Stem cell-based therapeutic strategies for rotator cuff tendinopathy

Abstract

Rotator cuff tendinopathy is a common musculoskeletal disorder that imposes significant health and economic burden. Stem cell therapy has brought hope for tendon healing in patients with final stage rotator cuff tendinopathy. Some clinical trials have confirmed the effectiveness of stem cell therapy for rotator cuff tendinopathy, but its application has not been promoted and approved. There are still many issues that should be solved prior to using stem cell therapy in clinical applications. The optimal source and dose of stem cells for rotator cuff tendinopathy should be determined. We also proposed novel prospective approaches that can overcome cell population heterogeneity and standardize patient types for stem cell applications.

The translational potential of this article

This review explores the optimal sources of stem cells for rotator cuff tendinopathy and the principles for selecting stem cell dosages. Key strategies are provided for stem cell population standardization and recipient selection.

Graphical abstract

1. Introduction

Rotator cuff comprises the supraspinatus, subscapularis, infraspinatus and teres minor tendons, which surrounds the humeral head and contributes to movement, stability and sensory motor control of the glenohumeral joint [1]. Tendinopathy describes a complex multifaceted pathology of the tendon, characterized by pain, decline in function and reduced exercise tolerance [2]. Rotator cuff tendinopathy is common in the general population and manifests in terminal stages as tendon tears, leading to daily pain and disability [3]. Over 200,000 repair procedures are performed annually for rotator cuff tear in the United States, resulting in approximately 474 million dollars in health care costs [4]. In addition to the heavy social costs, rotator cuff tendinopathy also leads to a decline in the quality of life, as evidenced by the significant increase in depression, anxiety, and insomnia in patients after rotator cuff tendinopathy [4].

Surgical treatment can significantly improve the postoperative functional scores and pain severity in patients with rotator cuff tendinopathy [5]. However, there is a high risk of non-healing and re-tearing after surgery. The re-tear rate at 2 years after single-row rotator cuff repair was 23.5%, while the re-tear rate at 2 years after double-row rotator cuff repair could reach 38.1% [6,7]. In summary, patients with rotator cuff tendinopathy have a large proportion of secondary tears after surgical treatment, and it is urgent to strengthen healing with appropriate regeneration adjuvant therapy.

Stem cells are cells that can proliferate, self-renew, and differentiate into numerous other functional cells under specific conditions [8]. 3 main modalities of action mediate the beneficial effects of stem cells once transplanted into recipients: (1) living cell expansion and differentiation into tissues of mesodermal origin; (2) close interactions with neighboring cells through cell-to-cell contact and release of paracrine factors and extracellular vesicles (EVs); (3) apoptotic phenomena involving both MSCs and immune cells [9]. In tendinopathy, stem cells have been shown to promote tendon healing by paracrine secretion of EVs regulating various biological processes [[10], [11], [12], [13], [14], [15], [16]](Fig. 1). Stem cell therapy is a therapeutic method using stem cells and mixed biological agents containing stem cells [17]. Stem cell therapy based on bone marrow-derived stem cells (BMSCs) and adipose-derived stem cells (ADSCs) has been applied in clinical treatment of rotator cuff tendinopathy and has demonstrated feasibility. A case report provided immunohistochemical evidence of rotator cuff tendon repair following stem cell injection [18]. In another trial, a 10-year follow-up showed that the rotator cuff tear rate in the stem cell injection group was significantly lower than that in the control group [19]. However, in some clinical trials, stem cell therapy has not achieved the expected effect. A randomized controlled clinical trial showed that stem cell therapy was not more effective than control [20]. Another cohort study showed that stem cell therapy could not provide long-term clinical benefits for patients undergoing shoulder arthroscopy [21].

Fig. 1.

Summary of stem cells' paracrine effect on tendinopathy.

1. Stem cells regulate the polarization balance of M1 and M2 macrophages through the TGF-β1-Smad2/3 signaling pathway; 2. Activation of M1 macrophages can resist apoptosis; 3. Stem cells inhibit the formation of fibrosis; 4. Stem cells regulate tenocytes and TSPCs; 5. Stem cells regulate matrix metalloproteinases to remodel the ECM; 6. Stem cells promote healing of rotator cuff tendinopathy by improving blood supply. Abbreviation: TSPCs: tendon stem/progenitor cells; ECM: extracellular matrix.

