Mesenchymal Stem Cells: Sources, Mechanisms, and Clinical Applications

What Are Mesenchymal Stem Cells?

Mesenchymal stem cells (MSCs) are a type of adult stem cell first identified by Alexander Friedenstein in the 1960s when he discovered that bone marrow contained a population of plastic-adherent, fibroblast-like cells capable of forming bone and cartilage [1]. The term “mesenchymal stem cell” was popularized by Arnold Caplan in 1991, though Caplan himself later advocated renaming them “medicinal signaling cells” to better reflect their primary mechanism of action [2].

MSCs are defined by the International Society for Cellular Therapy (ISCT) based on three minimum criteria: they must adhere to plastic under standard culture conditions; they must express surface markers CD73, CD90, and CD105 while lacking expression of CD34, CD45, and other hematopoietic markers; and they must be capable of differentiating into osteoblasts (bone cells), adipocytes (fat cells), and chondroblasts (cartilage cells) under appropriate in vitro conditions [3].

While the term “stem cell” implies unlimited self-renewal and pluripotency, MSCs are technically multipotent, meaning they can differentiate into a limited range of cell types within the mesenchymal lineage. This distinction is scientifically precise but often lost in popular discussion, where MSCs are sometimes portrayed as having the broad regenerative capacity of embryonic stem cells. Understanding their actual capabilities is essential for setting realistic expectations about what MSC therapies can and cannot accomplish.

Sources of Mesenchymal Stem Cells

Bone Marrow

Bone marrow was the first identified source of MSCs and remains the most extensively studied. MSCs are harvested from bone marrow via aspiration, typically from the iliac crest (hip bone). The procedure is performed under local anesthesia and takes approximately 30 to 60 minutes. Bone marrow aspirate contains a heterogeneous mixture of cells, including hematopoietic stem cells, and MSCs represent only about 0.001% to 0.01% of the total nucleated cell population [4].

Bone marrow-derived MSCs (BM-MSCs) have the most extensive clinical track record and are often considered the “gold standard” for comparison. However, their concentration in bone marrow declines with age, and the harvesting procedure, while generally safe, can be uncomfortable and carries a small risk of infection or bleeding at the aspiration site.

Adipose Tissue

Fat tissue contains a rich population of MSCs that can be isolated through lipoaspiration (a procedure similar to liposuction). Adipose-derived MSCs (AD-MSCs) are present at concentrations roughly 100 to 500 times greater than in bone marrow, making adipose tissue an attractive source for clinical applications [5]. The stromal vascular fraction (SVF) obtained from enzymatic digestion of lipoaspirate contains MSCs along with endothelial progenitor cells, pericytes, immune cells, and other stromal components.

AD-MSCs share most biological properties with BM-MSCs but may differ in certain differentiation capacities and cytokine secretion profiles. Some research suggests AD-MSCs have superior angiogenic properties, while BM-MSCs may have advantages in osteogenic (bone-forming) differentiation [6]. The relative ease of harvesting and the abundance of cells make adipose tissue an increasingly popular source in clinical practice.

Umbilical Cord Tissue

Wharton’s jelly, the gelatinous connective tissue within the umbilical cord, is a rich source of MSCs that can be collected non-invasively at the time of birth. Umbilical cord MSCs (UC-MSCs) are younger and more proliferative than adult-derived MSCs, with longer telomeres and higher expansion potential in culture [7]. They also appear to have stronger immunomodulatory properties, making them particularly attractive for allogeneic (donor-to-recipient) applications.

Because UC-MSCs are collected from tissue that would otherwise be discarded, their use avoids the ethical concerns associated with embryonic stem cells and the procedural burden of bone marrow or adipose harvesting. UC-MSCs are the most commonly used cell type in the growing market of allogeneic “off-the-shelf” stem cell products, though it is worth noting that the quality and cell viability of commercially available products vary widely.

Other Sources

MSCs have also been isolated from dental pulp, synovial fluid, peripheral blood, placental tissue, and amniotic fluid. While these sources are subjects of active research, bone marrow, adipose, and umbilical cord remain the dominant clinical sources.

