Mesenchymal stem cells are a type of adult stem cell found throughout the body, most abundantly in bone marrow and fat tissue, that can develop into bone, cartilage, and fat cells. But their real therapeutic promise has less to do with what they become and more to do with what they release: a cocktail of signaling molecules that calm inflammation, recruit repair cells, and nudge damaged tissue toward healing. This distinction between what researchers once thought MSCs did and what they actually seem to do has reshaped the field over the past two decades and sits at the center of ongoing debates about their name, their clinical potential, and the gap between lab results and real-world outcomes.
Identifying MSCs in the Lab
Unlike blood cells, which can be sorted by a single reliable marker, MSCs lack a unique molecular fingerprint. Instead, researchers define them by a combination of traits: they stick to plastic surfaces in culture, they carry certain surface proteins (CD105, CD73, and CD90), they lack the markers found on blood-forming cells (CD45 and CD34), and they can be coaxed in a dish to turn into bone, cartilage, or fat cells.1PubMed Central. Cell Surface Markers of Mesenchymal Stem Cells: Current Knowledge and Advances in Characterization Technologies These criteria were established by the International Society for Cellular Therapy as a minimum standard, but they are loose enough that cells from different tissues and different donors can all qualify while behaving quite differently in practice.
This lack of a single definitive marker is more than a technical annoyance. It means two labs can each claim to be studying “MSCs” while working with meaningfully different cell populations. A batch harvested from bone marrow and a batch harvested from umbilical cord tissue may both check the same boxes, yet respond differently to the same signals once transplanted. That variability is one reason clinical trials have produced inconsistent results and why standardization remains a persistent challenge.
Where They Live in the Body
MSCs were first isolated from bone marrow, and for years that was considered their primary home. We now know they reside in many tissues. Fat, umbilical cord blood, dental pulp, placenta, and synovial fluid all harbor cells meeting the MSC criteria.2Bioactive Materials. Adipose-derived mesenchymal stem cells (MSCs) are a superior cell source for bone tissue engineering Research has traced their origins to the perivascular niche, the tissue wrapping around blood vessels. Cells lining this space in skeletal muscle, pancreas, fat, and placenta have been shown to behave like MSC precursors once expanded in culture.3PubMed Central. Mesenchymal stem cells: from roots to boost
The source tissue matters. MSCs from fat tend to be easier to harvest in large numbers (a routine liposuction procedure yields plenty of starting material), while bone marrow MSCs have historically been more studied in clinical settings. Umbilical cord-derived MSCs have gained attention because they come from young tissue and can be banked without an invasive procedure for the donor. Each source has trade-offs in yield, expansion capacity, and therapeutic potency, and the field has not settled on a single “best” source for all applications.
The Paracrine Effect
The early excitement around MSCs centered on their ability to differentiate. The idea was simple: inject stem cells into a damaged knee and they become new cartilage; put them into a fractured bone and they become new bone. That picture turned out to be mostly wrong, or at least incomplete. Most transplanted MSCs do not survive long enough at the injury site to turn into replacement tissue. Instead, they appear to work primarily by secreting a complex mix of proteins, growth factors, and tiny membrane-bound packages called extracellular vesicles that influence nearby cells.
This secreted mix, known as the secretome, modulates inflammation, encourages blood vessel formation, reduces scarring, and pushes local immune cells toward a repair-friendly state.4PubMed Central. Effects of Mesenchymal Stem Cell-Derived Paracrine Signals and Their Delivery Strategies Among the most studied components of the secretome are exosomes, nano-sized vesicles that carry small RNA molecules, proteins, and lipids from one cell to another. These exosomes can deliver molecular instructions to wounded tissue, and research suggests they account for a large share of the therapeutic activity attributed to MSCs themselves.5PubMed Central. Exosomal microRNAs from Mesenchymal Stem Cells: Novel Therapeutic Effect in Wound Healing
The realization that MSCs work mainly through what they release rather than what they become has opened up a practical question: do you even need the cells? Some researchers are exploring whether purified exosomes or concentrated secretome fractions could deliver the same benefits without the complications of handling live cells. That research is still early, but it reflects how dramatically the field’s understanding of MSC mechanisms has shifted.
