Prospective application of stem cells to prevent post-operative skeletal fibrosis

Abstract

Post-operative skeletal fibrosis is considered one of the major complications causing dysfunction of the skeletal system and compromising the outcomes of clinical treatment. Limited success has been achieved using current therapies; more effective therapies to reduce post-operative skeletal fibrosis are needed. Stem cells possess the ability to repair and regenerate damaged tissue. Numerous studies show that stem cells serve as a promising therapeutic approach for fibrotic diseases in tissues other than the skeletal system by inhibiting the inflammatory response and secreting favorable cytokines through activating specific signaling pathways, acting as so-called medicinal signaling cells. In this review, current therapies are summarized for post-operative skeletal fibrosis. Given that stem cells are used as a promising therapeutic approach for fibrotic diseases, little effort has been undertaken to use stem cells to prevent post-operative skeletal fibrosis. This review aims at providing useful information for the potential application of stem cells in preventing post-operative skeletal fibrosis in the near future.

1. Introduction

Fibrosis, a progressive fatal disease which accounts for more than one third of deaths worldwide, is considered the end-stage of many diseases which can occur in numerous organ systems, including but not limited to lung, skin and kidney ^{1, 2}. Of course, the skeletal system is no exception. Post-operative skeletal fibrotic diseases, including epidural fibrosis, intraarticular fibrosis and peritendinous fibrosis, often occur after surgery, trauma or injury of the skeletal system ^3-5. Post-operative skeletal fibrosis is considered a major complication which can cause skeletal system dysfunction, increase medical costs for patients, reduce the satisfaction rate of clinical treatment and even lead to patient depression. Due to these potential complications, post-operative skeletal fibrosis has drawn great attention. Following the elucidation of in-depth cellular and molecular mechanisms underlying the formation and progression of post-operative skeletal fibrosis, many therapeutic strategies have been applied to reduce this condition, such as microsurgical techniques, effective hemostasis, various drugs and biomaterial mediated strategies ^{3, 4}. However, side effects of the current approaches diminish the efficacy of treatments and more effective therapeutic strategies are needed.

With the development of regenerative medicine, stem cells have proven to possess great potential as a therapy for various diseases⁶. Increasing evidence indicates that stem cells have shown anti-fibrotic effects in numerous fibrotic tissues, including kidney, lung, heart, liver and skin, in both animal models and human studies. For example, jugular vein injection of bone marrow-derived stem cells (BMSCs) could ameliorate bleomycin-induced lung fibrosis in a rat model ⁷ and tail injection of BMSCs could reduce interstitial fibrosis in the Col4α3^−/− deficient mouse model ⁸. Stem cells are also proven to be a therapy for fibrotic diseases of the skin ⁹, cardiovascular system ¹⁰ and liver ¹¹. Recently, stem cells have also been found to have an anti-fibrotic effect in skeletal fibrotic diseases. For instance, resident stem cells could attenuate muscle fibrosis by intronic polyadenylation of platelet-derived growth factor alpha (PDGFα) ¹² and adipose-derived stem cells (ADSCs) delivered by poly lactic-co-glycolic acid (PLGA) scaffold could prevent epidural fibrosis adhesion by reconstructing epidural fat ¹³.

The above-mentioned studies indicate that stem cells show anti-fibrotic effects by secreting immunomodulatory and trophic factors, which is in line with a new term for mesenchymal stem cells, Medicinal Signaling Cells coined by Caplan¹⁴. Since several kinds of stem cells are discussed throughout the article, “stem cells” are used to indicate the broad scope of this concept while tissue-specific stem cells were detailed in individual studies. The primary aim of this review is to illustrate the current understanding and achievements of stem cells as an approach for anti-fibrotic therapy, which might provide useful information to develop a therapeutic strategy for skeletal fibrotic diseases.

2. Definition of post-operative skeletal fibrosis and common mechanisms

Post-operative skeletal fibrotic disease mainly consists of three subtypes: epidural fibrosis, intraarticular fibrosis and peritendinous fibrosis. Epidural fibrosis after laminectomy is a major reason for failed back surgery syndrome which is defined by persistent chronic pain of the spinal nerve root and lower back. The progressive formation of fibrotic scar between the dura mater and surrounding muscles can result in poor outcomes after spine surgery, which is a challenge for physicians. Intraarticular fibrosis is a serious post-operative complication of knee surgery and has gained the attention of adult reconstruction surgeons. Progressive scar formation limits the function of the knee joint by altering the range of motion, resulting in side effects such as knee joint pain, cartilage degeneration and muscle atrophy. Peritendinous fibrosis adhesion occurs after tendon repair, finger fracture fixation and other hand injuries, and limits the glide of the tendon and interphalangeal joint range of motion.

