading via electroporation or lipofection incorporates chemotherapeutic agents, enabling targeted delivery to tumors with minimal off-target effects, as shown in melanoma xenografts where exosome-delivered doxorubicin reduced metastasis by 60%.[150]Hybrid engineering of exosomes, such as fusion with synthetic nanovesicles, enhances payload capacity and specificity. A 2024 study fused plant-derived exosomes with MSC nanovesicles for synergistic autoimmune skin therapy, improving penetration and cytokine modulation.[151] Clinical translation includes phase I trials for MSC exosomes in COVID-19-induced lung injury, reporting safety and preliminary efficacy in reducing inflammation markers like IL-6 by 40%.[152] Challenges persist, including scalable production and standardization, but these derivatives circumvent cell-based risks like senescence, positioning them as next-generation therapeutics.[153]
Historical Development
Early Discovery (1960s-1980s)
In the mid-1960s, Alexander Friedenstein and colleagues initiated foundational experiments demonstrating that bone marrow contains a subpopulation of non-hematopoietic cells capable of forming ectopic bone and supporting hematopoiesis. By transplanting dispersed bone marrow cells from guinea pigs or mice into diffusion chambers or under the renal capsule of allogeneic hosts, they observed the development of ossicles comprising donor-derived osteoblasts, stroma, and reticular elements, distinct from hematopoietic progenitors.[7] These findings established bone marrow as a reservoir for stromal progenitors, with the process requiring cell-cell interactions but not vascular invasion, highlighting the cells' intrinsic osteogenic potential.[154]By the late 1960s and early 1970s, Friedenstein's group advanced to in vitro characterization, identifying plastic-adherent, spindle-shaped cells that proliferated to form fibroblast colonies (colony-forming unit-fibroblasts, or CFU-F) at a frequency of approximately 1 in 10,000 to 100,000 marrow mononuclear cells.[155] These colonies exhibited self-renewal and could differentiate into osteoblasts, chondrocytes, and adipocytes when reimplanted in vivo, though early assays emphasized osteogenic and reticular lineages supporting hematopoiesis.[33] Key publications, such as Friedenstein et al. (1970), detailed the monolayer culture method, revealing the cells' fibroblastic morphology and capacity for rapid expansion while maintaining clonogenicity.[156]Through the 1970s and into the 1980s, collaborations with researchers like Maureen Owen refined the concept of a common stromal progenitor, termed "bone marrow osteogenic stem cells" in 1987 work by Friedenstein et al., emphasizing their role in skeletal and hematopoietic microenvironments.[7] Owen and Friedenstein's 1988 framework proposed these cells as multipotent stem cells for mesenchymal tissues, capable of both self-maintenance and lineage commitment, based on diffusion chamber assays showing donor-origin stroma in heterotopic sites.[157] This period solidified empirical evidence for their rarity, adherence-based isolation, and functional assays, laying groundwork for later expansions despite initial focus on bone marrow exclusivity and osteogenic bias in differentiation studies.[158]
Standardization and Expansion (1990s-2000s)
In 1991, Arnold I. Caplan introduced the term "mesenchymal stem cells" (MSCs) to describe a rare population of plastic-adherent cells derived from bone marrow and periosteum, capable of self-renewal and differentiation into skeletal tissues such as bone, cartilage, and fat, positioning them as progenitors for mesenchymal lineages in repair and regeneration.[159] This conceptualization built on prior observations of colony-forming unit-fibroblasts (CFU-Fs) in bone marrow but emphasized their stem-like properties under defined culture conditions, including adherence to tissue culture plastic and expansion in media supplemented with fetal bovine serum.[33]Throughout the 1990s, standardization efforts focused on isolation protocols from human bone marrow aspirates, typically involving density gradient centrifugation (e.g., Ficoll or Percoll) to separate mononuclear cells, followed by adherence-based selection and serial passaging to enrich for fibroblast-like colonies, yielding frequencies of approximately 0.001-0.01% of total nucleated cells.[7] Key demonstrations of multipotency included Pittenger et al.'s 1999 report of single-cell-derived MSCs differentiating into osteogenic, adipogenic, chondrogenic, and myogenic lineages in vitro, using specific induction media with dexamethasone, ascorbic acid, and beta-glycerophosphate for osteogenesis, among others.[3] These protocols highlighted challenges in maintaining homogeneity, as expanded populations exhibited donor variability and senescence after 10-20 passages, limiting scalability for therapeutic use.[160]The early 2000s saw accelerated expansion of MSC research, with preclinical studies validating ex vivo expansion yields of up to 10^14 cells from initial inocula under optimized conditions like hypoxia or growth factor supplementation (e.g., FGF-2), though heterogeneity persisted due to lack of uniform markers.