Introduction
Wound healing is a complex process. There are many biological pathways and factors interfering with stages of the process, including oxygenation, infection and microbial agents, age and sex, hormones, stress, diabetes, obesity, medications, alcohol consumption, smoking, and diet [1]. This complex process is composed of four profoundly coordinated and overlapping phases: hemostasis, inflammation, proliferation, and tissue remodeling or resolution.
Hemostasis: the initial phase of wound healing in which blood vessels constrict and platelets form a clot to stop bleeding, providing a scaffold for subsequent tissue repair.
Inflammation: which begins within 6–8 hours after injury, due to platelet migration and release of chemo-attractive agents, macrophages organize the next stage by phagocytosis and debridement of the damaged tissue.
Proliferation: 5–7 days following injury, cytokines including platelet-derived growth factor (PDGF), transforming growth factor (TGF)-α/β, and fibroblast growth factor (FGF) are secreted from macrophages. In this phase, angiogenesis leads to leukocyte migration and supplies the granulation tissue with oxygen and nutrients.
Tissue remodeling: over several months, wound contraction and extracellular matrix reorganization occur, transforming and rearranging the healed scar into mature scar tissue (Fig. 1) [2].
Complicated interactions cooperate to accomplish the process of wound healing. Among the players in these interactions are the extracellular matrix, resident cells, solvent mediators and infiltrating leukocyte subtypes [3].
Stem cells are multi-potential undifferentiated cells that have not yet been specialized. These versatile cells are responsible for generating all mature cells in the human body, making them the foundation of various cells, tissues, and organs of the body [4]. Stem cells are divided into several categories: (1) totipotent, such as cells of the zygote which can develop to any cell type and to extraembryonic structures; (2) pluripotent, such as embryonic cells which can develop to any cell in the germ layer; (3) multipotent, such as hematopoietic stem cells which can differentiate into several cell types within specific lineages; (4) oligopotent which can develop to limited cell types; and (5) unipotent, which develop to a specific cell type [5].
In 1998, James Thomson of the University of Wisconsin-Madison created the first human embryonic stem cell cultures. Since then, the therapeutic and regenerative potential of stem cells has been notable in the treatment of degenerative defects [6]. A significant branch of regenerative medicine is stem cell-based therapy that aims to expand the body’s self-healing system via arousing and aligning endogenous stem cells and restoring tissue equilibrium [7].
Immune system hyper-responses, such as cytokine storms, are more limited in mesenchymal stem cell (MSC) therapy where the body’s natural healing capacity is promoted due to the repairing properties of stem cells [8]. MSCs are a trending item of immunoregulation and tissue repair, with 10 MSC products currently sanctioned worldwide for myocardial infarction, Crohn’s disease, graft-versus-host disease (GVHD), and osteoarthrosis [9].
MSCs are mainly harvested from bone marrow, adipose tissue, and umbilical cords [10]. Human umbilical cord MSCs (hUCMSCs) originate from Wharton’s jelly of the umbilical cord, and are noted for immunoregulatory, self-renewal, and multipotency characteristics comparable to other MSCs [11]. In defense of this supposition, Gao et al. [12] analyzed the characteristics of MSCs from different tissue sources through single-cell transcriptomic and proteomic sequencing. They suggested that perinatal MSCs have stronger immunosuppressant potency than bone marrow-derived MSCs (BMMSCs) and adipose-derived MSCs (ADMSCs).
In a meta-analysis conducted by Ding et al. [13], it was found that among MSC from various tissue sources, BMMSCs likely exhibit more beneficial therapeutic effects in knee osteoarthritis improvement compared to other sources, particularly outperforming ADMSCs. However, extracting BMMSCs requires invasive methods. Additionally, their presence in bone marrow is quite scarce, making up only 0.001% to 0.01% of the total cellular composition. ADMSCs are more easily accessible and offer greater number of cells compared to bone marrow [14].
Perinatal MSCs can be obtained from five distinct regions within the placentome: the cord lining, Wharton’s jelly, the cord-placenta junction, the chorion, and the maternal placenta. The utilization of MSCs derived from perinatal tissues offers several advantages over those obtained from adult tissues. They exhibit greater proliferative capacity than adult MSCs while presenting a lower risk of immunogenicity and tumorigenesis compared to fetal and embryonic stem cells. Moreover, their isolation does not necessitate invasive procedures, unlike adult MSCs [15].