Despite no uniform mechanism that causes post-operative skeletal fibrosis, several potential mechanisms exist for each type of skeletal fibrosis. Fibroblast activation, which is considered the major cause of post-operative skeletal fibrosis, can be recognized as the common mechanism. During the repair process, damaged cells release inflammatory cells followed by a change in inflammatory response from the acute to the chronic phase. Following secretion of growth factors, cytokines and chemokines to the damaged area during chronic inflammation ^{15, 16}, the fibroblasts activated by the secreted factors migrate to the damaged skeletal area to produce collagen and form fiber to assist in healing, during which collagen formation and collagen catabolism maintain a balance due to the existence of a fibrinolytic enzyme. Unfortunately, the newly formed collagen exceeds the degraded collagen and thus there is excess collagen deposited in the remodeling process ^{16, 17}. The formed collagen develops into fibrosis tissue followed by adhesion to adjacent tissues, such as tendon, joint, skin, dura and nerve. The adhesion between fibrosis tissue and adjacent tissue decreases the range of motion of the joint and limits the movement of tendons; it also can lead to radicular low back pain and loss of muscle function.

3. Current therapies for post-operative skeletal fibrosis

Given the above-mentioned common mechanisms, additional therapies have been undertaken to prevent post-operative skeletal fibrosis through inhibiting fibroblast proliferation and suppressing the pro-inflammatory response. In this section, we summarize current therapeutic strategies to prevent post-operative skeletal fibrosis in animals and humans, including topical application of anti-tumor drugs, immunosuppressant drugs, biomaterials and other anti-fibrotic drugs (Table.S1). The disadvantages of each approach are also highlighted.

3.1. Anti-tumor drugs

Owing to the anti-fibrotic effect of anti-tumor drugs, positive results have been achieved in reducing post-operative skeletal fibrosis. Previous studies have shown that topical application of anti-tumor drugs, including mitomycin C (MMC) ^18-24, 10-hydroxycamptothecine (HCPT) ^{4, 25-30}, colchicine ^{31, 32} and 5-fluorouracil (5-Fu) ^{18, 33}, could prevent post-operative skeletal fibrosis in animal models by inhibiting the inflammatory response, suppressing fibroblast proliferation and reducing collagen deposition. However, more research is needed to determine the exact effect of 5-Fu ^{34, 35}. Interestingly, a previous study has shown that topical application of 0.5 mg/mL MMC could prevent epidural fibrosis after microendoscopic discectomy surgery in 37 patients ³⁶. Further evidence showed that anti-tumor drugs could reduce post-operative skeletal fibrosis by regulating miR-200b and RhoE expression, upregulating NOXA expression, decreasing vascular endothelial growth factor (VEGF) expression, blocking the transforming growth factor beta (TGF-β) pathway and activating the endoplasmic reticulum (ER) stress signaling pathway ^{29, 30}.

3.2. Immunosuppressant drugs

Local immune response is considered critical for each type of post-operative skeletal fibrosis and some immunosuppressant drugs used to prevent post-operative skeletal fibrosis have achieved success, including rapamycin ^{5, 37, 38}, tacrolimus ^39-41, pimercrolimus ⁴², azithromycin ⁴³, methotrexate ⁴⁴ and cyclosporin A ¹⁸. Results showed that they could prevent post-operative skeletal fibrosis by inhibiting fibroblast proliferation ^37-39, suppressing interleukin 2 (IL-2) and TGF-β1 expression ⁴⁰, regulating miR-429 and RhoE expression ⁴¹ and activating autophagy ⁵ and the ER stress signaling pathway ⁴⁴.

3.3. Biomaterials

More evidence indicates that some biomaterials have been successfully used to prevent post-operative skeletal fibrosis in animal models, including hyaluronic acid (HA) and derivatives ^45-50, chitosan and derivatives ^51-53 and amniotic membrane and derivatives ^54-57. Specifically, topical application of HA derivatives could prevent peritendinous fibrosis in patients ^{58, 59}. The underlying mechanisms include acting as a barrier, reducing inflammatory response and suppressing fibroblast proliferation. Other biomaterials, such as ADCON-L ^{60, 61}, collagen ^{62, 63}, seprafilm ⁶⁴ and silk-polyethylene glycol gel ⁶⁵, have also been applied to prevent skeletal fibrosis.

