Cell-cell fusion and stem cell plasticity: Emerging paradigms in tissue regeneration

Abstract

Cell-cell fusion is a fundamental biological process contributing to tissue development, homeostasis, and regeneration. Physiological fusion in the placenta, skeletal muscle, bone, and immune system is mediated by tightly regulated fusogens and microenvironmental cues. Dysregulated fusion, however, can lead to pathological outcomes, including malignant cell hybrids and viral syncytia. Stem cell-somatic cell fusion represents both a therapeutic opportunity and a safety concern, as fusion-derived heterokaryons and synkaryons can enhance tissue repair and plasticity while posing risks of genomic instability and unintended fusion events. In this review, we synthesize current knowledge on the molecular mechanisms, tissue-specific fusogens, and regulatory programs governing cell fusion. We also discuss translational strategies for harnessing stem cell fusion in regenerative therapies, highlighting opportunities to maximize repair while minimizing potential risks. Our goal is to provide a unified framework that integrates physiological and pathological perspectives, offering guidance for future research and clinical application.

Key Words: Cell fusion; Stem cells; Fusogen; Heterokaryon; Synkaryon; Regenerative therapy; Tissue regeneration

Core Tip: This review summarizes the status quo of cell-cell fusion across development, regeneration, and disease. We highlight key topics including membrane merger mechanisms, tissue-specific fusogens, and microenvironmental cues that influence fusion competence and post-fusion behavior. Stem cell-somatic cell fusion presents a dual-edged opportunity: Fusion-derived heterokaryons (multinuclear cells) and synkaryons (nuclear-mixed hybrids) may enhance repair and plasticity, yet raise safety concerns including genomic instability and unintended fusion events. Major progress has clarified core fusion stages and identified physiological fusogens, but critical problems remain in vivo gating, fate/stability prediction, and pathological risks (malignant hybrids, viral syncytia).

INTRODUCTION

Cell fusion is the process by which two or more cells merge into a single cell bound by a common plasma membrane, allowing the intermingling of their cytoplasmic and nuclear contents. In multicellular organisms, fusion underlies key physiological events, ranging from fertilization - the fusion of sperm and oocytes into a zygote - to the formation of multinucleated muscle fibers, osteoclasts, placental syncytiotrophoblasts (STBs), and macrophage-derived giant cells[1-4]. Through these events, cell fusion contributes to the development, immune responses, bone remodeling, and maintenance of tissue architecture throughout life[4-7]. Conversely, dysregulated fusion can perturb these processes and has been linked to myopathies, osteopetrosis, preeclampsia, and other disorders[8-10].

Stem cells are a particularly important fusion-competent population[11,12]. Beyond their well-established paracrine and differentiation-mediated roles in tissue repair, stem cells can fuse with other stem cells or differentiated cells in embryonic and postnatal tissues, thereby supporting tissue growth, regeneration, and homeostasis[12,13]. Hematopoietic stem cells and other bone marrow-derived cells (BMDCs) widely circulate and engraft in diverse organs, suggesting that they participate in regeneration by fusing with host parenchymal cells[14]. Indeed, in vivo studies have documented the fusion-mediated contributions of BMDCs to the central nervous system (CNS) as well as to retinal, hepatic, and skeletal muscle repair, where fusion-derived hybrids acquire features of injured tissue and support functional recovery[15-18].

Cell fusion can promote stem cell plasticity by establishing a mixed cytoplasmic environment that reprograms transcriptional networks and epigenetic states[19]. In heterokaryons formed immediately after fusion, distinct nuclei coexist within one cytoplasm. Even before nuclear fusion occurs, a window of plasticity opens, allowing cell fate to change[20,21]. Over time, heterokaryons may undergo cell division and karyogamy to form synkaryons, generating genome-mixed hybrid cells. While such hybrids can establish novel transcriptional networks and functional traits. However, they simultaneously face a greater propensity for aneuploidy and genomic instability[22-25]. In this context, stem cell fusion is direct mechanism of plasticity, by enabling fate conversion through fusion-mediated reprogramming.

These observations highlight cell-cell fusion as a complementary route for tissue regeneration; however, the ability to harness cell-cell fusion therapeutically remains limited. A deeper understanding of the molecular mechanisms governing membrane fusion and of post-fusion nuclear reprogramming, major fusogens, and fusion determinants in distinct tissues, as well as the conditions under which stem cells fuse with somatic cells in vivo is needed. Equally important are strategies to increase beneficial, tissue-restricted fusion while avoiding pathological events, such as fusion with cancer cells or virus-driven syncytia[26,27]. In this review, we summarize the physiological and pathological roles of cell fusion, outline current knowledge on the underlying mechanisms and fusogenic machinery, and discuss how these insights can be leveraged to design safer and more effective fusion-based regenerative therapies.