How MSCs Work: The Paracrine Paradigm

Early enthusiasm for MSCs centered on the idea that transplanted cells would engraft in damaged tissue and directly differentiate into the cell types needed for repair. However, decades of research have revealed a more nuanced picture. The vast majority of transplanted MSCs do not survive long-term in the recipient tissue. Tracking studies using labeled cells have shown that most MSCs disappear from the transplant site within days to weeks [8].

Despite their poor engraftment and survival, MSCs consistently produce measurable therapeutic effects. This apparent paradox is explained by the paracrine hypothesis: MSCs exert their primary therapeutic effects by secreting a complex array of bioactive molecules, including growth factors, cytokines, chemokines, and extracellular vesicles (exosomes), that influence the behavior of surrounding cells and tissues [2].

Key Paracrine Mechanisms

• Anti-inflammatory signaling: MSCs secrete anti-inflammatory cytokines such as IL-10, TGF-beta, and prostaglandin E2 (PGE2) that suppress excessive inflammatory responses and promote tissue homeostasis
• Angiogenesis promotion: MSCs release vascular endothelial growth factor (VEGF) and other angiogenic factors that stimulate new blood vessel formation in ischemic or damaged tissue
• Anti-apoptotic effects: MSC-derived factors can reduce cell death in stressed tissues, helping to preserve viable cells during injury or disease
• Tissue remodeling: MSCs secrete matrix metalloproteinases and other factors that help remodel scar tissue and promote organized tissue repair
• Recruitment of endogenous progenitors: The signaling molecules released by MSCs can attract the body’s own stem and progenitor cells to the site of injury, amplifying the regenerative response

Immunomodulation

MSCs possess remarkable immunomodulatory properties that have made them attractive candidates for treating autoimmune and inflammatory conditions. They can suppress T cell proliferation, inhibit B cell function, modulate dendritic cell maturation, and promote the generation of regulatory T cells (Tregs) [9]. These effects are mediated by both direct cell-to-cell contact and paracrine signaling.

Importantly, MSCs appear to be “immune privileged” to some degree, meaning they express low levels of MHC class II molecules and co-stimulatory molecules, which reduces the likelihood of immune rejection when used in allogeneic (donor) applications. This property, while not absolute, is one reason why allogeneic MSC products have been feasible in clinical settings without the need for immunosuppressive drugs in many cases [10].

Clinical Applications

Orthopedic Conditions

Orthopedic applications are the most common clinical use of MSCs. Research and clinical practice have explored MSC therapy for osteoarthritis, cartilage defects, tendon injuries, bone fractures, and avascular necrosis.

In osteoarthritis, intra-articular injection of MSCs has shown promise in reducing pain, improving function, and potentially slowing cartilage degeneration. A systematic review of clinical trials found that MSC injections for knee osteoarthritis produced significant improvements in pain scores and functional outcomes compared to baseline, with some studies showing evidence of cartilage preservation on MRI [11]. However, well-controlled trials comparing MSCs to placebo (saline injection) have produced mixed results, and the optimal cell source, dose, and preparation method remain active areas of investigation.

Autoimmune Conditions

The immunomodulatory properties of MSCs have led to clinical trials in graft-versus-host disease (GVHD), systemic lupus erythematosus, multiple sclerosis, Crohn’s disease, and type 1 diabetes.

The most advanced application is in GVHD, a life-threatening complication of bone marrow transplantation. An MSC product (Remestemcel-L, marketed as Ryoncil in some jurisdictions) has received conditional approval in certain countries for steroid-refractory acute GVHD in pediatric patients. Clinical data showed meaningful response rates in patients who had failed standard immunosuppressive therapy [12].

For Crohn’s disease, Darvadstrocel (Alofisel) received European Medicines Agency approval for the treatment of complex perianal fistulas, representing one of the few regulatory-approved MSC products globally. The product uses expanded allogeneic adipose-derived MSCs injected locally around the fistula tract [13].

Neurological Conditions

MSC therapy for neurological conditions, including stroke, traumatic brain injury, spinal cord injury, and neurodegenerative diseases, is in earlier stages of development. Preclinical data is extensive, showing that MSCs can reduce neuroinflammation, promote axonal regeneration, and support neuronal survival through paracrine mechanisms. Clinical trials in stroke and amyotrophic lateral sclerosis (ALS) have generally shown safety and some signals of benefit, but definitive efficacy data from large randomized trials is still awaited [14].