3.4. Other methods

Given the anti-fibrotic effect, traditional Chinese drugs have achieved positive effects in preventing post-operative fibrosis, including salvianolic acid B ⁶⁶, angelica sinensis ⁶⁷, safflower yellow ⁶⁸ and daidzein ⁶⁹. Non-steroidal anti-inflammatory drug (NSAIDs), including ketoprofen ⁷⁰, aceclofenac ⁷¹, indomethacin ⁷² and ibuprofen ⁷³, could also modify local inflammation response and prevent post-operative fibrosis.

To help alleviate the side effects of drugs, controlled release systems have been developed to reduce post-operative skeletal fibrosis, including MMC controlled-release films ^{74, 75}, liposome-encapsulated HCPT ⁷⁶ and gelatin combined with dexamethasone ⁷⁷, decorin ⁷⁸ and parecoxib ⁷⁹. Moreover, other drugs and proteins are used to prevent post-skeletal fibrosis, including simvastatin ⁸⁰, rosuvastatin ⁸¹, resveratrol ⁸² and all-trans retinoic acid ⁸³, verapamil ^{84, 85}, melatonin ⁸⁶, taurine ⁸⁷, citicoline ^{88, 89}, pentoxifylline ⁹⁰ and transduction with lentivirus carrying extracellular signal-regulated kinase 2 ^91-93. Additionally, oral application of pentoxifylline ⁹⁴ and percutaneous lysis of epidural fibrosis ⁹⁵ have achieved success in patients.

3.5. Disadvantages of current therapies for skeletal fibrosis

Despite some promising results from current approaches for the treatment of skeletal fibrosis, unexpected side effects still exist. For example, MMC could reduce epidural fibrosis in animal studies, but no clinically meaningful benefit was observed ³⁶. In addition, as an anti-tumor drug, MMC might cause dose-related complications after topical application, such as in a previous report of ophthalmologic surgery including epithelial defects, delayed wound healing, corneal perforation, necrotizing keratitis and persistent hypotony maculopathy ¹⁸. Another case is that ADCON has been proven to prevent post-operative skeletal fibrosis in animal models, but failed to prevent peritendinous fibrosis in patients ⁹⁶. Moreover, some unexpected adverse effects were found in the application of ADCON-L for patients, including back pain, spontaneous intracranial hypotension and subdural hematoma ⁹⁷. In additional to these serious side effects, most of these therapies achieved success in animals but less success was obtained in patients, indicating of the limited benefits of these therapies clinically.

4. Current stem cell therapy for tissue fibrosis

More evidence indicates that stem cells play a significant role in on preventing fibrosis in many animal models, as well as achieving good clinical outcomes. In this section, the current literature on stem cells as a therapy for fibrotic diseases is summarized, including kidney, lung, cardiac, liver and skin fibrosis (Table.S2).

4.1. Kidney fibrosis

Kidney fibrosis commonly appearing interstitially is considered a feature of chronic kidney disease. In a recent study, stem cells were tested for the effect on kidney fibrosis models, including Col4α3^−/− and Col4α5^{−/− 8, 98, 99}, unilateral ureteral obstruction (UUO) ^100-102, remnant kidney ^{103, 104}, renal artery stenosis ^{105, 106}, transplantation ¹⁰⁷, unilateral ischemia-reperfusion (IR) injury ¹⁰⁸ and nephrectomy IR cyclosporine ¹⁰⁹. Evidence indicates that stem cells could reduce renal fibrosis through some potential mechanisms, including inhibition of the renin-angiotensin system ⁹⁹, suppression of the inflammatory response ^{100, 102, 103, 105-108}, activation of the paracrine system ¹⁰⁴ and regeneration of podocytes ⁹⁸. Interestingly, stem cells reducing renal fibrosis and promoting survival do not delay the progression of kidney failure ⁸.