Characteristics of UCMSCs
The human umbilical cord is, in general, 65 cm in length and 1.5 cm in diameter. It attaches the fetus to the mother, ensuring the delivery of vital nutrients. One or more layers of squamous cuboidal epithelial cells, derived from the amnion, coat the cord. Two arteries and one vein located within the cord provide the blood supply for the fetus and placenta. A mucoid connective tissue known as Wharton’s jelly covers this part [16].
Wharton’s jelly is composed of hyaluronic acid, chondroitin sulfate, and most significantly, primitive MSCs. The main role of Wharton’s jelly is to endure “torsional and compressive” stresses on the umbilical vessels during the fetal growth process [17]. Unlike when obtaining MSCs from bone marrow, the conventional view of umbilical cord tissue as a waste product after birth eliminates ethical concerns when extracting MSCs from it [18].
Human umbilical cord blood mononuclear cells are the largest cells in the human body, including a diverse range of stem cells, lymphocytes, and monocytes [19]. A quiescent MSC is defined as a major histocompatibility complex (MHC) class I-expressing cell lacking expression of the MHC class II, or co-stimulatory molecule. Interferon gamma (IFN-γ) stimulates MSCs and causes vigorous upregulation of markers such as MHC class I and II molecules, immune modulatory molecules (CD200, CD274/PD-L1/B7-H1), cytokine/chemokine receptors (CXCR3, CXCR4, CXCR5, CCR7, CD119/IFN-γ receptor), adhesion molecules (CD54, CD106), DNAX accessory molecule-1 (DNAM) ligands (CD112, CD155), natural killer group 2 member D (NKG2D) ligands (macrophage inflammatory complex A/B, UL binding protein 1, 2, 3), and Notch receptors (Jagged-1) [7,20].
Because UCMSCs come from newborn infants, there are no significant differences between donors, thereby eliminating age-associated variations and confirming their suitability for cell-based therapies [21]. MSCs from the umbilical cord have a higher rate of proliferation and can be collected without harming adult tissues. This reinforces the notion that regenerative medicine could potentially utilize these cells [22]. Since umbilical cords are commonly discarded after labor, the process of collecting umbilical cord MSCs is both painless and noninvasive, with few ethical restrictions on its use [23]. Other tissue sources harbor challenges such as invasive harvesting techniques, low cell yield, declining quality over time, and the risk of abnormal matrix formation. For example, ossification has been observed in BMMSCs [24].
Pivotal wound healing-mediated growth factors were found in exosomes derived from BMMSCs, ADMSCs, and UCMSCs. Some of these factors, such as FGF-2, vascular endothelial growth factor-A (VEGF-A), hepatocyte growth factor (HGF), and PDGF-BB are expressed in all three MSC origins, whereas UCMSCs were the only source that expressed TGF-β [25].
Among MSC’s gene expression profile, UCMSCs have more qualities in common with embryonic stem cells (ESCs) and faster self-renewal than BMMSCs and ESCs. One of the few drawbacks of this source is that the donor baby must undergo genomic and chromosomal examinations to confirm the donor’s health [26].
Mechanisms of action in wound healing
MSCs contribute to wound healing through diverse immune mechanisms and their immunomodulatory properties. For instance, hUCMSCs suppress the proliferation of some immune cells such as B and T lymphocytes, thereby dampening excessive immune activation. Additionally, they modulate macrophage polarization by shifting pro-inflammatory (M1) phenotypes toward anti-inflammatory (M2) phenotypes, which play a critical role in tissue repair. This phenotypic switch is further supported by the increased secretion of anti-inflammatory cytokines, including interleukin (IL)-10 and IL-4, which collectively alleviate inflammation and promote a regenerative microenvironment [27]. MSC spheroids secrete various immunomodulatory factors such as tumor necrosis factor-inducible gene 6 protein (TSG-6), prostaglandin E2 (PGE2), leukemia inhibitory factor, indoleamine-pyrrole 2,3-dioxygenase (IDO), and IL-6, which contribute to their therapeutic effects [28,29].