The heterogeneity of stem cell therapy leads to their efficacy instability. The heterogeneity of stem cell therapy is determined by various factors, including but not limited to donor and tissue sources, cell population, culture conditions, cell isolation techniques, cryopreservation, thawing protocols, and administration regimens [[22], [23], [24]]. In this review, we discuss the key issues that must be addressed for the clinical translation of stem cell therapy for rotator cuff tendinopathy: (1) What tissue-derived stem cells are suitable for rotator cuff tendinopathy? (2) How to select the optimal stem cell dose for patients? To address these concerns, we conducted a thorough review of clinical trials and preclinical studies. We hope to identify the optimal stem cell tissue source and dose selection strategy to help patients with rotator cuff tendinopathy receive more efficient and stable stem cell therapy.

2. Suitable stem cell source for rotator cuff tendinopathy

Stem cells are typically obtained from four sources: embryonic tissue, fetal tissues, adult tissue and differentiated somatic cells that have been genetically reprogrammed [25]. For the purposes of stem cell therapy, adult mesenchymal stem cells (MSCs) are regarded as the most advantageous type of stem cells. MSCs are located in practically all organs and tissues of the adult organism, e.g. skin, brain, heart, blood vessels, skeletal muscle, intestine, liver, kidneys, reproductive organs, adipose tissue and bone marrow, as well as body fluids such as blood and urine [26]. The use of MSCs avoids the ethical and legal issues associated with the use of stem cells derived from human embryos and fetuses [27]. MSCs can overcome the mutational and other adverse effects associated with genetically reprogrammed differentiated somatic cells [28]. In addition, MSCs can be isolated and applied in autologous form, avoiding potential immune reactions caused by allogeneic implantation [29]. An exceptional safety profile has been shown in a large number of cell therapy clinical trials that use MSCs [30].

The difference between the sources of MSCs cannot be ignored. Camila et al. found that the difference between MSCs tissue sources is far greater than that between donor sources at single cell resolution [31]. Multiple studies have shown that specific MSCs sources can better achieve specific functions. ADSCs displayed a stronger inhibitory effect on T cell growth than BMSCs, both under resting conditions and after being stimulated with IFN-γ and TNF-α. This enhanced inhibition was attributed to the higher activity of IFN-γ-dependent IDO [32]. In another comparative study, the in vitro suppressive effect of resting BMSCs and ADSCs did not differ, but it was found to be more pronounced than the suppressive effect of stem cells derived from umbilical cord. However, umbilical cord and ADSCs induced a higher regulatory T-cell/T helper 17 ratio [33]. Stem cells derived from bone marrow and umbilical cord enriched with different functional pathways in gene set enrichment analysis [31]. Another study found that ADSCs promoted hematopoiesis in vitro and in vivo better than BMSCs [34]. In addition, there is already clinical evidence that stem cells from specific sources can better treat specific diseases. Zhou et al. demonstrated that ADSCs are a better cell source for the treatment of osteoarthritis than BMSCs through meta-analysis of clinical trials [35].

2.1. Clinical clues: the efficacy of BMSCs and ADSCs is uncertain

We collected 11 articles published from 2012 to 2023 concerning clinical trials of stem cell-based therapy for rotator cuff tendinopathy. Of these 11 clinical trials, 3 used ADSCs, 3 used biological agents containing ADSCs and 5 used biological agents containing BMSCs (Table 1). Only one trial used allogeneic transplantation, while the other trials used autologous transplantation [20]. To explore whether the clinical effects of stem cells derived from heterogeneous tissues are different, we screened and summarized seven clinical trials with evidence levels 2 to 3 (Fig. 2).

Table 1.

Clinical effects of stem cell therapy on rotator cuff tendinopathy.