4.2. Lung fibrosis

Lung fibrosis is a serious progressive lung disease which leads to respiratory failure. Bleomycin-induced lung fibrosis is used as a classic animal model for lung fibrosis research ^{7, 110-113}. Other frequently used models include hyperoxia-induced ¹¹⁴, silica-induced ¹¹⁵ and irradiate-induced ¹¹⁶ lung fibrosis. Stem cells have proven to reduce lung fibrosis through suppressing T-cell function ¹¹², inhibiting expression of IL-1β ¹¹³, IL-6 ^{113, 114}, TGF-β ^{110, 111, 113, 114}, tumor necrosis factor alpha (TNF-α) ^{110, 113, 114} and alpha smooth muscle actin (α-SMA) ^{114, 115}, but increase matrix metalloproteinase (MMP) expression ¹¹⁰. Moreover, tail vein injection of BMSCs transfected with FLK1 (VEGF receptor-2) reduced irradiation-induced fibrosis by promoting differentiation into functional lung cells ¹¹⁶.

4.3. Cardiac fibrosis

Cardiac fibrosis is defined as an over-deposition of collagen in the heart. Many causes may contribute to the fibrosis formation process, including hypertrophic cardiomyopathy, toxic insults and metabolic disturbances. The left anterior descending artery (LAD) fibrosis model is widely used for evaluation of stem cell treatment ^{10, 117-119}. Increasing evidence indicates that stem cells have the ability to improve cardiac function by reducing cardiac fibrosis ^{10, 117-120}. Interestingly, VEGF, a pro-angiogenic factor, which contributes to fibrosis formation, is upregulated after stem cell application ^{118, 119}.

4.4. Liver fibrosis

Liver fibrosis is a common stage for secondary liver disease, including chronic hepatitis B, chronic hepatitis C and alcoholism. A CCL₄-induced liver fibrosis model is often used for assessing the effect of stem cells on liver fibrosis ^{11, 121-126}. Stem cells could act as an anti-fibrotic agent on liver fibrosis by decreasing collagen deposition ^{123, 124}, inhibiting expression of VEGF ¹²¹ and TGF-β1 ¹²², preventing the formation of epithelial to mesenchymal transition (EMT) ¹²² and increasing expression of MMPs ^{11, 124, 126}. Encouraging reports have shown that human umbilical cord derived mesenchymal stem cells (uMSCs) could reduce hepatitis B virus (HBV)-induced liver fibrosis and promote liver function by inhibiting the serum laminin level and increasing hepatocyte growth factor (HGF) level in patients ¹²⁷.

4.5. Skin fibrosis

Skin fibrosis is a pathological outcome of various injuries including wound injuries, exogenous-induced fibrosis and autoimmune skin diseases. There are three fibrosis models, including bleomycin-induced fibrosis ^{9, 128}, radiation-induced fibrosis ^129-131 and excisional-induced fibrosis ^{132, 133}. These models are used to detect the effect of stem cells on skin fibrosis. Results have revealed that stem cells could reduce skin fibrosis and promote skin healing by decreasing collagen deposition ^{9, 133}, suppressing macrophage and neutrophil infiltration ^{9, 130}, inhibiting expression levels of TGF-β1 ^{9, 128, 129, 133}, IL-1 ¹³⁰, VEGF ¹²⁸, α-SMA ^{9, 133} and TNF-α ^{130, 133} and increasing expression levels of MMPs ⁹ and IL-10 ¹³⁰.

5. Potential mechanisms underlying anti-fibrosis of stem cells

As summarized above, stem cells have shown an anti-fibrotic effect in animal models and patients (Figure 1). Therefore, stem cells could be potential therapies for post-operative skeletal fibrosis. In this section, we propose several potential mechanisms, which may contribute to forthcoming stem cell therapy of post-operative skeletal fibrosis (Figure 2).

Figure 1.

Potential interference of stem cells on fibrosis formation. Cellular and molecular pathways include tissue injury and/or repair triggering secretion of cytokines (VEGF, TGF-β, IL-1, TNF-α, IFN-γ, PDGF) by mast cells, macrophages, lymphocytes, monocytes, basophils and eosinophils that lead to activation of fibroblasts via trans-differentiation of epithelial cells and fibrocytes and further into myofibroblasts. Cytokines and exosomes secreted by stem cells could inhibit fibroblast activation and other cell types to trans-differentiate into myofibroblasts, resulting in upregulation of MMPs and downregulation of TIMPs and α-SMA which leads to inhibition of fibrosis formation.

Figure 2.