How MSCs Manage the Immune System
One of the most striking properties of MSCs is their ability to dial down immune responses. They do this on multiple fronts. On the innate side, they can push macrophages from an aggressive, inflammation-driving state toward a calmer, tissue-repair-promoting state.6PubMed Central. Mesenchymal Stromal Cells Affect Disease Outcomes via Macrophage Polarization On the adaptive side, they suppress the proliferation of T cells through several pathways, including producing enzymes that starve T cells of essential amino acids and generating nitric oxide that interferes with T cell signaling.7Cell Stem Cell. How Do Mesenchymal Stromal Cells Suppress T Cells?
What makes this immunomodulation unusual is that MSCs are not simply immunosuppressive in a blanket way. They respond to their environment. In the presence of strong inflammatory signals, MSCs ramp up their anti-inflammatory output. In a less inflamed environment, they behave differently. This context-sensitivity is part of why MSCs have attracted interest for autoimmune conditions, transplant rejection, and other diseases where the immune system is causing the damage.
The Lung Trap and the Homing Problem
If you inject MSCs into a vein, most of them never reach the intended target. The majority get stuck in the lungs on their first pass through the circulatory system.8PubMed Central. Pulmonary passage is a major obstacle for intravenous stem cell delivery: the pulmonary first-pass effect The reason is mechanical: MSCs are relatively large cells, and pulmonary capillaries are narrow. Studies using microspheres have confirmed that the average suspended MSC is bigger than the diameter of lung capillaries, so the cells physically cannot squeeze through in large numbers.9Transplantation Proceedings. Stem Cell Transplantation: The Lung Barrier
Once trapped, the cells do gradually migrate out to organs like the liver and spleen, and when an injury is present, inflammatory signals can redirect some of them toward the wound site.10PubMed Central. Mesenchymal Stem Cells Home to Sites of Injury and Inflammation But the efficiency of this homing process is poor. A key molecular pathway involved is the interaction between a receptor on MSCs and a signaling molecule released by damaged tissue, but even with this guidance system in place, only a small fraction of injected cells actually reach the injury.11PubMed Central. Engineering MSC Migration: Roles of Nanoparticles in Activating Migratory Pathways and Functions Researchers are experimenting with nanoparticles and genetic modifications to boost migration, and some clinical protocols sidestep the problem entirely by injecting MSCs directly into the affected joint, organ, or tissue rather than into the bloodstream.
Clinical Applications So Far
The immunomodulatory properties of MSCs have found their most established clinical foothold in graft-versus-host disease, a life-threatening complication that can follow bone marrow transplants when donated immune cells attack the recipient’s body. A phase II study of 55 patients with steroid-resistant acute GVHD found that 30 achieved a complete response after MSC infusions, with no side effects during or immediately after treatment. Complete responders had substantially higher two-year survival compared to those who did not respond fully.12PubMed. Mesenchymal stem cells for treatment of steroid-resistant, severe, acute graft-versus-host disease: a phase II study In Japan, a commercial MSC product called Temcell was approved for acute GVHD and has been used in over 380 patients in a real-world registry, with an overall response rate of about 56% by day 28.13PubMed. Off-the-shelf bone marrow-derived mesenchymal stem cell treatment for acute graft-versus-host disease: real-world evidence Those numbers are meaningful for a disease where patients have already failed steroids, but they also highlight that MSC therapy is not a guaranteed fix.