Reference	Level of evidence	Cell dose (x106 ​cells)	Treatment group intervention	Comparison	Number treated	Outcome measures	Time points	Outcomes
Autologous ADSCs
Kim et al. (2017) [21]	III cohort study	4.46	Arthroscopic rotator cuff repair with an injection of ADSCs loaded in fibrin glue	Arthroscopic rotator cuff repair	10 (MSC);	MRI, VAS, UCLA, Constant, ROM	1, 3, 6, and 12 months and then yearly thereafter (VAS, ROM); 6 and 12 months and then yearly thereafter (UCLA, Constant); a minimum of 1 year after surgery (MRI)	Significant structural enhancement
					10 (control)
Jo et al. (2018) [36]	IV case series	10, 50, 100	ADSCs injection	Baseline	3 (1.0 ​× ​107 dose);	VAS, ROM, electronic scale, SPADI, ASES, UCLA, SST, DASH, SANE, MRI	6,12 and 24 months (MRI); 1, 3, 6,12 and 24 months (other measures)	Significant structural and functional enhancement
					3 (5.0 ​× ​107 dose);
					12 (1.0 ​× ​108 dose)
Allogeneic ADSCs
Chun et al. (2022) [20]	Ⅱ randomized controlled trial	10	Injection of ADSCs loaded in fibrin glue	Saline control, saline/fibrin glue mixture control	8 (MSC);	MRI, VAS, ASES	12 weeks, 12 months (MRI); 6 weeks and 3, 6, 12, and 24 months (VAS, ASES)	No significant effects
					8 (saline control);
					8 (saline/fibrin glue mixture control)
Biological agents containing ADSCs
Hurd et al. (2020) [37]	Ⅱ randomized controlled trial	11.4(UA-ADRCs)	UA-ADRCs injection	Glucocorticoid control	8 (UA-ADRCs);	ASES, RAND Short Form-36 Health Survey, VAS, MRI	3, 6, 9, 12, 24, 32, 40 and 52 weeks (ASES, RAND Short Form-36 Health Survey, VAS); 24 and 52 weeks (MRI)	Significant functional enhancement
					8 (glucocorticoid control)
Alt et al. (2021) [18]	V case report	75(UA-ADRCs)	UA-ADRCs injection	Baseline	1	Immunohistochemical and microscopic analysis	2.5 months	Formation of new tendon tissue
Randelli et al. (2022) [38]	Ⅱ randomized controlled trial	/	Arthroscopic rotator cuff repair with microfragmented adipose tissue	Arthroscopic rotator cuff repair	22 (microfragmented adipose tissue);	MRI, ASES, SST, VAS, dynamometer	3, 6, 12, 18, and 24months (ASES, SST, VAS. dynamometer); 18 months (MRI)	Significant functional enhancement
					22 (control)
Biological agents containing BMSCs
Hernigou et al. (2014) [19]	III case control study	0.51 (progenitor cells)	Arthroscopic rotator cuff repair with BMAC	Arthroscopic rotator cuff repair	45 (BMAC);	MRI	Every month to the 24 months	Significant structural enhancement
					45 (control)
Ellera Gomes JL et al. (2012) [39]	IV case series	5.65(CD34+ cells)	Arthroscopic rotator cuff repair with BMAC	Baseline	14	MRI, UCLA	12 months	No significant effects
Centeno et al. (2020) [40]	Ⅱ randomized controlled trial	810 (nucleated cells)	BMAC and PRP injection	Exercise therapy	25 (BMAC and PRP);	DASH, MRI, NPS, SANE	1, 3, 6, 12, and 24months (DASH, NPS, SANE); 12 months (MRI)	Significant functional enhancement
					25 (exercise therapy)
Schoch et al. (2022) [41]	III cohort study	/	Arthroscopic rotator cuff repair with BMAC	Arthroscopic rotator cuff repair	114 (BMAC);	Incidence of revision surgery	24 months	Significant decrease in the incidence of revision surgery
					3800 (control)
Kim SJ et al. (2018) [42]	III case control study	/	BMAC and PRP injection	Exercise therapy	12 (BMAC and PRP);	VAS, MMT, ASES, MRI	3 weeks and 3 months	Significant functional enhancement
					12 (exercise therapy)

Abbreviation: ADSCs: adipose-derived stem cells; VAS: visual analogue scale; UCLA: Los Angeles score; ROM: range of motion; SPADI: shoulder pain and disability index; ASES: American shoulder and elbow surgeons standardized shoulder assessment form; SST: simple shoulder test; DASH: shoulder and hand score; SANE: single assessment numeric evaluation; NPS: numeric pain scale; PRP: platelet-rich plasma; MMT: manual muscle test.

Fig. 2.

Clinical efficacy of stem cells from different sources for rotator cuff tendinopathy.

The tear size evaluated by MRI was used as the structural assessment index. The Constant score, UCLA score, ASES score, and DASH score were used as functional assessment indexes. Within one year is considered a short time point, and one year or more is considered a long time point. Abbreviation: ADSCs: adipose-derived stem cells; SVF: stromal vascular fraction; BMAC: bone marrow aspirate concentrate.