[161] Initial clinical translations emerged, including Phase I trials for osteogenesis imperfecta (1990s onward) using autologous or allogeneic bone marrow-derived MSCs infused post-chemotherapy, achieving engraftment rates of 1-5% in bone but variable efficacy.[70] Standardization advanced significantly in 2006 when the International Society for Cellular Therapy (ISCT) established minimal criteria: adherence to plastic, expression of CD105, CD73, and CD90 (>95% of cells), absence of CD45, CD34, CD14, or CD11b (<2%), and trilineage differentiation capacity in vitro, providing a benchmark despite ongoing debates over true stemness versus stromal progenitor identity.[162] These criteria facilitated broader adoption but did not fully resolve potency assays, as functional readouts like immunomodulation (e.g., IDOinduction) gained prominence alongside differentiation.[7]
Contemporary Shifts and Recent Milestones (2010s-2025)
In the 2010s, mesenchymal stem cell (MSC) research transitioned toward clinical translation, with a surge in trials emphasizing immunomodulatory and paracrine effects over direct differentiation into tissue types, as evidenced by increased focus on cytokine secretion and extracellular vesicles for anti-inflammatory actions.[4] This period marked the first regulatory approvals for MSC therapies outside the United States, beginning with South Korea's Ministry of Food and Drug Safety granting commercialization for Hearticell-AMI, an autologous bone marrow-derived MSC product, in July 2011 for acute myocardial infarction.[70] In January 2012, the same agency approved Cartistem, using umbilical cord MSCs, for degenerative arthritis, and Cuepistem, employing adipose-derived MSCs, for Crohn's disease fistulas.[70]By the mid-2010s, over 500 clinical trials involving MSCs were registered globally, expanding applications to autoimmune conditions, graft-versus-host disease (GVHD), and orthopedic injuries, though many demonstrated safety without consistent efficacy in meeting primary endpoints.[4] In March 2018, the European Medicines Agency approved Alofisel, an adipose-derived MSC product, for complex perianal fistulas in Crohn's disease, representing the first EU authorization, though it was withdrawn in December 2024 following a failed confirmatory trial.[70] The decade's trial proliferation—reaching approximately 1,000 by 2019—highlighted manufacturing challenges, including cell sourcing variability from bone marrow, adipose, or perinatal tissues, prompting refinements in International Society for Cell & Gene Therapy criteria for MSC potency and identity.[4]The 2020s accelerated MSC evaluation amid the COVID-19 pandemic, with trials targeting acute respiratory distress syndrome (ARDS) via intravenous MSCs to curb cytokine storms, showing reduced proinflammatory markers like IL-6 and TNF-α in phase 2 studies, though large-scale efficacy remained variable.[70] By October 2024, clinicaltrials.gov listed over 1,500 MSC trials, including 339 phase 1, 280 phase 2, and 36 phase 3 studies, underscoring a shift to allogeneic, off-the-shelf products for scalability.[4] A pivotal milestone occurred on December 18, 2024, when the U.S. Food and Drug Administration approved Ryoncil (remestemcel-L-rknd), an allogeneic bone marrow-derived MSC therapy from Mesoblast, for steroid-refractory acute GVHD in pediatric patients aged two months and older—the first such U.S. approval, based on trials demonstrating a 70% overall response rate.[95] As of 2025, twelve MSC-based therapies had received worldwide approvals, predominantly in Asia (e.g., South Korea and Japan) for indications like myocardial infarction, arthritis, and fistulas, with projections for up to 50 by 2040 amid ongoing efforts to address potency assays and trial standardization.[163][164]
Controversies and Critical Perspectives
Disputes Over True Stemness and Functionality
The designation of mesenchymal stem cells (MSCs) as true stem cells has faced significant scrutiny, primarily due to insufficient evidence of robust self-renewal and multilineage differentiation in vivo, which are hallmarks of stem cell identity. Critics, including Arnold Caplan—who coined the term "mesenchymal stem cells" in 1991—argue that MSCs fail to meet these criteria, as they exhibit limited serial transplantation potential and primarily derive from pericyte-like progenitors rather than exhibiting unlimited proliferative capacity.[18] In 2010, Caplan proposed reclassifying MSCs as "Medicinal Signaling Cells" to emphasize their therapeutic role in secreting bioactive factors, a view he reiterated in subsequent works, highlighting that the "stem cell" label misleads expectations of direct tissue regeneration.[165] This perspective is supported by observations that MSCs, when isolated via plastic adherence, represent heterogeneous populations of stromal cells, including fibroblasts and pericytes, rather than a uniform clonogenic stem cell subset.[166]Further disputes center on the multipotency of MSCs, with in vitro demonstrations of differentiation into osteogenic, adipogenic, and chondrogenic lineages often failing to replicate in physiological contexts. Studies indicate that bulk-cultured MSCs rarely form single-cell-derived clones capable of multilineage differentiation, suggesting that observed plasticity may arise from population-level heterogeneity or culture-induced artifacts rather than intrinsic stemness.