Thus, the immune response of MSCs collaborates in tissue repair. “Licensing” of MSCs is the process where inflammatory cytokines like IFN-γ and tumor necrosis factor alpha (TNF-α) activate their immunomodulatory functions, enhancing secretion of anti-inflammatory factors (IDO, PGE2) to suppress immune cells (T-cells) and promote macrophage polarization from M1 to M2 phenotypes, optimizing their therapeutic effects in tissue repair and inflammation control. Through the Licensing process, MSCs are drawn to injury sites and activated by the inflammatory environment. Following the interaction of MSCs with the immune system, these cells release various cytokines and growth factors, contributing to wound healing and tissue regeneration [30].
Type 2 macrophages play a crucial role in wound healing. M2 macrophages are subclustered into steady-state macrophages, M2a (high expression of fibronectin 1), and M2c (high expression of IL-10) macrophages. M2a macrophages are dominant in hUCMSC. M2a macrophages concentrate on collagen synthesis, migration, phagocytosis, autophagy, and hypoxia-inducible factor 1 (HIF-1) signaling [31]. Xu et al. [32], in their study, demonstrated that HIF-1α overexpression suppresses reactive oxygen species, protects DNA, meliorates growth and survival of ADMSC, and boosts ADMSC paracrine function, leading to modified therapeutic effects in diabetic wound healing.
hUCMSC transplantation modulates TGF-β mRNA expression in the TGF-β/Smad signaling pathway. TGF-β binds to serine/threonine kinase cell-surface receptors and directly phosphorylates the carboxyl-terminal of Smad2 and Smad3, which causes activation of the TGF-β/Smad signaling pathway. This pathway induces overgrowth of fibroblasts and overaccumulation of extracellular matrix, resulting in fibrosis and scar formation [33].
Preclinical studies on UCMSCs in wound healing
In rat models with severe burns treated with UCMSCs, the migration of UCMSCs into the wound reduces immune cell recruitment and lowers pro-inflammatory cytokines (IL-1, IL-6, TNF-α), while increasing anti-inflammatory factors (IL-10, TSG-6), enhancing burn wound healing [34].
Yang et al. [35] administered hUCMSC exosomes encapsulated in a thermosensitive PF-127 hydrogel to a full-thickness skin wound in a diabetic rat model. They discovered that hUCMSC exosomes speed up the rate at which wounds heal, raise the levels of substances like CD31, Ki-67, VEGF, and TGF-β1, and help granulation tissue grow again. Wang et al. [36] induced apoptosis in UCMSCs cultured in vitro, extracted apoptotic extracellular vesicles (ApoEVs), and used them in a type 2 diabetes mellitus (T2DM) mouse skin wound model. They found that UCMSC-derived ApoEVs decrease oxidative stress by suppressing macrophage pyroptosis in T2DM mice under both in vivo and in vitro high-glucose conditions. Overall, these effects facilitated wound healing in the T2DM model mice.
Liu et al. [37] concluded that hUCMSCs enhance corneal epithelial cell proliferation and migration in vitro as well as wound healing in vivo. hUCMSC exosomes reverse the role of phosphatase and tensin homolog (PTEN) in suppressing the phosphoinositide 3-kinase/protein kinase B (PI3K/Akt) signaling pathway in human corneal epithelial cells through the induction of miR-21.
In a study by Hendrawan et al. [38], diabetic rats with excision wounds were split into three groups (in vivo part): those that did not receive treatment, those that received antibiotics, and those that received hypoxic hUCMSC conditioned media (CM). The aim of the study was to assess fibroblast cell proliferation and collagen deposition in vitro, as well as wound area reduction, re-epithelialization capacity, and collagen formation in vivo. The study found that hypoxic hUCMSC CM contained higher levels of growth factors linked to wound healing and led to greater re-epithelialization and collagen production compared to both the antibiotic-treated and untreated groups (Table 1).
Lu et al. [39] demonstrated that exosomes derived from hUCMSCs (hUCMSCs-Exo) significantly accelerated healing in a rat model of complex perianal fistulas. The study revealed that hUCMSCs-Exo at a medium dose (10 μg/100 μL) optimally promoted wound closure by enhancing collagen synthesis, angiogenesis, and macrophage polarization from M1 to M2 phenotypes. Notably, treated rats showed reduced fistula diameter on ultrasound and improved histopathological scores.