The therapeutic effect of stem cell therapy based on ADSCs is uncertain. A cohort study of 35 patients treated with arthroscopic rotator cuff repair and 35 patients treated with arthroscopic rotator cuff repair plus ADSCs injection found that ADSCs significantly improved the structural results in terms of re-tear rate. However, there were no clinical differences over the 28-month follow-up period [21]. In another trial, 24 patients with shoulder pain lasting for more than 3 months and partial tears in the supraspinatus tendon were divided into three groups, which were injected with stem cells in fibrin glue, saline/fibrin glue mixture and saline only. The participants were followed up at 6, 12 weeks, 6, 12 months and 2 years after injection and found that there was no difference in post-injection pain duration or severity [20]. A randomized controlled trial used fresh, uncultured, unmodified, autologous adipose-derived regenerative cells (UA-ADRCs) to treat symptomatic, partial-thickness rotator cuff tears (12 patients received UA-ADRCs and 6 patients received glucocorticoid as control). After treatment, the average American Shoulder and Elbow Surgeons Standardized Shoulder Assessment Form (ASES) total score of 24 and 52 weeks in UA-ADRCs group was significantly higher than that in corticosteroid group [37]. Another trial included 44 patients (22 per group) who underwent single-row arthroscopic rotator cuff repair. The results showed that autologous microfragmented lipoaspirate tissue containing ADSCs could only improve the short-term clinical and functional results after operation [38].

The limited number of evidence make the curative effect of stem cell therapy based on BMSCs equally unconvincing. A cohort study compared 114 patients who received bone marrow aspirate concentrate (BMAC) during an rotator cuff repair with 3800 rotator cuff repair-only patients and found that the use of BMAC was associated with a significantly lower incidence of revision surgery [41]. A randomized controlled trial compared the effects of BMAC and platelet product injection with exercise therapy in the treatment of partial and full-thickness supraspinatus tears (11 controls and 14 treatments) [40]. Compared to exercise therapy, BMAC treatment significantly improved function and pain at 3 months. Another study found that patients who received BMAC as an adjunct in single-row rotator cuff repair had significantly improved tendon integrity compared to the control group. However, clinical function was not followed up in this study [19]. A case control study compared the clinical outcomes with shoulder cuff exercises and BMAC-enriched platelet-rich plasma (PRP) injection therapy (12 controls and 12 treatments) [42]. The visual analog scale (VAS) improvement in the treatment group was significantly better than the control group at 3 months but there was no difference at 3 weeks. Tear size reduction was observed at 3 weeks or 3 months after BMAC-PRP injection, but there was no significant difference compared to the control group.

The clinical effects of stem cell therapy based on ADSCs and BMSCs in the treatment of rotator cuff tendinopathy are uncertain. Due to the limited number of trials, we are unable to further analyze and compare the clinical efficacy of ADSCs and BMSCs. We will further explore the source of stem cells suitable for rotator cuff tendinopathy from preclinical studies.

2.2. Preclinical clues: tendon is the most promising source

In addition to bone marrow and adipose tissue, stem cells from other sources such as tendon, umbilical cord, subacromial bursal, and urine have been used in preclinical studies for the treatment of rotator cuff tendinopathy [43]. For the ability to form tendon in vivo and in vitro, evidence suggests that tendon-derived stem cells (TDSCs) are the most promising stem cells (Fig. 3).

Fig. 3.

Tendon is the most promising source of stem cells for treating rotator cuff tendinopathy.

A smiley face indicates that stem cells from this source have an advantage in comparison. A straight face indicates that stem cells from this source were included in comparison but did not show any superiority.

The tendinous differentiation of TDSCs was significantly superior to that of BMSCs and ADSCs. Youngstrom et al. systematically compared tenogenesis of bone marrow-, adipose-, and tendon-derived stem cells in a dynamic bioreactor [44]. The results showed that TDSCs showed higher gene expression profile and the most mature tendon-like phenotype in three kinds of stem cells. Tan et al. found that TDSCs exhibited higher clonogenicity, proliferated faster, and expressed higher tenomodulin, scleraxis, collagen 1 α 1 (Col1A1), decorin, alkaline phosphatase, Col2A1, and biglycan messenger RNA levels than BMSCs [45]. TDSCs had the potential of spontaneous tenonic differentiation [46]. When human TDSCs is placed in a completely heterogeneous environment, it can still form a tissue structure similar to that found in human body [47]. In the absence of differential induction, BMSC may lead to tendon ossification [48].

