Exosomes derived from tendon stem/progenitor cells enhance tendon-bone interface healing after rotator cuff repair in a rat model

The rate of retear after surgical repair remains high.Mesenchymal stem cells(MSCs)have been extensively employed in regenerative medicine for several decades.However,safety and ethical concerns constrain their clinical application.Tendon Stem/Progenitor Cells(TSPCs)-derived exosomes have emerged as promising cellfree therapeutic agents.Therefore,urgent studies are needed to investigate whether TSPC-Exos could enhance tendon-bone healing and elucidate the underlying mechanisms.In this study,TSPC-Exos were found to promote the proliferation,migration,and expression of fibrogenesis markers in BMSCs.Furthermore,TSPC-Exos demonstrated an ability to suppress the polarization of M1 macrophages while promoting M2 macrophage polarization.In a rat model of rotator cuff repair,TSPC-Exos modulated inflammation and improved the histological structure of the tendon-bone interface,the biomechanical properties of the repaired tendon,and the function of the joint.Mechanistically,TSPC-Exos exhibited high expression of miR-21a-5p,which regulated the expression of PDCD4.The PDCD4/AKT/mTOR axis was implicated in the therapeutic effects of TSPC-Exos on proliferation,migration,and fibrogenesis in BMSCs.This study introduces a novel approach utilizing TSPC-Exos therapy as a promising strategy for cell-free therapies,potentially benefiting patients with rotator cuff tear in the future.

Dental pulp stem cells express tendon markers under mechanical loading and are a potential cell source for tissue engineering of tendon-like tissue

Postnatal mesenchymal stem cells have the capacity to differentiate into multiple cell lineages. This study explored the possibility of dental pulp stem cells （DPSCs） for potential application in tendon tissue engineering. The expression of tendon- related markers such as scleraxis, tenascin-C, tenomodulin, eye absent homologue 2, collagens I and VI was detected in dental pulp tissue. Interestingly, under mechanical stimulation, these tendon-related markers were significantly enhanced when DPSCs were seeded in aligned polyglycolic acid （PGA） fibre scaffolds. Furthermore, mature tendon-like tissue was formed after transplantation of DPSC-PGA constructs under mechanical loading conditions in a mouse model. This study demonstrates that DPSCs could be a ootential stem cell source for tissue enEineerin~ of tendon-like tissue.

Mechanotransduction of stem cells for tendon repair

Tendon is a mechanosensitive tissue that transmits force from muscle to bone.Physiological loading contributes to maintaining the homeostasis and adaptation of tendon,but aberrant loading may lead to injury or failed repair.It is shown that stem cells respond to mechanical loading and play an essential role in both acute and chronic injuries,as well as in tendon repair.In the process of mechanotransduction,mechanical loading is detected by mechanosensors that regulate cell differentiation and proliferation via several signaling pathways.In order to better understand the stem-cell response to mechanical stimulation and the potential mechanism of the tendon repair process,in this review,we summarize the source and role of endogenous and exogenous stem cells active in tendon repair,describe the mechanical response of stem cells,and finally,highlight the mechanotransduction process and underlying signaling pathways.

Mesenchymal stem cells for the treatment of tendon disorders

Tendon disorders are associated with increased morbidity and a reduction in the quality of life, especially in people of working age. Recently, a new approach, cell-based therapy, offers promising potential to treat tendon injuries. Mesenchymal stem cells are the most suitable candidates for such therapies due to their capacity to differentiate into cells of mesodermal origin, their paracrine properties and their potential use in autologous transplantation. This review summarizes experimental as well as clinical data focusing on the use of mesenchymal stem cells to treat tendinophaties.

Advanced glycation end productions and tendon stem/progenitor cells in pathogenesis of diabetic tendinopathy

Tendinopathy is a challenging complication observed in patients with diabetes mellitus.Tendinopathy usually leads to chronic pain,limited joint motion,and even ruptured tendons.Imaging and histological analyses have revealed pathological changes in various tendons of patients with diabetes,including disorganized arrangement of collagen fibers,microtears,calcium nodules,and advanced glycation end product(AGE)deposition.Tendon-derived stem/progenitor cells(TSPCs)were found to maintain hemostasis and to participate in the reversal of tendinopathy.We also discovered the aberrant osteochondrogenesis of TSPCs in vitro.However,the relationship between AGEs and TSPCs in diabetic tendinopathy and the underlying mechanism remain unclear.In this review,we summarize the current findings in this field and hypothesize that AGEs could alter the properties of tendons in patients with diabetes by regulating the proliferation and differentiation of TSPCs in vivo.