Liu et al. [40] investigated the effects of hUCMSCs on wound healing in a rat model of severe burns. The results showed that intravenous injection of hUCMSCs significantly accelerated wound closure, reduced inflammation (lower IL-1, IL-6, TNF-α, higher IL-10 and TSG-6), and enhanced neovascularization (increased VEGF and blood flow). Additionally, hUCMSCs improved collagen deposition (higher collagen I/III ratio). The findings suggest that hUCMSCs therapy promotes burn wound healing by modulating inflammation and tissue regeneration, supporting its potential clinical application.
A study by Wu et al. [42] explored whether genetically modified hUCMSCs overexpressing anti-inflammatory cytokines (IL-4, IL-10, IL-13, termed MSCs-3IL) could enhance diabetic wound healing by modulating macrophage activity. hUCMSCs were modified using lentiviral vectors, showing significantly increased cytokine expression (IL-4 and IL-10 mRNA by 15,000- and 800,000-fold; protein secretion at 400 and 200 ng/mL). The modified hUCMSCs promoted macrophage polarization and improved wound healing in diabetic models, assessed through healing rate, histology, and immunohistochemistry.
Xu et al. [43] developed a hybrid hydrogel composed of gelatin methacrylate and chitosan-catechol to encapsulate hUCMSCs for diabetic wound healing. In diabetic mice, the hydrogel-hUCMSCs combination significantly accelerated wound closure by reducing inflammation (suppressing TNF-α and IL-1β), promoting angiogenesis (increased CD31 and VEGF expression), and improving collagen deposition. This approach demonstrates a promising strategy for treating diabetic wounds by integrating stem cell therapy with advanced biomaterials.
Zi et al. [44] demonstrated that hUCMSC-Exos significantly enhance skin wound healing by promoting elastin production via the TGF-β1-Smad signaling pathway. In vitro experiments revealed that hUCMSC-Exos upregulated elastin, collagen I, and fibronectin expression in human foreskin fibroblasts by activating SP1 binding to the elastin promoter, as confirmed by chromatin immunoprecipitation assays. In vivo, hUCMSC-Exos combined with hydrogel accelerated wound closure in mice, improved collagen organization, increased elastic fiber formation, and enhanced angiogenesis. These findings highlight the therapeutic potential of hUCMSC-Exos for clinical wound healing applications, particularly through extracellular matrix remodeling and targeted molecular mechanisms.
Liu et al. [31] investigated the role of hUCMSC-Exos in promoting murine skin wound healing through modulation of neutrophils and macrophages, as revealed by single-cell RNA sequencing. Their study demonstrated that hUCMSC-Exos significantly accelerated wound closure by enhancing re-epithelialization and collagen deposition. hUCMSC-Exos increased neutrophil recruitment and reduced inflammation while promoting anti-inflammatory M2 macrophage polarization, particularly the M2a subtype, which supports tissue repair. These findings provide insights into the cellular and molecular mechanisms underlying hUCMSC-Exos-mediated wound healing, offering potential therapeutic targets for enhancing tissue regeneration.
Clinical applications and current research
In a randomized clinical trial conducted by Hashemi et al. [47], chronic diabetic wounds were treated with an acellular amniotic membrane seeded with Wharton’s jelly MSCs. This significantly reduced the duration of wound healing and lesion size (P<0.002). Kim et al. [48] performed a double-blinded, randomized, prospective, split-face comparison study on twenty-five patients with Fitzpatrick skin types III or IV. Twenty-three of the patients who received ablative fractional laser treatments on both cheeks used a cream hUCMSC CM, with or without an additional stem cell-containing serum. Administration of both serum and cream led to a reduced microcrust area and a decreased global improvement score of post-treatment erythema, with no evidence of adverse events linked to the use of either hUCMSC-containing serum or cream, presenting more evidence of the wound healing acceleration properties of hUCMSC.