Animal experiments have also shown that TDSCs are more effective than BMSCs in treating tendinopathy. An in vivo study of patellar tendon defect showed that the biomechanical outcome of tendon repaired by TDSCs was better than that of BMSCs [48]. Another experiment using autologous TDCS and BMSC implantation to treat Achilles tendon rupture also confirmed that the tendon treated with TDCSs had higher ultimate failure load and greater fibrous structure [49].

The use of TDSCs is considered an ideal process for the treatment of tendinopathy. According to the United States Food and Drug Administration (FDA) guidelines, it can be reasonably expected that homologous cells will perform the appropriate function for the treatment of tendinopathy, while non-homologous use of BMSCs and ADSCs increases safety and efficacy issues [50].

3. Optimal stem cell dose for rotator cuff tendinopathy

Dosage is a key factor affecting the curative effect of stem cell therapy. In a Phase I/IIa clinical trial, local injection of 40 million adipose-derived MSCs was found to result in a higher rate of healing for complex perianal fistulas in Crohn's disease compared to a dose of 20 million [51]. In the treatment of leukemia, dosage was found to have a significant impact on the clinical outcome [52]. In a clinical trial using stem cell therapy for rotator cuff tendinopathy, the researchers found that the successful patients received a higher dose of stem cells than the failed patients [19]. In addition, the healing area at 3 months was positively correlated with the number and concentration of stem cells. Dosage may affect the therapeutic effect by regulating the implantation, differentiation, maturation and lineage-specific changes of stem cells. Piltti et al. found that increasing human neural stem cell transplantation dose alters oligodendroglial and neuronal differentiation after spinal cord injury, thus affecting the motor function of mice [53].

The cell viability and delivery determine the number of stem cells that can truly function in the pathological microenvironment. Ineffective dosing due to low cell viability post-thaw has been the postulated reason in some studies that have failed to show the efficacy of stem cells. For instance, in moderate to severe acute respiratory distress syndrome, only patients who received intravenous infusion of cells with the highest viability (70%–85%) showed improved oxygenation compared to placebo [54]. In stem cell therapy for rotator cuff tendinopathy, many trials have not provided information on cell viability. Stem cell therapeutic potency is dependent upon delivery. In a murine model of toxic colitis, a direct comparison of freshly harvested stem cells in the log phase of growth showed a substantial effect only when delivered subcutaneously or intraperitoneally, while maximum tolerated intravenous bolus dosing failed to do so [55]. Stem cell therapy for rotator cuff tendinopathy all used local administration. Besides direct injection, some studies have attempted to load stem cells onto fibrin glue in order to improve delivery efficiency [20,21]. Local administration provides a targeted method for delivering paracrine factors directly to the affected tissue [55]. In contrast, systemic delivery is not an effective treatment option for rotator cuff tendinopathy. Stem cells trapped in filter organs such as the lungs and liver are likely to be more abundant than those homing to the site of tendinopathy after intravenous administration [56]. In addition, mechanical stimulation of the blood flow can induce stem cells to differentiate in the incorrect direction, such as endothelial and smooth muscle cells [57].

3.1. Clinical clues: high-dose ADSCs provide better therapeutic effects

Jo explored the appropriate dosage of ADSCs in an open-label, single-center trial with no active control trial [36]. Three dose-escalation ADSCs in 3 ​ml of saline was injected into the anterior, center and posterior one third of the torn end of rotator cuff (1 ​× ​10⁷, 5 ​× ​10⁷, 1 ​× ​10⁸). In various forms of shoulder function assessment, including the Constant score, Los Angeles (UCLA) score, the Simple Shoulder Test (SST) and Shoulder and Hand (DASH) score, all evaluations in the low-dose group did not show significant improvement, some evaluations in the medium-dose group showed significant improvement, and all evaluations in the high-dose group showed significant improvement [58]. In another clinical trial using ADSCs, a dose of 1 ​× ​10⁷ ​cells with 80% viability were injected into the center of the tendon defect. Patients receiving stem cell therapy did not show significant improvement in VAS scores, ASES scores, or tear size compared to those receiving saline at 3 months [20]. For ADSCs, a dose of 1 ​× ​10⁷ ​cells appear insufficient for effective treatment of rotator cuff tendinopathy.