SEARCH STRATEGY

We conducted the literature search using PubMed and Google Scholar database to identify relevant studies published up to July 2025. The search strategy combined terms including “cell fusion”, “stem cell fusion”, “heterokaryon”, “synkaryon”, “fusogen”, “reprogramming”, “tissue regeneration”, and “genomic instability”, with additional terms tailored to specific tissues and model systems. Key concepts and mechanistic frameworks in the cell fusion field were established by seminal early studies. Given that direct therapeutic exploitation of stem cell-cell fusion remains relatively limited, we performed a broader search without date limitations. Instead, we prioritized mechanistic studies, in vivo evidence, and proof-of-concept translational reports, supplemented by recent advances when available. Additional papers were identified by manual screening of reference lists from relevant reviews and primary studies.

MECHANISMS OF CELL FUSION

Cell-cell membrane fusion process

Certain cell types can undergo cell-cell fusion, a biological process in which the lipid bilayers of two adjacent cells merge to form a single continuous membrane. Fusion is initiated by specific recognition and tight adhesion between neighboring cells. Upon contact, cells engage in surface recognition to selectively identify compatible fusion partners, primarily through ligand-receptor interactions[28]. For instance, in human placental trophoblasts, the fusogenic proteins syncytin-1 and syncytin-2, which are envelope glycoproteins of endogenous retroviral origin, are expressed on the cell surface and initiate fusion by binding to their respective receptors, alanine serine cysteine transporter 2 (ASCT2) and major facilitator superfamily domain-containing protein 2 (MFSD2A)[29,30]. In myoblasts, intercellular adhesion is mediated by cadherins with extracellular matrix components, promoting close apposition of neighboring cells[31]. Cells brought into close contact activate the small guanosine triphosphatases (GTPases) cell division cycle 42 (CDC42) and Rac family small GTPase 1 (Rac1), which regulate cytoskeletal organization, polarity, and plasma membrane dynamics, thereby preparing cells for fusion by driving actin remodeling and membrane reorganization[32-35]. In their native physiological context, the intermembrane distance between two cells typically ranges from 10 nm to several tens of nanometers. For membrane fusion to occur, lipid bilayers must overcome hydration-mediated repulsive forces and approach within less than approximately 2 nm. Achieving this degree of proximity requires the partial dehydration of the water bound to the membrane surface; otherwise, the strong polar repulsion between the two membranes prevents lipid mixing[28,36]. Fusogens facilitate the dehydration process by drawing membranes into nanoscale proximity and displacing the hydration shell at the interface[37-40]. Once sufficiently dehydrated, local disruption and reorganization of the outer leaflets permit the merging of the two outer monolayers, generating a hemifusion intermediate in which the inner leaflets and cytoplasmic contents remain separated[41,42]. Continued lipid rearrangement then destabilizes both the outer and inner leaflets, and the expansion of this destabilized region allows the bilayer components to intermix and retract from the contact site, thereby nucleating a nascent fusion pore. Subsequent pore enlargement enables cytoplasmic continuity and ultimately yields a single fused cell[43,44]. In summary, this sequence of: (1) Partner recognition; (2) Tight adhesion; (3) Cytoskeletal rearrangement; (4) Membrane apposition and dehydration; (5) Hemifusion; and (6) Fusion pore formation and expansion represents a generalized membrane fusion cascade that is co-opted by developmental fusogens, viral fusogens, and engineered fusogenic systems (Figure 1).

Figure 1 Schematic illustration of the cell fusion process. The diagram shows the sequential steps of cell-cell fusion: (1) Cell recognition and adhesion: Two distinct cells recognize each other through receptor-ligand interactions, initiating adhesion; (2) Cytoskeletal rearrangement: Actin cytoskeleton remodeling promotes membrane proximity and prepares for fusion; (3) Membrane apposition and dehydration: The opposing plasma membranes are brought into close contact, and local dehydration of the intermembrane space occurs; (4) Hemifusion: The outer leaflets of the lipid bilayers merge, forming an intermediate hemifusion structure; and (5) Fusion pore formation: A pore opens between the two cells, enabling cytoplasmic continuity and resulting in a fully fused multinucleated cell.

From heterokaryon-to-synkaryon

Immediately following cell-cell fusion, the resultant hybrid - termed a heterokaryon -contains distinct nuclei that coexist within a shared cytoplasm. During this stage, the nuclear envelopes remain intact and no direct karyogamy occurs. Over time, heterokaryons may follow divergent fates: (1) Maintain a binucleated configuration; (2) Undergo ploidy reduction via selective nuclear loss or micronucleation; or (3) Proceed toward synkaryon formation through karyogamy[45-47]. The synkaryon is a transient, genetically unstable intermediate harboring a full tetraploid (4N) chromosomal complement derived from both parental cells. During subsequent mitoses, random segregation of parental alleles often occurs, and defects in spindle assembly or checkpoint control may result in aneuploidy or chromosomal mosaicism[48,49]. Nevertheless, when chromosome segregation proceeds accurately, diploid progeny may arise that either: (1) Retain the genomic identity of one parent; or (2) Acquire a hybrid diploid genome, representing recombination between both fusion partners (Figure 2)[50].