[3] The International Society for Cellular Therapy (ISCT) has recommended the term "mesenchymal stromal cells" since 2006 to denote their supportive, non-stem role in the marrow niche, reserving "stem cell" for rigorously proven progenitors that demonstrate sustained engraftment and tissue reconstitution.[166]In vivo tracking in preclinical models reveals minimal long-term engraftment or differentiation into host tissues, with MSCs instead modulating local microenvironments through transient presence.[167]Functionality debates underscore that MSCs' clinical effects—such as immunomodulation in graft-versus-host disease or anti-inflammatory actions in autoimmune conditions—derive predominantly from paracrine mechanisms, including exosome-mediated delivery of cytokines like VEGF and trophic factors, rather than cellular replacement.[167] Over 700 clinical trials registered as of 2017 primarily leverage these signaling properties, with rare evidence of stable multilineage contribution, prompting calls to prioritize secretome-based therapies over whole-cell administration.[168] This shift reflects causal evidence that MSCs act as transient "drugstores" stimulating endogenous progenitors, not as self-renewing engrafters, challenging claims of broad regenerative potency and highlighting risks of overhyping unverified stem-like attributes in therapeutic development.[18][3]
Efficacy Skepticism and Trial Failures
Skepticism surrounding the efficacy of mesenchymal stem cell (MSC) therapies has grown due to repeated failures in demonstrating clinically meaningful outcomes in randomized controlled trials, despite preclinical promise and apparent safety profiles. A 2021 review of MSC applications identified major challenges including inconsistent potency, limited engraftment, and failure to replicate animal model benefits in humans, contributing to trial disappointments across indications like graft-versus-host disease (GvHD) and cardiovascular conditions.[137] By 2025, analyses of over 1,000 MSC trials underscored that while safety is consistently affirmed, efficacy endpoints are met in fewer than 20% of phase II/III studies, often due to heterogeneous cell populations and unoptimized dosing.[70][4]Specific trial failures exemplify these issues. The CHART-1 phase III trial (initiated 2016, results 2020) evaluating allogeneic MSCs for chronic heart failure patients yielded neutral results on primary endpoints like 6-minute walk distance and quality of life, despite positive phase II data; investigators attributed this to batch-to-batch variability in cell potency and inadequate patient stratification.[169] In GvHD treatment, a phase III trial by Prochymal (remestemcel-L) failed in 2012 to show superiority over placebo in pediatric steroid-refractory acute GvHD, leading to non-approval by the FDA despite European conditional approval based on smaller studies; subsequent U.S. re-trials confirmed inconsistent responses linked to disease severity mismatches.[170]COVID-19 MSC trials, such as a 2021 randomized study of umbilical cord MSCs, reported no significant reduction in mortality or ventilator-free days compared to controls, highlighting over-optimism from early observational data.[171]Underlying causal factors for these failures include rapid apoptosis of infused MSCs—often exceeding 90% within hours—limiting paracrine signaling and tissue repair; donor variability in secretome profiles; and incomplete mechanistic understanding, where immunomodulation fails under inflammatory stress.[172][173] Heterogeneity in manufacturing protocols exacerbates outcomes, as evidenced by a 2022 meta-analysis showing no dose-response correlation in neurological trials due to unstandardized expansion methods.[174] These patterns have prompted calls for refined potency assays and engineered MSC derivatives to address empirical gaps, though commercial approvals remain limited to niche indications like refractory GvHD in Japan (2015).[142]
Safety Risks Including Tumor Promotion
Mesenchymal stem cells (MSCs) carry potential safety risks related to oncogenesis, primarily stemming from their immunomodulatory properties and interactions with the tumor microenvironment. Preclinical studies have demonstrated that MSCs can promote tumor growth and metastasis in certain models by secreting pro-angiogenic factors such as vascular endothelial growth factor (VEGF) and by facilitating epithelial-mesenchymal transition in cancer cells.[175] For instance, in mouse models of breast and lung cancer, intravenously administered MSCs homed to tumor sites and enhanced metastasis through extracellular matrix remodeling and immune evasion mechanisms.[176] These effects arise causally from MSCs' ability to polarize macrophages toward an M2 pro-tumor phenotype and suppress cytotoxic T-cell responses, potentially exacerbating existing malignancies or creating permissive niches for dormant cancer cells.