Moreira et al. [49] conducted a randomized controlled trial to assess the feasibility and effectiveness of applying hUCMSCs through the intranasal route. hUCMSC was administered intranasally to a hyperoxia-induced rat model of bronchopulmonary dysplasia. The study demonstrated restoration of lung alveolarization, vascularization, and pulmonary vascular remodeling in these animals. The authors concluded that intranasal delivery of hUCMSCs is a noninvasive way of administering cell-based therapies that can be applied as an adjunct or alternative to other routes.
Mohseni et al. [50] designed a randomized clinical trial to evaluate the safety and efficacy of platelet gel-containing dressings for pediatric patients with ulcerative sclerotic skin chronic GVHD (cGVHD) allogeneic hematopoietic stem cell transplantation. The study concluded that umbilical cord blood-derived platelet gel is a safe and effective treatment for skin ulcers associated with sclerotic skin cGVHD in pediatric patients (Table 2).
Future perspectives and potential developments
Exosome-based therapy
Emerging technologies are constantly being developed to improve the efficacy of hUCMSCs. Extracellular vesicles (EVs) can significantly alter the microenvironment of tissues containing UCMSCs. These EVs act as carriers of bioactive molecules that support tissue regeneration and modulate various cellular activities. EVs provide a cell-free alternative that minimizes the potential risks of direct cell transplantation without disrupting the beneficial properties of stem cells [51]. These vesicles facilitate cell-to-cell communication, prevent apoptosis, stimulate angiogenesis and cell proliferation, and promote the differentiation of cells required for tissue matrix production [52].
Exosomes are a specific subtype of EVs characterized by their small size (30–150 nm) and their forming process inside multivesicular bodies [53]. These are emerging as a promising treatment option due to their low immunogenicity, strong stability, and precise targeting ability. Their unique properties, shaped by their source, contribute to their diversity, allowing them to serve specialized functions in medical applications [54].
Research has shown that injecting exosomes in and around wound sites in rats significantly enhances skin cell proliferation, migration, angiogenesis, and promotes faster wound closure [34,55]. Studies have demonstrated that hUCMSC-Exos can deliver Wnt4 to promote wound healing and protect skin cells from heat-induced apoptosis by activating the Akt pathway [34]. Zhang et al. [56] demonstrated that the 14-3-3ζ protein carried by hUCMSC-Exos facilitates the recruitment of phosphorylated large tumor suppressor, forming a complex that promotes Ser127 phosphorylation of yes-associated protein (YAP). This mechanism plays a key role in regulating skin cell proliferation by precisely modulating Wnt4 activity in a self-regulating manner. In essence, hUCMSC-Exos function as a regulatory “brake,” modulating YAP signaling to ensure balanced and controlled dermal regeneration.
HUCMSCs-Exos decreases scar formation and myofibroblast buildup in mouse models of skin wounds. These exosomes contain key microRNAs, including miR-21, miR-23a, miR-125b, and miR-145, which play a crucial role in suppressing myofibroblast accumulation by targeting the TGF-β2/Smad2 signaling pathway [55].
In vivo studies have demonstrated that hUCMSC-Exos promote angiogenesis under blue light irradiation by upregulating specific miRNAs, including miR-135b-5p and miR-499a-3p in endothelial cells. Furthermore, these exosomes play a key role in modulating inflammatory responses by reducing levels of TNF-α and IL-1β while increasing IL-10 expression [41]. Hypoxically preconditioned hUCMSC-Exos, combined with an efficient hydrogel delivery platform, enhance diabetic wound healing via the miR-486-5p/SERPINE1 axis [57].
However, despite the strong potential of hUCMSCs-Exos as cell-free therapies for wound healing, further research remains necessary to standardize isolation methods, enhance retention time, and assess clinical efficacy [58].
Genetic modifications
Gene editing and cell engineering technologies enable the modification of stem cell characteristics and survival. Genetic interventions targeting key players within the immune system to reduce organ and tissue transplant rejection can be a promising method of using MSCs. Wound healing mechanisms like angiogenesis are mediated through diverse genetic interventions, including gene augmentation, silencing, and precise editing using CRISPR/Cas9 (guided by gRNA for targeted DNA modifications), zinc finger nucleases, transcription activator-like effector nucleases (TALEN) and base editors [59]. Srifa et al. [60] demonstrated that CRISPR-Cas9/AAV6-mediated genome editing can effectively engineer human MSCs (hMSCs) to enhance wound healing in diabetic mice. By integrating PDGF-BB or VEGF-A expression cassettes into the hemoglobin subunit beta safe harbor locus, they generated hMSC lines capable of sustained growth factor hypersecretion. These engineered cells, when transplanted into full-thickness wounds of db/db mice, significantly accelerated wound closure compared to wild-type hMSCs, with PDGF-BB-secreting cells promoting granulation tissue formation and VEGF-A-secreting cells enhancing angiogenesis. This work establishes a versatile platform for precision-engineered MSCs therapies targeting tissue repair.