Figure 2 Heterokaryon-to-synkaryon fates after cell-cell fusion. Following membrane fusion and cytoplasmic continuity, the immediate product of cell-cell fusion is a heterokaryon, in which parental nuclei coexist within a shared cytoplasm. From this state, several fates are possible. In one trajectory, heterokaryons persist as stable binucleated cells, maintaining separate nuclei but integrating cytoplasmic signaling and metabolic networks, which can enable functional complementation between fusion partners. In a second trajectory, ploidy reduction occurs through selective nuclear loss, nuclear budding, or micronucleation, ultimately yielding near-diploid revertant cells that retain genetic material from one or both partners. In a third trajectory, karyogamy leads to formation of a single synkaryon harboring a combined tetraploid (4N) genome. Subsequent cell divisions from synkaryons can result either in chromosomal instability and aneuploid or mosaic progeny, or in error-corrected segregation and stable diploid hybrid clones that carry recombined parental genomes. These divergent fates underlie both beneficial outcomes, such as regeneration and lineage plasticity, and pathological consequences, including genomic instability and tumor progression.

Synkaryon-derived hybrids embody a mixed chromosomal complement from both parents and frequently display shared phenotypic and genetic traits. For example, interspecies fusion between murine embryonic stem (ES) cells and human BJ fibroblasts produces hybrid cells that can be propagated for more than 50 passages while maintaining ES cell - like proliferative immortality and the expression of key pluripotency markers, such as octamer-binding transcription factor 4. Notably, the viral vector originally introduced into BJ fibroblasts remains detectable in the hybrid cells, indicating preservation of somatic cell-derived genetic elements[51]. Similar observations have been reported in other fusion systems, where recipient cells acquire functional enhancements, including genetically engineered traits[52-55]. Moreover, fusion can restore deleted genomic regions, and cells carrying chromosomal deletions can regain missing loci following fusion with genetically intact partners[56,57]. These generic biophysical and nuclear events are implemented by distinct sets of fusogens in different tissues and can be experimentally manipulated using physical, chemical, or biological fusion-inducing methods.

INDUCING CELL-CELL FUSION

Cell-cell fusion creates a single continuous cytoplasm, enabling direct sharing of intracellular components between distinct cells. This event is governed by genetic programs, fusogenic protein expression, and downstream signaling cascades. Fusion requires membrane remodeling, such as increased curvature and local bending, to overcome the substantial energetic barriers; consequently, spontaneous fusion is rare under normal physiological conditions[58-60]. For example, bone marrow-derived mesenchymal stem cells typically exhibit spontaneous fusion frequencies of approximately 2%[61]. These low baseline rates complicate the mechanistic dissection of stem cell-cell fusion. To address these challenges, a range of induction strategies have been developed, including physical (e.g., electric pulses), chemical [e.g., polyethylene glycol (PEG)], and biological (e.g., inactivated/replication-defective viruses) approaches.

Physical fusions

Physical methods for inducing fusion include electrofusion, microfluidic confinement, microcompression, and acoustic (ultrasound)-based strategies[62-65]. Electrofusion delivers short high-voltage pulses that require close membrane contact and promote fusion by transiently reorganizing the lipid bilayer in response to an electric field[66]. High-field pulses lower the bilayer energy barrier and generate hydrophilic pores, thereby increasing membrane permeability and facilitating cytoplasmic mixing and, in some cases, nuclear fusion[67,68]. Fusion efficiency and cell viability strongly depend on the pulse parameters and must be tailored to the target cells. As electroporation scales with cell size, marked size disparities between partner cells can yield asymmetric permeabilization and reduce fusion efficacy[69-71]. Electrofusion offers high yields with relatively low cytotoxicity and is amenable to standardization across laboratories; however, it requires specialized instrumentation and precise electrical tuning.

Chemical fusions

PEG-mediated fusion is the most widely used chemical fusion method. PEG is a simple water-soluble polymer that has long been used for hybridoma generation (immune cell-myeloma fusion). By creating a thermodynamically unfavorable environment for interfacial water, PEG rapidly dehydrates the adjacent membranes, bringing them into molecular-scale proximity, destabilizing the bilayer, and initiating lipid mixing. However, completion of a stable fused state typically requires additional time after the initial membrane fusion[72,73]. PEG fusion requires minimal equipment and is operationally straightforward[74-76]; however, its performance depends on multiple variables, including cell type and PEG purity, concentration, and molecular weight. Higher PEG concentrations can increase the fusion probability but at the cost of cytotoxicity and reduced viability. Moreover, PEG often produces random, nonselective pairings, limiting the precise formation of targeted hybrids[77-81]. Thus, although it is experimentally simple, PEG fusion requires careful optimization to balance efficiency, viability, and selectivity.