3.2. Dosage selection Strategy:Moderation and individualization

The dosage of stem cells is not necessarily the higher the better. Preclinical experiments suggest that an excessive amount of stem cells can lead to poor therapeutic efficacy. Yang used second near-infrared fluorescence imaging with biocompatible PbS quantum dots to observe the cell fate of three doses of stem cells (1 ​× ​10⁴, 5 ​× ​10⁴, 1 ​× ​10⁵) during the treatment of supraspinatus tendon tear in mice [59]. They found that the moderate dose group showed the longest residence time and highest cell retention rate around the footprint during the repair stage. Histological analysis also indicated that the moderate dose group achieved the optimal therapeutic effect. Kwon et al. investigated the therapeutic effects and optimal dose of human umbilical cord blood-derived stem cell (UCSCs) injection in chronic full-thickness rotator cuff tendon tears. Rabbits were allocated into three groups: normal saline, 1 ​× ​10⁶ UCSCs, and 2 ​× ​10⁶ UCSCs. The results showed that there were no significant differences in tear size and motion analysis parameters between low and high dose groups four weeks after injection, indicating that the benefits of UCSCs are not dose-dependent [60].

Studies in other fields also suggest that there is an optimal dosage for stem cell therapy. Research on intra-articular injection of stem cells for knee osteoarthritis has found that higher doses of stem cells do not result in better therapeutic outcomes [61]. Currently, there is a lack of sufficient dose-finding clinical trials in the field of stem cell therapy. In certain conditions, such as age-related frailty and heart disease, a therapeutic window has been suggested with an intramyocardial total dose of 100 million cells, resulting in improved left ventricular end-systolic volume and 6-min walk test [62]. The selection of stem cell dosage must consider specific cell sources and recipient characteristics. From the perspective of stem cells, the significant differences between stem cells derived from different tissues dictate that they should each have their own optimal dosage. From the patient's perspective, we should choose the dosage based on individualized factors such as weight and the size of the rotator cuff tear. The majority of trials have used doses ranging between 1 and 2 million cells per kilogram recipient weight in other fields of stem cell therapy [30].

4. Challenges and prospects for clinical translation of stem cell therapy

4.1. Cell population heterogeneity

Among the prominent parameters controlling the efficacy of stem cells in clinical applications, the standardization of stem cells populations is on the top of the list [63]. BMAC and stromal vascular fraction of the adipose tissue are highly heterogeneous cell populations [64]. The stromal vascular fraction of the adipose tissue includes not only adipose stromal and hematopoietic stem and progenitor cells but also endothelial cells, erythrocytes, fibroblasts, lymphocytes, macrophages and pericytes, among others [65]. A clinical trial of ADSCs for rotator cuff tendinopathy evaluated the ability of stromal vascular fraction to produce ADSCs and found that ADSCs only accounted for 9.6% of stromal vascular fraction [21]. The variability of mixed biologics containing stem cells poses a significant obstacle to the stability of clinical applications.

Purified stem cell products also exhibit significant heterogeneity. Taking TDSCs as an example, TDSCs are heterogeneous and consist of subpopulations with different proliferation and differentiation potentials. The CD105⁻TDSCs subpopulation shows excellent potential for cartilage formation in vitro [66], while the CTSK⁻TDSCs subpopulation is the main contributor to tendon ectopic ossification [67]. These subpopulations that tend to differentiate into non-tendon lineages are not suitable for the treatment of rotator cuff tendinopathy. On the other hand, NES⁺TDSCs exhibit strong tendon differentiation and regeneration potential and may be an ideal subpopulation of TDSCs for tendinopathy [68].

The cell microenvironment has emerged as a key determinant of cell behavior and function in development, physiology, and pathophysiology [69]. Chemical and physical modulation of the microenvironment can effectively induce changes in the subpopulation structure of stem cells [70,71]. The application of these environmental factors in customized combinations or magnitudes can be used to selectively mature pluripotent stem cell derived endothelial cells into an arterial or venous subtype [72]. Pre-implantation modulation of the physical and chemical microenvironment can be used to select specific subpopulations of stem cells, resulting in a more homogeneous population of high-quality stem cells. In the future, it will be crucial to identify the specific subpopulation of stem cells from specific source that are most suitable for treating rotator cuff tendinopathy, and to produce these stem cells by developing appropriate stem cell culture systems.