Stem cell therapies in tendon-bone healing

Tendon-bone insertion injuries such as rotator cuff and anterior cruciate ligament injuries are currently highly common and severe.The key method of treating this kind of injury is the reconstruction operation.The success of this reconstructive process depends on the ability of the graft to incorporate into the bone.Recently,there has been substantial discussion about how to enhance the integration of tendon and bone through biological methods.Stem cells like bone marrow mesenchymal stem cells(MSCs),tendon stem/progenitor cells,synovium-derived MSCs,adipose-derived stem cells,or periosteum-derived periosteal stem cells can self-regenerate and potentially differentiate into different cell types,which have been widely used in tissue repair and regeneration.Thus,we concentrate in this review on the current circumstances of tendon-bone healing using stem cell therapy.

Tendon stem/progenitor cell ageing: Modulation and rejuvenation

Tendon ageing is a complicated process caused by multifaceted pathways and ageing plays a critical role in the occurrence and severity of tendon injury.The role of tendon stem/progenitor cells(TSPCs)in tendon maintenance and regeneration has received increasing attention in recent years.The decreased capacity of TSPCs in seniors contributes to impaired tendon functions and raises questions as to what extent these cells either affect,or cause ageing,and whether these age-related cellular alterations are caused by intrinsic factors or the cellular environment.In this review,recent discoveries concerning the biological characteristics of TSPCs and age-related changes in TSPCs,including the effects of cellular epigenetic alterations and the mechanisms involved in the ageing process,are analyzed.During the ageing process,TSPCs ageing might occur as a natural part of the tendon ageing,but could also result from decreased levels of growth factor,hormone deficits and changes in other related factors.Here,we discuss methods that might induce the rejuvenation of TSPC functions that are impaired during ageing,including moderate exercise,cell extracellular matrix condition,growth factors and hormones;these methods aim to rejuvenate the features of youthfulness with the ultimate goal of improving human health during ageing.

A Novel Low Air Pressure-Assisted Approach for the Construction of Cells-Decellularized Tendon Scaffold Complex

Objective The goal of this study was to develop a decellularized tendon scaffold(DTS)and repopulate it with adipose-derived stem cells(ADSCs)assisted by low air pressure(LP).Methods The porcine superficial flexor tendons were processed into the DTSs using a combination of physical,chemical,and enzymatic treatments.The effectiveness of decellularization was verified by histological analysis and DNA quantification.The properties of the DTSs were evaluated by quantitative analysis of biochemical characterization,porosimetry,in vitro biocompatibility assessment,and biomechanical testing.Subsequently,the ADSCs-DTS complexes were constructed via cell injection assisted by LP or under atmospheric pressure.The differences in cell distribution,biomechanical properties,and the total DNA content were compared by histological analysis,biomechanical testing,and DNA quantification,respectively.Results Histological analysis confirmed that no cells or condensed nuclear materials were retained within the DTSs with widened interfibrillar space.The decellularization treatment resulted in a significant decrease in the content of DNA and glycosaminoglycans,and a significant increase in the porosity.The DTSs were cytocompatible in vitro and did not show reduced collagen content and inferior biomechanical properties compared with the fresh-frozen tendons.The assistance of LP promoted the broader distribution of cells into the adjacent interfibrillar space and cell proliferation in DTSs.The biomechanical properties of the scaffolds were not significantly affected by the recellularization treatments.Conclusion A novel LP-assisted approach for the construction of cells-DTS complex was established,which could be a methodological foundation for further bioreactor and in vitro studies.

First immunohistochemical evidence of human tendon repair following stem cell injection: A case report and review of literature

BACKGROUND Current clinical treatment options for symptomatic,partial-thickness rotator cuff tear(sPTRCT)offer only limited potential for true tissue healing and improvement of clinical results.In animal models,injections of adult stem cells isolated from adipose tissue into tendon injuries evidenced histological regeneration of tendon tissue.However,it is unclear whether such beneficial effects could also be observed in a human tendon treated with fresh,uncultured,autologous,adipose derived regenerative cells(UA-ADRCs).A specific challenge in this regard is that UA-ADRCs cannot be labeled and,thus,not unequivocally identified in the host tissue.Therefore,histological regeneration of injured human tendons after injection of UA-ADRCs must be assessed using comprehensive,immunohistochemical and microscopic analysis of biopsies taken from the treated tendon a few weeks after injection of UA-ADRCs.CASE SUMMARY A 66-year-old patient suffered from sPTRCT affecting the right supraspinatus and infraspinatus tendon,caused by a bicycle accident.On day 18 post injury[day 16 post magnetic resonance imaging(MRI)examination]approximately 100 g of abdominal adipose tissue was harvested by liposuction,from which approximately 75×10^(6) UA-ADRCs were isolated within 2 h.Then,UA-ADRCs were injected(controlled by biplanar X-ray imaging)adjacent to the injured supraspinatus tendon immediately after isolation.Despite fast clinical recovery,a follow-up MRI examination 2.5 mo post treatment indicated the need for open revision of the injured infraspinatus tendon,which had not been treated with UAADRCs.During this operation,a biopsy was taken from the supraspinatus tendon at the position of the injury.A comprehensive,immunohistochemical and microscopic analysis of the biopsy(comprising 13 antibodies)was indicative of newly formed tendon tissue.CONCLUSION Injection of UA-ADRCs can result in regeneration of injured human tendons by formation of new tendon tissue.