Biological fusions

Biological fusion refers to the merging of cells under normal physiological conditions and is most commonly mediated by fusogenic proteins. Viral cell-cell fusion follows the conserved merger cascade (Figure 1) but includes virus-specific hallmarks. Viral fusogens expressed on infected cells engage specific receptors on neighboring targets, and fusion proceeds through a conserved sequence: (1) Receptor-ligand recognition and fusogen activation; (2) Exposure and insertion of the fusion peptide into the opposing membrane; (3) Clustering of fusogens at the contact site; (4) Large-scale refolding that pulls membranes into nanometer proximity; (5) Formation of a hemifusion intermediate; and (6) Nucleation and expansion of a fusion pore, which leads to complete membrane fusion. Iterative execution of this program yields extensive syncytia containing multiple nuclei[82-86]. Virus-mediated cell-cell fusion has been documented across many enveloped viruses, including human immunodeficiency virus (HIV), severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and respiratory syncytial virus (RSV)[87-90].

Although physical, chemical, and biological fusion methods are widely used to induce fusion in vitro, their feasibility in vivo is fundamentally constrained by delivery problems, off-target fusion, cytotoxicity, and monitoring difficulty. Therefore, these limitations suggest that fusion-based regenerative strategies should focus less on directly inducing fusion in vivo and more on strategies that transient restricted fusogen/receptor expression to defined cell types, using inducible or transient systems, and genomic surveillance to minimize unintended hybrid formation.

MAJOR FUSOGENS AND FUSION DETERMINANTS

Cell-cell fusion is executed by fusogens - specialized membrane proteins that lower the substantial energy barrier limiting lipid bilayer merging. Fusogens do so by displacing hydration, inducing local curvature, reorganizing lipids and nucleating and enlarging a fusion pore in which the cytoplasmic contents can mix[91-93]. Depending on the system, fusogens can operate in unilateral (present on only one of the two fusion partners) or bilateral (present on both partners) modes and in homotypic or heterotypic pairings. These topological configurations impose directionality and specificity on fusion reactions[44,94].

Placental trophoblast fusion: Syncytin-1 and syncytin-2

In the human placenta, the best-characterized fusogens are syncytin-1 and syncytin-2 that are essential for the formation and maintenance of the STB layer. Syncytin-1 and -2 mediate the fusion of villous cytotrophoblasts (CTBs) into multinucleated STBs by engaging their cognate receptors, ASCT2 and MFSD2A, respectively, followed by a retroviral-like cascade of receptor binding, fusion-peptide insertion, large-scale conformational refolding, hemifusion, and fusion-pore opening[95-99].

Recent studies have expanded this view by demonstrating that trophoblast fusogenicity can be tightly regulated at multiple levels. Transcription factors, such as glial cells missing 1, upstream signaling pathways (including the neuropeptide FF/neuropeptide FF receptor 2 signaling pathway), and epigenetic regulation factors cooperate to control syncytin expression[100,101], whereas host restriction factors (such as guanylate-binding proteins) and proteolytic processing steps differentially modulate syncytin-1 vs syncytin-2 activation[102]. In addition, co-factors such as galectin-1 can stabilize syncytin-2-MFSD2A interactions and enhance fusion efficiency[103]. Together, these studies indicate that placental fusion is not driven by syncytins alone but rather emerges from a regulated fusogenic module composed of viral-derived fusogens, their receptors, and accessory regulators.

Skeletal muscle fusion: Myomaker and Myomixer (Myomerger)

Skeletal muscle development and regeneration rely on the fusion of mononucleated myoblasts into multinucleated myofibers, a process orchestrated by two muscle-restricted fusogens: Myomaker (TMEM8C/Mymk) and Myomixer (also known as Myomerger, Minion, or Mymx)[104,105]. These factors function at distinct, yet interdependent, stages of the membrane fusion pathway. Myomaker, a multi-pass transmembrane protein with a critical palmitoylated cytoplasmic tail, primes membranes and acts at or before the hemifusion stage, where the outer leaflet lipids begin to mix[106,107]. In contrast, Myomixer is a small fusogenic micropeptide whose ectodomain drives the transition from hemifusion to full fusion by promoting fusion pore opening and expansion[108].