Han et al. [61] demonstrated that microencapsulated VEGF gene-modified hUCMSCs significantly enhance vascularization and wound healing in tissue-engineered dermis (TED). By combining a collagen-chitosan laser-drilled acellular dermal matrix scaffold with hUCMSC-derived fibroblasts, they successfully constructed a functional TED. Their transplantation experiments in pig skin defects revealed elevated VEGF expression, increased microvessel density, and accelerated re-epithelialization compared to control groups. These findings suggest this combined approach could improve TED survival and healing outcomes, offering promising clinical applications for burn wounds and chronic ulcers.
Chang et al. [62] used TALEN genome editing to engineer hUCB(blood)MSCs that could controllably secrete HGF when treated with doxycycline. The modified cells showed improved migration, stress resistance, and promoted blood vessel formation. When tested in mice with limb ischemia and delivered using special microgels, these engineered cells effectively restored blood flow. This approach combines precise genetic modification with biomaterial delivery to overcome the short lifespan of therapeutic proteins and enhance stem cell therapies for vascular diseases.
In general, combining advanced gene-editing tools with stem cell engineering offers great potential for regenerative medicine. Improving techniques like CRISPR and delivery methods could make modified MSCs more effective in treating chronic wounds, vascular conditions, and tissue repair. Future studies should aim to make these therapies safer, more scalable, and longer-lasting, while also exploring combined strategies to activate multiple healing pathways. These developments could lead to new, practical treatments that overcome current challenges in healing and transplantation.
Challenges and limitations
Like other therapeutic approaches, the use of MSCs also presents certain limitations that must be addressed. Only about 20% of MSCs are isolated from umbilical cord blood, likely due to the limited number of these cells entering the bloodstream [14].
He et al. [63] found that phosphatidylserine released from dead MSCs exerts an immunomodulatory effect similar to that of live MSCs. Since cell-based products inevitably contain dead cells, their potential impact on patient health remains a concern.
The nature of MSCs as a “product-by-process” means that their final characteristics are determined by the specific manufacturing techniques used for their generation or enrichment. As a result, variations in processing methods lead to differences in the final product, complicating identification based on a single marker. This approach primarily emphasizes in vitro procedures rather than the functionality of the cells within a living system [64].
When introduced into a complex living system without controlled conditions, MSCs exhibit low survival rates, posing a challenge for their therapeutic effectiveness. Additionally, cellular senescence alters their properties, further compromising their potential as a treatment [65].
Conclusion
hUCMSCs represent a promising advancement in wound healing therapy, offering significant advantages over traditional treatments and other cell-based approaches. Their unique combination of accessibility, ethical acceptability, and robust therapeutic properties makes them particularly valuable for clinical applications.
The mechanisms through which UCMSCs facilitate wound healing are comprehensive, including immunomodulation, angiogenesis promotion, and tissue regeneration, contributing to more effective wound repair. Both preclinical and clinical studies have demonstrated encouraging results, particularly in treating challenging conditions such as diabetic wounds and burn injuries. The development of advanced delivery systems, including hydrogel-based applications and exosome therapy, has further enhanced their therapeutic potential. Additionally, the documented safety profile of UCMSC-based treatments, with minimal reported adverse effects, supports their clinical implementation.
While challenges remain in optimizing preparation methods and standardizing treatment protocols, ongoing technological advances continue to expand the potential applications of these cells. The integration of emerging technologies, such as gene editing and tissue engineering, may further enhance their therapeutic efficacy. As research progresses, UCMSCs are poised to play an increasingly important role in regenerative medicine, offering new hope for patients with chronic wounds and other tissue repair needs.
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