Main cytokine pathways regulating tissue fibrosis. Cytokines (TGF-β, IL-1, TNF-α, IFN-γ, PDGF) secreted by inflammatory cells mediate the activation of several intracellular signaling pathways. As a major fibrogenic cytokine, TGF-β activates the canonical and noncanonical pathway resulting in induction of fibrogenic genes, such as collagen I/III/IV, TIMPs and α-SMA. Other cytokines, such as PDGF, IFN-γ, IL-1 and TNF-α, are also involved in regulating fibrosis formation primarily through their binding to specific receptors. Cytokines secreted by stem cells, including but not limited to HGF, IL-1Ra, MMPs and TSG-6, could inhibit profibrotic effect of fibrogenic cytokines.

Abbreviation: α-SMA: alpha-smooth muscle actin; HGF: hepatocyte growth factor; IFN-γ: interferon gamma; IGF-I: insulin-like growth factor I; IL-1: interleukin 1; IL-1Ra: interleukin 1 receptor antagonist; TSG-6: tumor necrosis factor-stimulated gene-6; MMP: matrix metalloproteinase; mTOR: mammalian target of rapamycin; NF-κB: nuclear factor κB; PDGF: platelet-derived growth factor; PI3K: phosphoinositol 3-kinase; Ras: renal artery stenosis; TGF-β: transforming growth factor beta; TIMPs: tissue inhibitor of metalloproteinases; TNF-α: tumor necrosis factor alpha.

5.1. Suppression of inflammation response

Chronic inflammation is considered the major factor contributing to fibrosis. During the chronic inflammation period, newly generated lymphocytes and macrophages migrate to the target organ followed by the pathological fibrosis process triggered by activated myofibroblasts. Stem cell application could modify organ microenvironment, including promotion of anti-inflammatory cytokine secretion and suppression of pro-inflammatory cytokine secretion, to prevent fibrosis formation. In the fibrosis model, use of stem cells suppressed infiltration of T lymphocytes and neutrophils ^{9, 112} and macrophage function, including inhibiting pro-inflammatory CD68- and CD80-positive macrophages and increasing anti-inflammatory CD163-positive macrophages ^{9, 130}.

TNF-α is a pro-inflammatory cytokine which plays a role in the fibrosis process. Numerous studies have shown that TNF-α expression, at both mRNA and protein levels, is decreased after application of stem cells ^{100, 103, 105, 110, 113, 114, 130, 133}. In the same manner as TNF-α, interleukins, including IL-1α ¹³⁰, IL-1β ^{113, 130} and IL-6 ^{103, 107, 113, 114}, are suppressed following stem cell application in several fibrosis models. As a cytotoxic factor, interferon gamma (IFN-γ) could also be diminished after stem cell application ¹¹⁰.

EMT is proven to be a central role for the relationship between inflammation and fibrosis formation ¹³⁴. Previous reports showed that stem cells could reduce renal and liver fibrosis by inhibiting EMT ^{100, 108, 122}. Vimentin, a marker of EMT, is proven to be involved in post-operative skeletal fibrosis and, when downregulated, fibrosis is prevented. Increasing evidence shows that vimentin is downregulated by stem cell application in fibrosis models ^{103, 122}, indicative of suppression of inflammation by stem cells through inhibition of EMT.

5.2. Inhibition of the TGF-β1 pathway

TGF-β1 is considered the major profibrotic cytokine which contributes to fibrosis formation. TGF-β1 promotes profibrotic cell proliferation and induces extracellular matrix (ECM) formation ¹³⁵. Moreover, TGF-β1 mRNA level was reported to be significantly higher in 28-day scar tissue ¹³⁶ and in the tissue of the post-operative skeletal fibrosis model ^{25, 40, 66, 84}. α-SMA, a myofibroblast marker, could be upregulated by TGF-β1. Downregulation of the α-SMA expression level was found in a fibrosis model treated by stem cells ^{9, 114, 115, 124, 126, 133}, suggesting that stem cells can inhibit proliferation of myofibroblasts. Furthermore, inhibition of the TGF-β pathway has proven to be responsible for the effect of stem cells on fibrotic diseases ^{103, 110, 113, 128, 129}. A similar effect has also been found in transplanted exosomes isolated from stem cell-conditioned medium ¹²². Stem cell-conditioned medium from in vitro studies could suppress fibroblast differentiation and activity by inhibiting the TGF-β/Smad pathway ^{137, 138}.