Neither factor alone is sufficient to complete myoblast fusion: Myomaker expression enables heterotypic fusion between myoblasts and Myomaker-negative fibroblasts but does not support fibroblast-fibroblast fusion, whereas co-expression of Myomaker and Myomixer confers robust fusogenic capacity to non-fusogenic cells[109]. Genetic loss-of-function studies have further demonstrated that Myomaker is required by both fusion partners for efficient myoblast-myoblast fusion, indicating a bilateral requirement[110]. More recent work has shown that dynamic, stage-specific expression of Myomaker and Myomixer is essential not only during embryonic myogenesis but also during adult muscle repair. Moreover, evidence suggests that gene mutations associated with muscle-restricted fusogens lead to myopathy phenotypes, emphasizing Myomaker and Myomixer status as core vertebrate muscle fusogens rather than mere fusion “modulators”[106,109,111-113].

Gamete fusion: Izumo sperm-oocyte fusion 1-JUNO as an adhesion hub in a multi-component fusion machinery

In mammalian fertilization, the fusion of sperm and oocytes represents the terminal and most selective step of the reproductive process. A central molecular module in this context is the high-affinity ligand-receptor pair formed by izumo sperm-oocyte fusion 1 (IZUMO1) in sperm and IZUMO1 receptor (JUNO/IZUMO1R/FOLR4) in oocytes[114]. IZUMO1, an immunoglobulin superfamily protein, translocates to the sperm plasma membrane following the acrosomal reaction, enabling tight binding to JUNO on the egg surface[115,116]. JUNO is a glycophosphatidylinositol-anchored protein that mediates species-specific recognition and membrane apposition. Additional oocyte factors, such as CD9, shape the microvillar architecture of the oolemma and indirectly promote sperm-egg fusion competence[117-120].

However, IZUMO1-JUNO binding alone is not sufficient to trigger lipid fusion, and neither protein behaves as a classical fusogen when ectopically expressed[121,122]. In recent years, genome-wide and gene knockout studies have identified several sperm membrane proteins - sperm acrosome associated 6, transmembrane protein 95, sperm-oocyte fusion required 1, fertilization influencing membrane protein, and DC-STAMP domain containing -1 and -2 - that are individually essential for sperm-oocyte fusion but not for initial adhesion. This specificity suggests that they assemble with IZUMO1 into a larger, multi-component “fusion machinery” at the sperm-egg interface[123-127]. Despite this progress, a bona fide mammalian gamete fusogen equivalent to the hapless 2/generative cell-specific 1 fusogen characterized in plants and many protists has not yet been conclusively identified. Current evidence supports a model in which IZUMO1-JUNO functions as an adhesion nexus that recruits and organizes additional sperm factors into a fusion-competent complex, with actual fusogenic activity likely distributed across multiple cooperating proteins rather than residing in a single dominant fusogen[128,129].

Although the roles of fusogens have been studied predominantly in the context of development and fertilization, similar or related fusogenic programs have been repurposed in injury settings, where stem cells fuse with somatic cells and contribute to tissue regeneration. These mechanistic insights provide a foundation for understanding how stem cells engage in cell-cell fusion in vivo and how fusion-derived hybrids can contribute to, or complicate, attempts at tissue regeneration. Table 1 summarizes representative fusogens and their receptors.

Table 1 Key fusogens and receptors involved in physiological cell-cell fusion.

Tissue/context	Fusogen	Receptor/binding partner	Key determinants/notes
Placenta (CTBs → STBs)	Syncytin-1	ASCT2	ERV-derived fusogens; fusion is gated by trophoblast differentiation and transcriptional programs (e.g., GCM1)
	Syncytin-2	MFSD2A
Skeletal muscle (myoblast fusion)	Myomaker (TMEM8C/Mymk)	No single defined receptor	Myomaker/Myomixer execute fusion after adhesion and actin remodeling; Myomaker is often required in both partners
	Myomixer (Myomerger, Minion, Mymx)
Mammalian fertilization (sperm-egg)	No conclusively identified single fusogen	Validated adhesion pair: IZUMO1 (sperm) ↔ JUNO (oocyte)	IZUMO1-JUNO mediates essential adhesion; multiple sperm factors and oocyte CD9 enable fusion competence
Osteoclast fusion	No classical fusogen established	DC-STAMP, OC-STAMP	Multinucleation depends on DC-STAMP/OC-STAMP within RANKL-driven differentiation and contact-dependent remodeling

CTBs: Cytotrophoblasts; STBs: Syncytiotrophoblasts; ASCT2: Alanine serine cysteine transporter 2; MFSD2A: Major facilitator superfamily domain-containing protein 2; ERV: Endogenous retrovirus; GCM1: Glial cells missing 1; IZUMO1: Izumo sperm-oocyte fusion 1; RANKL: Receptor activator of nuclear factor kappa B ligand.

STEM CELL-DRIVEN CELL FUSION IN INJURED TISSUES

Among intercellular interactions, cell-cell fusion plays a potential role in tissue regeneration. During injury, damaged parenchymal cells may fuse with stem cells, thereby enhancing cellular function and facilitating damaged tissue repair[5,130,131]. Stem cells have been reported to display relatively high fusion competence compared to that of many other somatic cell types, particularly in injury or inflammatory settings[132-134]. Stem cells can fuse with other stem cells or differentiated cells during embryogenesis, and in adult tissues, they may contribute to tissue growth, regeneration, and homeostasis (Figure 3).