Allogeneic vs. Autologous MSCs

One of the most significant practical questions in MSC therapy is whether to use autologous cells (from the patient’s own body) or allogeneic cells (from a donor).

Autologous Approach

Autologous MSCs eliminate the risk of immune rejection and disease transmission from a donor. However, they require a harvesting procedure (bone marrow aspiration or liposuction), and the quality and quantity of cells obtained may be compromised by the patient’s age, health status, and underlying disease. Autologous cells also typically require culture expansion to reach therapeutic doses, which adds time, cost, and regulatory complexity.

Allogeneic Approach

Allogeneic MSCs, particularly those derived from umbilical cord tissue, offer the advantage of “off-the-shelf” availability. Young donor cells tend to be more proliferative and potent than those from older patients. The relative immune privilege of MSCs makes allogeneic use feasible without immunosuppression in many cases. The main concerns with allogeneic MSCs include variability in donor cell quality, the potential for immune sensitization with repeated dosing, and questions about long-term safety that have not been fully resolved by existing clinical data [10].

Culture Expansion: Benefits and Concerns

Because MSCs represent a small fraction of the cells in bone marrow or adipose tissue, culture expansion in a laboratory is often used to increase cell numbers to therapeutic doses (typically tens of millions to hundreds of millions of cells). This process involves growing the cells on plastic surfaces in specialized culture media for several weeks.

Culture expansion raises several concerns. Extended passage in culture can lead to changes in cell surface marker expression, reduced differentiation capacity, and cellular senescence. There have also been theoretical concerns about chromosomal instability during prolonged culture, though clinical data has not shown increased malignancy risk in patients receiving culture-expanded MSCs [15]. Regulatory agencies consider culture expansion to constitute “more than minimal manipulation,” which triggers the requirement for an Investigational New Drug (IND) application or Biologics License Application (BLA) for clinical use in the United States.

FDA Regulation

The FDA’s regulatory framework for cell-based therapies distinguishes between minimally manipulated and more-than-minimally-manipulated cell products. Under 21 CFR Part 1271, a cell product is exempt from premarket review if it meets all of the following criteria: it is minimally manipulated; it is intended for homologous use; it is not combined with another article; and (for autologous cells) it does not have a systemic effect or depend on metabolic activity for its primary function.

Most MSC applications do not meet all of these criteria. Culture expansion is considered more than minimal manipulation, and many proposed uses (such as treating autoimmune disease with cells injected intravenously) are arguably non-homologous. This means that the vast majority of MSC therapies in the United States legally require FDA premarket approval, and clinics offering expanded MSC products outside of clinical trials may be operating in violation of federal regulations [15].

The regulatory landscape is complex, and patients should be aware that a product being marketed commercially does not mean it has been FDA-approved. As of early 2026, only a small number of MSC products have received regulatory approval anywhere in the world, and no MSC product has received full FDA approval in the United States.

Current Clinical Trials

As of 2026, hundreds of clinical trials involving MSCs are registered on ClinicalTrials.gov, spanning indications from osteoarthritis and GVHD to heart failure, COPD, diabetic foot ulcers, and COVID-19 recovery. Phase 3 trials are underway for several indications, and the next few years are expected to bring additional regulatory decisions on MSC products in the United States and Europe.

Key areas of active investigation include:

• Knee osteoarthritis: Multiple phase 2 and phase 3 trials comparing different MSC sources and doses to placebo
• Heart failure: Trials evaluating intramyocardial and intravenous MSC delivery for ischemic and non-ischemic cardiomyopathy
• Inflammatory bowel disease: Ongoing trials for Crohn’s disease fistulas and luminal disease
• Spinal cord injury: Early-phase trials evaluating intrathecal MSC delivery for chronic spinal cord injury
• Aging and frailty: Novel trials exploring MSC infusions for age-related decline in function and immune competence

Patients interested in MSC therapy should consider enrolling in a registered clinical trial, which provides access to therapy under rigorous medical oversight and contributes to the evidence base needed for regulatory approval.

Related Reading

• Stem Cell Therapy: Complete Guide
• Stem Cell Injections: Types, Costs, and What to Expect
• Peptide Therapy: Complete Guide
• Exosome Therapy: Mechanisms and Research

References

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