4.2. Recipient selection

The pathogenesis of tendinopathy is multifactorial and complex. Various risk factors including excess mechanical overuse as well as environmental factors including smoking, metabolic diseases and certain medications can trigger the development of tendinopathy [2]. The specific pathophysiological mechanisms underlying tendinopathy are currently not clear. Inflammation, oxidative stress, matrix degeneration, and hypoxia are believed to be involved in tendinopathy [73,74]. Rotator cuff tendinopathy is a heterogeneous disease. The transcriptomics of the subacromial bursa tissue in rotator cuff tendinopathy caused by trauma and degeneration exhibit significant differences [75]. Elevated transcripts in traumatic rotator cuff tendinopathy represented metabolic and degradation processes, as well as transmembrane protein transport, while processes associated with the cell cycle were mainly enriched in degenerative tendinopathy. There were also significant differences in the molecular profiles of rotator cuff tendon tissue between different genders [76].

The different diseases treated with stem cells range widely regarding the underlying pathology and degree. It is reasonable to assume that stem cells differently affect the various conditions. Nevertheless, is stem cell therapy suitable for the treatment of all end-stage rotator cuff tendon tendinopathy? Are there some patients who are more suitable for receiving stem cell therapy? Detailed analyses of patient molecular subtypes, clinical phenotype and responses to stem cell treatment is needed. Analyses of patient factors could involve synovial fluid, blood, urine, synovium and pathological tendon. Baseline samples may offer valuable information on the clinical phenotype and molecular subtypes of patients, based on a diverse range of biomarkers, imaging techniques, and patient-reported outcome measures. This data could inform future patient stratification for stem cell therapy. Samples collected at follow-up timepoints can be used to help determine the effects of stem cell therapy on inflammatory or matrix degeneration. Defining baseline patient disease subtypes along with clinical phenotyping will help identify and stratify patients who are receptive to receiving stem cell therapy.

5. Conclusion

To provide patients with more efficient and stable stem cell therapy for rotator cuff tendinopathy, we have explored the two key factors that affect stem cell efficacy: cell source and dosage. BMSCs and ADSCs have been preliminarily applied, but their clinical efficacy is not yet certain. TDSCs have demonstrated unique advantages in both in vitro and in vivo experiments. Regarding dosage selection, there is ample evidence to suggest that dose selection can determine the success or failure of clinical treatment. We should specify the dosage of stem cells based on the principles of moderation and individualization. In the unique pathological environment of rotator cuff tendinopathy, a thorough comparison of stem cells from various sources and further research on how to personalize dosing is warranted. Overcoming challenges in cell population heterogeneity will be crucial in advancing the clinical translation of stem cell therapy for rotator cuff tendinopathy. Furthermore, recipient selection based on molecular subtypes, clinical phenotypes, and responses to stem cell treatment will help personalize and optimize stem cell therapy for better clinical outcomes.

Author contributions

All authors have made substantial contributions to all of the following: (1) the design of the work, (2) the drafting of the article or its critical revision for important intellectual content, (3) the final approval of the version to be submitted and agreed to be accountable for all aspects of the work.

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Indicate the specific contributions made by each author (list the authors initials followed by their surnames, e.g., Y.L. Cheung). The name of each author must appear at least once in each of the three categories below.

Conception and design of study: W.L. Shen, J.Y. Li, Z. Yin, X. Chen, acquisition of data: Z.T. Wang, C.L. Fang, H.Z. Liu, X.A. Mo, Z.C. Wang, L.F. Shen, analysis and/or interpretation of data:

Z.T. Wang, J.C. Luo, J.J. Wang Drafting the manuscript: Z.T. Wang, Y.G. Liao, C.Q. Tang, revising the manuscript critically for important intellectual content: Y.G. Liao, Z. Yin, C.L. Wang Approval of the version of the manuscript to be published (the names of all authors must be listed):Z.T. Wang, Y.G. Liao, C.L. Wang, C.Q. Tang, C.L. Fang, J.C. Luo, H.Z. Liu, X.A. Mo, Z.C. Wang, L.F. Shen, J.J. Wang, X. Chen, Z. Yin, J.Y. Li, W.L. Shen.

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