Figure 3 Stem cell fusion-mediated regeneration in diverse tissues. Stem cells and bone marrow-derived cells (BMDCs) can contribute to tissue repair not only through paracrine signaling and differentiation but also by fusing with host somatic cells in injured organs. In this schematic, a central node representing hematopoietic stem cells, mesenchymal stromal/stem cells, and other BMDCs is connected to multiple target tissues. In the liver, BMDC-hepatocyte fusion generates hybrid cells that can proliferate, repopulate the parenchyma, and support functional rescue in transplantation models. In the central nervous system, BMDCs fuse with Purkinje neurons to form binucleated heterokaryons that maintain neuronal firing properties and may contribute to circuit stability in the context of inflammation. In the heart, fusion between stem cells and cardiomyocytes can facilitate mitochondrial transfer, partial nuclear reprogramming, and integration into contractile syncytia, thereby supporting conduction and contractile function. In skeletal muscle, satellite cells and, under some conditions, BMDCs fuse with damaged myofibers to restore multinucleated fibers and maintain muscle architecture. Fusion events have also been reported in other tissues, including epithelium, retina, and pancreas, where their contribution to long-term regeneration remains to be fully defined. Across these contexts, tissue injury, inflammation, chemokine gradients, and extracellular matrix remodeling create fusion-permissive niches that recruit and prime stem cells to engage in fusion-mediated repair. CNS: Central nervous system; HSCs: Hematopoietic stem cells; MSCs: Mesenchymal stromal/stem cells; BMDCs: Bone marrow-derived cells.

Liver

The liver, known for its robust regenerative capacity, can employ fusion as a compensatory mechanism, in which BMDCs fuse with damaged hepatocytes to help restore functional hepatic tissue[135,136]. Beyond classical differentiation, stem cell-hepatocyte fusion generates reprogrammed hybrid cells that contribute to liver regeneration and functional recovery[137]. Notably, stem-hepatocyte hybrids within the liver can proliferate faster than resident hepatocytes[138]. These findings underscore the importance of a fusion-enabled route for liver repair. However, most available data are derived from transplantation and in vivo/in vitro injury models with a relatively small number of fusion events, and the extent to which such hybrids contribute to long-term liver homeostasis in humans remains uncertain.

Nervous system

While hepatic fusion supports regeneration after injury, stem cell fusion may also facilitate the replacement of damaged or nonfunctional neurons through the generation of functional hybrids. The fusion between stem cells and neurons has been demonstrated in human and animal models. Y-chromosome-positive Purkinje neurons have been identified in female patients after male bone marrow transplantation, and in vivo fusion between stem cells and cerebellar neurons has been observed, yielding multinucleated hybrids; these findings provide strong evidence of fusion events in the brain[139,140]. In mice with neurodegeneration, hybrids are electrically active and capable of forming functional synapses, suggesting their therapeutic potential in the CNS[141]. Inflammation enhances fusion efficiency, particularly between damaged Purkinje neurons and stem cells[133]. The resulting binucleated heterokaryons displayed a Purkinje-like morphology and preserved their firing properties[142]. Together, these observations indicate that stem cell-mediated fusion may support not only structural restoration but also the functional re-establishment of neuronal signaling, especially in inflammatory contexts. These studies support the idea that fusion-derived hybrids can be functionally integrated into neuronal circuits, although the frequency of such events and their long-term impact on human CNS diseases remain to be quantified.

Heart

Stem cell-cardiomyocyte fusion has been shown to promote cardiac regeneration through cellular reprogramming[143,144]. Stem cells can fuse with host cardiomyocytes in vivo, forming hybrids that acquire features of mature cardiomyocytes[139]. These fusion-derived hybrids display features of both stem cells and cardiomyocytes, supporting the concept of fusion as a mechanism for cardiac repair[145,146]. Spontaneously fused hybrids often exhibit electrophysiological properties that are intermediate between those of stem cells and cardiomyocytes. Hybrids that more closely resemble cardiomyocytes than conventional stem cells may better support action potential conduction in the damaged myocardium[147]. Furthermore, fusion facilitates the transfer of intact mitochondria from stem cells to injured cardiomyocytes, thereby prolonging cell survival and maintaining nuclear integrity. These hybrids exhibit partial reprogramming toward a cardiac progenitor-like state with enhanced proliferative capacity and expression of early cardiac transcription factors[148]. Collectively, these mechanisms indicate that cell fusion is not merely cellular supplementation but a multifaceted strategy for regeneration. Together, these observations suggest that cell-cell fusion is a plausible contributor to cardiac repair; however, direct evidence for a durable, fusion-mediated functional benefit in clinical settings remains limited.