Osteoarthritis is another area of intense interest. The logic is twofold: MSCs can differentiate into cartilage-forming cells, and their anti-inflammatory secretions could slow the joint destruction that drives the disease.14PubMed Central. Mesenchymal stem cells in osteoarthritis therapy: a review A systematic review of randomized controlled trials found that MSC injections significantly improved pain and function scores in knee osteoarthritis patients at both six and twelve months compared to control groups, with fat-derived MSCs and higher doses showing the most benefit.15PubMed Central. Efficacy and safety of mesenchymal stem cells in knee osteoarthritis: a systematic review and meta-analysis of randomized controlled trials A dose-escalation trial using umbilical cord-derived MSCs injected directly into the knee also reported safety across all dose levels and improvements in pain and function, though interestingly, lower and medium doses outperformed the highest dose in that study.16PubMed Central. A Phase I Dose-Escalation Clinical Trial to Assess the Safety and Efficacy of Umbilical Cord-Derived Mesenchymal Stromal Cells in Knee Osteoarthritis Results like these are encouraging but still preliminary. No country has broadly approved MSC injections as a standard osteoarthritis treatment, and the optimal dose, cell source, and patient selection criteria remain unsettled.
The Unregulated Clinic Problem
The gap between what research has shown and what some clinics promise is enormous. Hundreds of businesses sell stem cell injections directly to consumers for conditions ranging from back pain to autism, often with no rigorous evidence that the specific product they administer works for the condition being treated. Published case reports document patients suffering serious harm from unapproved stem cell products, including blindness and infections, and the FDA has acknowledged that adverse events from these products are likely underreported.17Cell Stem Cell. The American stem cell sell in 2021: U.S. businesses selling unlicensed and unproven stem cell interventions The regulatory landscape has struggled to keep up. The FDA alone cannot adequately police the direct-to-consumer market, and enforcement actions have been sporadic.18PubMed. Adverse events related to unapproved stem cell products and other regenerative interventions: recommendations for more robust regulation of the direct-to-consumer marketplace
If you are considering an MSC-based treatment, the distinction between a registered clinical trial and a pay-to-play clinic matters enormously. A legitimate trial is listed on a registry like ClinicalTrials.gov, has ethics committee oversight, monitors for adverse events systematically, and does not charge you for the experimental product. A clinic that asks for several thousand dollars upfront and promises results for a broad menu of conditions is selling hope, not evidence-based medicine.
Why They Do Not Always Work the Same Way Twice
One of the persistent frustrations in the MSC field is inconsistency. A therapy that looks promising in one trial may disappoint in the next, and part of the reason traces back to how MSCs are manufactured. Growing cells in culture to get enough of them for treatment changes the cells over time. Research comparing MSCs from young and aged animal donors against MSCs passaged many times in culture found that prolonged time in a dish may affect the cells more than the aging of the donor itself, degrading their growth potential, their ability to form new blood vessels, and their differentiation capacity.19PubMed Central. Comparison of the Donor Age-Dependent and In Vitro Culture-Dependent Mesenchymal Stem Cell Aging in Rat Model
Cryopreservation adds another layer of complexity. Freezing and thawing MSCs for off-the-shelf use causes not just cell death but also subtle changes in the surviving cells’ therapeutic function.20PubMed Central. Research advances in cryopreserved preparations of mesenchymal stem cells: technical innovations, application challenges, and quality control A product that performs well fresh from culture may lose potency after being stored and shipped. These manufacturing variables mean that two “MSC therapies” can share a label while delivering meaningfully different biological activity, which clouds the interpretation of clinical trial data across the board.
The Name Debate
Arnold Caplan, who originally coined the term “mesenchymal stem cells” in 1991, later argued the name should be changed. His reasoning was straightforward: calling them stem cells implies they work by turning into new tissue, which gives patients and even some physicians the wrong mental model. Because the evidence increasingly shows that MSCs function primarily by secreting therapeutic molecules at injury sites, Caplan proposed renaming them “medicinal signaling cells,” keeping the same abbreviation but shifting the emphasis from differentiation to signaling.21Stem Cells Translational Medicine. Mesenchymal Stem Cells: Time to Change the Name! 22Journal of ISAKOS. The Classic Review of Caplan (1991) on cell-based therapeutic technology using Mesenchymal Stem Cells
The International Society for Cellular Therapy had already pushed the field toward “mesenchymal stromal cells” years earlier, reserving “stem cell” for populations with demonstrated self-renewal and multipotency. In practice, all three names circulate in the literature, and the confusion has real consequences: the word “stem cell” carries outsized marketing power, which unregulated clinics exploit freely.