[177]Tumorigenicity risks are heightened by ex vivo expansion protocols, where prolonged culture can induce cytogenetic aberrations, including aneuploidy and chromosomal instability, leading to malignant transformation in rare cases. In preclinical assessments, bone marrow-derived MSCs passaged beyond 15-20 population doublings exhibited tumorigenic potential in immunodeficient mice, forming sarcomas upon subcutaneous injection.[178] Such transformations are linked to telomere shortening, oxidative stress, and replicative senescence bypass, with heterogeneity across donor sources—e.g., adipose versus umbilical cord MSCs—amplifying variability; younger donor cells show lower aberration rates but higher proliferation risks.[179] Regulatory bodies, including the International Society for Cell & Gene Therapy, emphasize karyotyping and genomic stability testing to mitigate these, yet inconsistencies in manufacturing standards persist, as evidenced by reports of undisclosed abnormalities in early-phase trials.[180]Clinical data, while indicating overall tolerability, reveal sparse but concerning signals of tumor promotion. A systematic review of over 200 MSC trials through 2023 found no direct causation of de novo tumors from injected cells, but noted increased metastasis rates in patients with occult cancers post-infusion, attributed to MSC-mediated vascularization.[181] Adverse events like thromboembolism and pulmonary fibrosis, occurring in up to 10% of intravenous administrations, indirectly heighten oncogenic risks by promoting inflammation and hypoxia—conditions that foster tumor initiation.[119] Long-term follow-up remains limited; a 2024 meta-analysis of COVID-19 trials reported no MSC-linked cancers over two years, yet urged vigilance in cancer-prone populations due to immunosuppression enabling opportunistic malignancies.[182] Critics highlight underreporting in industry-sponsored studies, where safety endpoints often prioritize short-term metrics over decade-long oncologic surveillance, underscoring the need for standardized potency assays to predict pro-tumorigenic subsets.[183]
Ethical, Regulatory, and Commercial Critiques
Ethical critiques of mesenchymal stem cell (MSC) therapies primarily center on informed consent deficiencies and the potential for patient harm in unproven applications, despite MSCs originating from adult or perinatal tissues that evade the embryo destruction controversies associated with pluripotent stem cells.[184][185] In clinical settings, particularly pay-to-participate trials or direct-to-consumer treatments, participants often receive incomplete disclosures about uncertain efficacy, long-term risks such as tumorigenicity, and the experimental nature of interventions, violating ethical standards like those outlined in the Declaration of Helsinki.[186][187] Critics argue that marketing unverified MSC products exploits vulnerable patients seeking cures for conditions like autism or multiple sclerosis, fostering therapeutic misconception where hope overrides evidence-based caution.[188][189]Regulatory challenges arise from the classification of MSCs as biologics or drugs under agencies like the U.S. Food and Drug Administration (FDA) and European Medicines Agency (EMA), necessitating rigorous investigational new drug applications, good manufacturing practices, and phase-based trials—processes many clinics circumvent by claiming minimal manipulation or autologous use exemptions.[169] As of 2023, only 12 MSC therapies achieved global approval, with nine from Asian regulators and limited Western successes, such as the FDA's 2025 authorization of Ryoncil (remestemcel-L) for pediatric steroid-refractory graft-versus-host disease.[190][96] In the U.S., over 1,480 businesses operated 2,754 clinics by March 2021 marketing unapproved stem cell interventions, including MSCs, prompting FDA enforcement actions like clinic closures and warnings for violations involving contaminated products that hospitalized at least 13 patients with bacterial infections.[191][123] Inconsistent international frameworks exacerbate "stem cell tourism," where patients travel to lax jurisdictions like Mexico for treatments lacking phase III validation, delaying legitimate advancements through diluted trial pools and safety data gaps.[192][193]Commercial critiques highlight the proliferation of for-profit entities prioritizing revenue over evidence, with unproven MSC infusions marketed at costs exceeding tens of thousands of dollars per treatment despite scant peer-reviewed support for broad indications.[194] These operations often amplify anecdotal testimonials while minimizing risks like immune rejection or inefficacy, undermining public trust and diverting resources from standardized products.[195] Even approved MSC therapies face "failure to launch" due to manufacturing scalability issues, potency variability across donors, and high production costs, as seen in divergent clinical outcomes between similar formulations.[196][197] Industry funding biases in trials, coupled with equitable access barriers—where therapies remain unaffordable for most—raise concerns that commercialization prioritizes niche markets over causal validation of MSC mechanisms like paracrine signaling, potentially stalling broader therapeutic maturation.[198][199]