Other tissues

Stem cell fusion has been observed in muscle cells[149], renal tubular cells[150], monocytes[54], epithelial cells[151,152], retinal neurons[16], and other stem cells[153,154], suggesting its broad applicability across tissues. Rather than solely forming new lineages, fusion-derived hybrids may act as functional intermediates to restore host cell performance in diverse disease contexts.

OBSTACLES AND IMPROVEMENTS IN CELL-CELL FUSION FOR REGENERATION

Cell-cell fusion is an attractive mechanism in stem cell therapy and tissue regeneration; however, several biological, technical, and clinical challenges remain (Figure 4).

Figure 4 Balancing beneficial and pathological cell-cell fusion for safer regenerative therapies. Cell-cell fusion can act as a double-edged sword in vivo, providing opportunities for tissue regeneration while also posing significant safety risks. A: On the beneficial side, controlled fusion between stem cells or bone marrow-derived cells and somatic cells in injured tissues can generate hybrids that participate in structural repair and functional recovery, as shown in the central nervous system, skeletal muscle, heart, liver, and other organs. In these contexts, fusion-derived heterokaryons or synkaryons can undergo nuclear reprogramming and expand stem cell plasticity beyond classical paracrine or differentiation-mediated mechanisms; B: On the pathological side, cancer cell-normal cell fusion can produce hybrid tumor cells with increased genomic diversity, metastatic potential, and therapy resistance; viral fusogens can drive syncytia formation that disrupts tissue barriers and leads to cell death; and dysregulated endogenous fusogens in tissues such as placenta, skeletal muscle, or bone can contribute to disorders including preeclampsia, myopathies, and bone remodeling defects; C: To safely harness fusion in regenerative medicine, future strategies must move beyond globally enhancing or suppressing fusion and instead aim to precisely control fusion partners, timing, and microenvironmental context. Key levers include: (1) Fusogen-level control, using tissue-specific, inducible, or transient modulation of fusogens and their receptors; (2) Context-aware delivery, aligning cell therapy with injury- or inflammation-induced fusion-permissive niches while minimizing exposure to malignant or otherwise vulnerable cells; and (3) Monitoring and safety, incorporating lineage tracing, clonal tracking, and genomic surveillance of fusion-derived clones. Together, these principles highlight how insights into both physiological and pathological fusion can guide the design of fusion-aware cell-based therapies with improved efficacy and safety.

Low frequency and translational gap

Spontaneous fusion of stem and somatic cells is rare in vivo, making detection, quantification, and functional attribution difficult. Most in vitro studies rely on physical or chemical triggers to induce fusion; however, these approaches are not directly applicable in vivo, especially in transplantation settings, because they lack the spatial specificity, temporal control, and safety required for clinical use. Consequently, in vitro fusion-inducing methods have limited translational potential.

Biologically grounded improvements

To promote physiologically relevant fusion while limiting risk, three complementary strategies are emerging: (1) Modulation of fusogen or receptor expression: Restrict and transiently induce fusogenic programs only in target cells[110,155]; (2) Use of fusion-permissive niches linked to tissue damage, or controlled inflammation by timing cell delivery to transient windows when endogenous cues elevate fusion probability[156,157]; and (3) Use of preconditioned donor cells, including hypoxic preconditioning, which improves survival, motility, and membrane dynamics, thereby increasing productive fusion per encounter[158,159]. Collectively, these approaches may enhance spontaneous in vivo fusion and support functional tissue repair.

Unintended fusion with cancer cells

While cell-cell fusion can support regeneration through stem cell plasticity, fusion involving cancer cells represents an obstacle, as it frequently enhances malignancy rather than tissue repair. Several studies have shown that cancer cells can fuse with diverse normal cells, including immune cells[160], myeloid cells[161], myofibroblasts[162], stem cells[163], endothelial cells[164] and epithelial cells[165], thereby generating hybrids. Following fusion, polyploid heterokaryons and subsequent synkaryons undergo extensive nuclear and epigenetic reprogramming, producing hybrids with increased genetic and phenotypic diversity[166-169]. Dendritic cell - tumor hybrids can elicit strong antitumor responses and are being explored in immunotherapy[170]. Nevertheless, most fusion-derived hybrids tend to acquire stem cell-like traits, increased metastatic capacity, enhanced drug resistance, and other malignant hallmarks, ultimately resulting in high-grade hybrids[171-176].

For example, syncytin-1 is expressed in certain breast cancers; its receptor, ASCT2, is present on tumor and endothelial cells, and antisense inhibition of syncytin-1 suppresses fusion between these cell populations[177]. This finding suggests that fusogen-level interventions can selectively block undesired cancer-related fusions. Importantly, fusogen expression appears to be tumor type-specific, and a broadly shared fusogen across cancers has not yet been identified[178,179]. Systematically mapping and targeting cancer-specific fusogenic circuits may deepen mechanistic insights and inform safer fusion-based regenerative strategies. However, selective approaches to prevent and modulate tumor-associated fusion events remain underexplored.