Mitochondrial Transfer and Tunneling Nanotubes
Beyond secreting molecules into the surrounding fluid, MSCs can also form direct physical connections with neighboring cells. These thin tubes, called tunneling nanotubes, act as bridges through which MSCs transfer mitochondria and other cellular components to damaged cells.23PubMed Central. Tunneling nanotubes and mesenchymal stem cells: New insights into the role of melatonin in neuronal recovery This organelle transfer can rescue cells whose own energy-producing machinery has been compromised by injury or disease, essentially giving them a metabolic jump-start.
Research published in Cell demonstrated that bone marrow stromal cells use these nanotubular connections to transplant mitochondria into immune cells called CD8+ T cells, boosting their anti-tumor activity.24Cell. Intercellular mitochondrial transfer as a therapeutic platform to boost antitumoral T cell immunity This finding is especially striking because it suggests MSC-derived mitochondrial transfer could potentially be harnessed in cancer immunotherapy, an area where MSCs have traditionally been viewed with caution.
MSCs and Tumors
That caution is warranted. MSCs are attracted to tumors much the way they are attracted to wounds, because tumors produce many of the same inflammatory signals. Once recruited to the tumor microenvironment, MSCs can play roles that help rather than hinder cancer: promoting blood vessel growth into the tumor, suppressing the immune response that would otherwise attack cancer cells, increasing the “stemness” of tumor cells, and even contributing to drug resistance.25PubMed Central. Mesenchymal stem cells in the tumor microenvironment This dual nature is a core challenge for the field. The same anti-inflammatory and tissue-supporting properties that make MSCs attractive for treating autoimmune disease or repairing injured joints could, in the wrong context, support tumor growth. Clinical trials increasingly exclude patients with active cancer or recent cancer history, and long-term follow-up for tumor safety remains an active area of study.
Engineering Better MSCs
Many of the current limitations of MSC therapy, variable potency, donor-to-donor differences, culture-related aging, and poor homing, have spurred efforts to engineer more reliable alternatives. One approach gaining traction is generating MSC-like cells from induced pluripotent stem cells. Because iPSCs can be grown indefinitely from a single well-characterized cell line, they represent a potentially unlimited, standardized starting material.26PubMed Central. Methods to produce induced pluripotent stem cell-derived mesenchymal stem cells These iPSC-derived MSCs share many functional properties with their tissue-derived counterparts but are genetically and functionally distinct in ways researchers are still characterizing.27PubMed Central. Induced Pluripotent Stem Cell-Derived Mesenchymal Stromal Cells Are Functionally and Genetically Different From Bone Marrow-Derived Mesenchymal Stromal Cells
Another engineering direction combines MSCs with biomaterial scaffolds for tissue repair. Rather than injecting cells into a joint or wound and hoping they stay put, researchers seed MSCs onto three-dimensional scaffolds designed to mimic the structure of natural tissue. A veterinary study demonstrated that combining bone marrow MSCs with a polymer-hydroxyapatite scaffold promoted bone cell growth and differentiation more effectively than either component alone.28PubMed Central. Osteogenic potentials in canine mesenchymal stem cells: unraveling the efficacy of polycaprolactone/hydroxyapatite scaffolds in veterinary bone regeneration Scaffold-based approaches are particularly relevant for structural repairs, such as critical-sized bone defects, where simply flooding an area with cells and signals is not enough and the tissue needs architectural support to regenerate properly.