Direct engraftment means that administered cells reach a target tissue, remain there as viable cells, and become durably incorporated into the tissue. Early regenerative-medicine models often assumed that substantial engraftment was necessary for benefit. For many mesenchymal stromal/stem cell (MSC) applications, however, biodistribution studies indicate limited long-term persistence after systemic administration, with many cells initially retained in the pulmonary circulation and subsequently cleared. Detection of labeled cells or cell-associated signal shortly after administration is not equivalent to durable engraftment. Demonstrating direct replacement requires evidence of viable donor cells integrated into tissue and contributing meaningfully to function over an appropriate time frame.
Figure 1 placeholder — Direct engraftment
Differentiation
Differentiation is the acquisition of a more specialized cellular phenotype. MSCs can display differentiation potential under defined experimental conditions, especially along osteogenic, chondrogenic, and adipogenic lineages, but in-vitro differentiation capacity does not prove that administered cells differentiate into the required tissue in vivo. Claims of transdifferentiation into cardiomyocytes, neurons, hepatocytes, or other specialized cells require particularly rigorous lineage evidence. Apparent donor-cell marker expression can be confounded by cell fusion, label transfer, phagocytosis of labeled material, or nonspecific staining. Functional tissue replacement requires more than expression of selected markers.
Figure 2 placeholder — Differentiation
Paracrine effects
Paracrine effects arise when administered cells influence neighboring or distant cells through released soluble factors, extracellular vesicles, metabolites, and other signals rather than by becoming permanent replacement tissue. MSC secretomes can contain cytokines, chemokines, growth factors, lipids, and EV-associated cargo capable of altering inflammatory, vascular, survival, and remodeling pathways in experimental systems. Paracrine signaling can occur even when cell persistence is brief. The specific signals depend on cell source, manufacturing, environmental sensing, dose, route, and recipient tissue. Mechanistic plausibility does not establish clinical effectiveness, and the active mediators may differ among products and diseases.
Figure 3 placeholder — Paracrine effects
Immune-mediated effects
Immune-mediated effects include changes produced through interactions between administered cells or cell-derived products and innate or adaptive immune systems. MSCs can interact with monocytes, macrophages, dendritic cells, T cells, B cells, natural killer cells, complement, and coagulation pathways. Some proposed therapeutic effects may result from recipient immune responses to administered cells, including recognition, apoptosis, phagocytic clearance, and downstream reprogramming of host immune cells. Thus, short persistence does not necessarily imply biological inactivity. Conversely, immunomodulatory findings in vitro or in animal models do not establish that the same magnitude or direction of effect occurs clinically.
Figure 4 placeholder — Immune-mediated effects
Matrix remodeling
Matrix remodeling refers to changes in extracellular matrix synthesis, degradation, organization, and mechanical properties. MSCs, EVs, immune cells, fibroblasts, and tissue-resident cells can influence collagen deposition, matrix metalloproteinases, tissue inhibitors of metalloproteinases, myofibroblast activity, scar formation, and other remodeling pathways. Matrix changes may support repair, reduce or worsen fibrosis, or alter tissue mechanics depending on context. Remodeling should not be equated automatically with regeneration: a tissue can remodel without restoring its original architecture or function. Clinical relevance requires structural and functional endpoints appropriate to the disease.
Figure 5 placeholder — Matrix remodeling
Clinical takeaway
Cell replacement is only one possible mechanism of regenerative-cell interventions. Direct engraftment and differentiation must be demonstrated rather than assumed; paracrine, immune-mediated, and matrix-remodeling mechanisms may operate even when administered cells persist only briefly.
Mechanisms can coexist
These mechanisms are not mutually exclusive. A product may produce transient local signaling, interact with immune cells, alter matrix remodeling, and show limited engraftment at the same time. Their relative importance can vary by product, route, dose, tissue, and disease.
Persistence versus effect
Cell persistence and biological effect are different variables. A short-lived cell can release signals or trigger host responses, whereas long persistence does not by itself prove therapeutic function.
Tracing and lineage evidence
Cell-tracking studies require careful interpretation. Fluorescent dyes, radionuclides, genetic labels, sex-mismatched markers, and other tracing methods have different limitations. Label persistence can outlast cell viability or transfer to host cells.
Product and disease specificity
Mechanisms demonstrated for one MSC source, manufacturing process, route, or disease cannot automatically be generalized to another. Mechanism-of-action evidence should be tied to the exact product and intended clinical context.
Clinical interpretation
A clinical improvement cannot by itself reveal mechanism. Mechanistic claims should be supported by direct measurements capable of distinguishing engraftment, differentiation, paracrine signaling, immune effects, and matrix changes.
Knowledge gaps
For many investigational cell and EV interventions, the dominant mechanism in humans remains uncertain. Better in-vivo tracking, validated biomarkers, tissue sampling where feasible, and mechanistically informative trials are needed.
Bottom-line evidence statement
The modern evidence base does not support treating durable cell replacement as the default explanation for every regenerative-cell effect. Mechanisms must be demonstrated product by product, and biological plausibility remains separate from proof of clinical benefit.
Additional controlled terminology
Cell persistence and biodistribution influence interpretation of the pulmonary first-pass effect and other trafficking patterns. Cell tracking and lineage tracing are required to distinguish true transdifferentiation from cell fusion or label transfer. Paracrine signaling reflects the secretome, including a cytokine, chemokine, growth factor, or extracellular vesicle. Innate immunity and adaptive immunity can contribute through phagocytic clearance and other host response pathways. The extracellular matrix changes through collagen deposition, matrix metalloproteinase activity, and myofibroblast behavior, which can alter fibrosis without necessarily producing regeneration. Each proposed mechanism of action requires direct evidence.