HGF secreted by stem cells was reported to inhibit fibroblast proliferation and reduce collagen deposition ^{117, 120, 127}. Moreover, stem cells overexpressing HGF exhibited a better anti-fibrotic effect than non-transfected cells in the bleomycin-induced lung fibrosis model ¹³⁹. Further studies indicate that HGF could inhibit TGF-β1 expression and promote fibrosis scar remodeling by increasing MMP-1 expression ¹⁴⁰.

Tumor necrosis factor-stimulated gene-6 (TSG-6) secreted from stem cells functions as a TNF-α inhibitor. Recent reports indicate that stem cells could prevent peritoneal fibrosis by secreting TSG-6; the same effect was found in reducing wound fibrosis and renal fibrosis by inhibiting TGF-β1 expression ^{133, 141, 142}.

5.3. Promotion of tissue remodeling

ECM has proven to play a critical role in fibrosis formation and to possess active TGF-β1 ability. Therefore, inhibiting ECM formation and inducing degradation are possible mechanisms to prevent fibrosis. Insufficiency of MMPs, the major regulator of ECM degradation, or an imbalance of MMPs/TIMPs (tissue inhibitor of metalloproteinases) fails to promote ECM degradation. In previous studies, stem cells have proven to reduce fibrosis by inducing degradation of ECM components, such as hydroxyproline content and collagen deposition ^{7, 9, 110, 115, 120, 122, 124} as well as laminin and fibronectin ^{107, 127}. Exosomes and microvesicles released from transplanted stem cells also possess the ability to degenerate ECM ^{115, 122}.

In fibrosis model studies, expression levels of MMP-2, MMP-9 and MMP-13 increased following stem cell application ^{9, 11, 117, 124}. MMP-2 and MMP-9 are gelatinases that have high activity against gelatin, the main component of ECM. Intriguingly, some studies found that expression levels of MMP-2 and MMP-9 were downregulated after stem cell application according to some in vivo studies ^{7, 120}. TIMPs, including TIMP-1, TIMP-2, TIMP-3 and TIMP-4, are suppressed by application of stem cells ¹¹⁰. Suppression of TIMP2 by stem cell-conditioned medium also suggests that stem cells possess a paracrine effect ¹¹⁷. The TIMP/MMP ratio was downregulated by application of stem cells indicating that there was a balance between degradation and synthesis of ECM ¹⁰³.

5.4. Other potential mechanisms

In addition to the above mentioned mechanisms, others include the p38/MAPK pathway ¹³², regulation of miR-let7c ¹⁰¹ and the ER stress pathway ¹⁰⁶. In particular, ER stress has been proven to be linked to post-operative skeletal fibrosis ^{30, 44, 143}. Induction of fibroblast apoptosis is considered a mechanism of preventing post-operative skeletal fibrosis. Interestingly, the anti-apoptotic effect of stem cells on fibrosis prevention was found in several fibrosis models, possibly suggesting that application of stem cells could modify the microenvironment to protect resident cells from damage and promote recovery ^{98, 103}. Furthermore, VEGF is considered a profibrotic factor for fibrosis; expression of VEGF is inhibited by stem cell application in some fibrosis models ^{121, 128}. However, other reports found that VEGF expression increased after stem cell application in fibrosis models ^{105, 118, 119}, indicating that stem cells could reduce fibrosis by promoting angiogenesis and vascular stability.

6. Conclusion and Perspective

Post-operative skeletal fibrosis contributes to poor clinical outcomes and current therapeutic strategies have provided unsatisfactory results in reducing clinical post-operative skeletal fibrosis. In this review, we assess current therapies for post-operative skeletal fibrosis and discuss the effect and mechanisms of stem cells on preventing fibrotic diseases in the kidney, lung, heart, liver and skin. Most current therapies for post-operative skeletal fibrosis can inhibit fibroblast proliferation to a certain degree but interfere with the physiological function of surrounding tissues. Fortunately, increasing evidence indicates that stem cells can contribute not only to a local tissue regeneration but also to modulation of a regional microenvironment when stem cells are applied as a therapy for anti-fibrosis. Therefore, we hypothesize that stem cells might be a better choice for reducing post-operative skeletal fibrosis. More experiments should be undertaken to help us gain a better understanding of the effectiveness of stem cells as a therapy for post-operative skeletal fibrosis, including but not limited to stem cell source, amount, delivery approach and treatment timing.