Pathological consequences of dysregulated cell-cell fusion

Although fusion underlies normal development and regeneration, dysregulated fusion can lead to multinucleation, functional impairment, immune activation, and tissue injury.

Viral syncytia: In infections such as RSV, HIV, measles, and SARS-CoV-2, infected cells fuse with neighboring uninfected cells to form syncytia[82]. Fusion is driven by viral proteins displayed on host membranes [RSV fusion (F)/attachment (G) glycoprotein[180], HSV glycoproteins gB/gD[181], HIV envelope glycoproteins gp41/gp120[182], measles F protein/hemagglutinin (H) protein[183], and SARS-CoV-2 spike glycoprotein (S)/angiotensin-converting enzyme 2][88] that engage receptors on adjacent cells. Newly formed syncytia continue to present fusogens, enabling iterative fusion and growth of large multinucleated structures. The consequences include disruption of intercellular junctions, endothelial barrier dysfunction, membrane destabilization, and cell death[184-187].

Placenta: The continuous fusion of CTBs generates multinucleated STBs, which are essential for maternal-fetal exchange[188]. Syncytin-1/2 and their receptors are required for the formation of SBTs; loss of receptor expression impairs CTB fusion and STB formation, leading to labyrinth thinning, spongiotrophoblast thickening, reduced vascular density, and complications such as preeclampsia and fetal growth restriction[189-193]. Conversely, syncytin overexpression may provoke excessive fusion and the aberrant retention of cell cycle proteins within STBs, suggesting structural and functional instabilities[194,195].

Skeletal muscle: Myoblast-myoblast fusion depends on Myomaker and Myomixer; loss-of-function prevents fusion and causes defective myogenesis[106,109]. However, excessive fusion produces large but disorganized multinucleated fibers. The increase in myonuclei does not linearly improve cell function; per-nucleus transcriptional output declines, and hypertrophic fibers fail to generate superior contractility compared to normal myofibers[196-198]. However, the complete functional effect of hyperfusion remains to be determined.

Bone: Osteoclasts are formed by the fusion of mononuclear precursors of the monocyte/macrophage lineage. Dysregulated fusion reduces multinucleation and impairs bone resorption capacity. In animal models, loss-of-function of DC-STAMP or OC-STAMP prevents osteoclast fusion, diminishes resorption, and contributes to osteopetrosis and abnormal bone remodeling[199-202].

Across organs, pathology converges on aberrant fusogenic machinery, such as downregulation, overexpression, receptor loss, or mutation, underscoring the need for target-restricted, temporally gated fusogen control, delivery timed around injury/inflammation, and preconditioning for future clinical translation.

CONCLUSION

Cell-cell fusion is a fundamental mechanism that contributes to tissue development, homeostasis, and regeneration; however, the same processes can drive pathology when dysregulated. As outlined in this review, physiological fusion in the placenta, skeletal muscle, bone, and immune system is mediated by tightly regulated tissue-specific fusogens and microenvironmental cues. Malignant cell fusion, viral syncytia, and defective placental or muscle fusion illustrate how similar programs can be co-opted in disease. Understanding these parallel physiological and pathological trajectories is essential for attempting to manipulate fusion in a therapeutic context.

Stem cell-somatic cell fusion can act as a potent driver of cellular plasticity, enabling rapid functional rescue through cytoplasmic sharing and, in some cases, longer-term reprogramming via genomic integration. At the same time, the properties that enable beneficial reprogramming and clonal expansion also confer risks of genomic instability, aberrant differentiation, and inadvertent fusion with premalignant or malignant cells. These safety risks also carry ethical considerations, underscoring the need for strict clinical oversight and genomic surveillance in any translational application of fusion-based approaches. Therefore, future efforts should focus less on globally enhancing or suppressing fusion and depend on strategies that selectively harness fusion-induced plasticity while more on precisely controlling fusion partners, timing, and tissue context. Priorities include systematic mapping of fusogens, receptors, and upstream signaling circuits in defined niches; development of in vivo models and quantitative lineage-tracing tools to resolve rare but functionally relevant fusion events; and rational integration of fusion biology into the design of cell-based therapies through donor cell engineering, context-aware delivery, and longitudinal monitoring of fusion-derived clones.

By providing a unified view of the physiological and pathological roles of cell-cell fusion and highlighting stem cell fusion as both a therapeutic opportunity and a safety concern, these findings of this review could lay a conceptual groundwork for strategies that exploit fusion-mediated stem cell plasticity while improving the safety and efficacy of future regenerative therapies.

ACKNOWLEDGEMENTS

We would like to thank all the professionals who contributed to the discussion and elaboration of this review.