兔肩袖腱骨愈合过程中基质细胞衍生因子1的表达及作用机制

背景:近年在腱骨损伤领域部分学者将基质细胞衍生因子1搭载在组织工程支架上用以促进腱骨愈合,取得了较好的成效,但在基质细胞衍生因子1促进腱骨愈合机制及自然愈合过程中其是否参与修复,目前尚未明确。目的:研究兔肩袖全层冈上肌断裂后腱骨愈合过程中基质细胞衍生因子1表达,及其在腱骨损伤时对干细胞的迁移作用和最佳体外促迁移浓度。方法:随机取成年新西兰大白兔18只建立肩袖损伤模型,另取3只为空白对照。于造模后3,5,7,14,21,28 d各处死3只并处死空白组兔,取腱骨连接处组织保存在-80℃冰箱。应用ELISA反应检测损伤后各时间点愈合处基质细胞衍生因子1表达。取幼兔股骨骨髓间充质干细胞分离培养鉴定,通过transwell实验验证基质细胞衍生因子1对干细胞的促迁移作用效果及体外促迁移最佳浓度,将培养到P3代的干细胞与Brdu共培养后注入兔耳缘静脉,通过免疫组化染色验证干细胞是否迁移至损伤处。结果与结论:①基质细胞衍生因子1在肩袖腱骨愈合过程呈双峰表达,于伤后3 d明显增高(P<0.01)随后下降,于伤后5 d达最低,后再次升高于伤后14 d达峰值(P<0.01),然后下降;②细胞免疫组化染色可见标记有Brdu的干细胞确有迁移至损伤处;③transwell实验结果表明60-80 ng/mL的基质细胞衍生因子1对干细胞促迁移效果最好,而200 ng/mL浓度反而会起到抑制迁移作用;④基质细胞衍生因子1参与了肩袖腱骨愈合的炎症反应期和增殖期的愈合过程,其作用机制可能为通过促进干细胞迁移至损伤处并分化为各类细胞促进修复,并且基质细胞衍生因子1的促迁移作用存在于一定浓度范围,超出范围则可能起到抑制作用。

损伤前交叉韧带修复及促进移植物愈合的策略

背景:近些年,对于前交叉韧带损伤的治疗日益成熟,但是,临床上对于前交叉韧带损伤的手术时机、手术方式的选择、移植物的选择及促进移植物愈合的方法等问题还存在争议。目的:总结前交叉韧带损伤的手术时机、手术方式、移植物选择和促进移植物愈合方法的最新研究进展,为前交叉韧带损伤寻找新的治疗方向。方法:从PubMed、中国知网、万方数据、维普、SinoMed、ScienceDirect、Springer和Cochrane图书馆数据库进行前交叉韧带损伤相关文献的检索,最终经过筛选纳入相关文献72篇进行综述分析。结果与结论:①在手术时机方面:前交叉韧带早期重建与延迟重建相比,早期重建可减缓半月板的损伤、提高生活质量、促进功能恢复,然而手术时机的不同是否会加速软骨损伤,目前还无定论。②在手术方式方面:关节镜下前交叉韧带重建术是前交叉韧带损伤的常用手术方式;前交叉韧带动态内稳定修复术在短期和长期疗效中,都可以带来和传统前交叉韧带重建术相似的结局。③在移植物的选择方面:自体腘绳肌肌腱是前交叉韧带移植物的首要选择;骨-髌腱-骨移植物和同种异体移植物作为次要选择。④在促进移植物愈合的策略方面:缝合带加强可以增加膝关节稳定性,保证移植物的愈合;干细胞通过抗炎作用、血管生成作用、抑制骨溶解和促进软骨细胞分化促进移植物的腱-骨愈合;保留前交叉韧带残端可以维持膝关节稳定、促进本体感觉恢复,为移植物的愈合提供先决条件;富血小板血浆促进移植物愈合的有效性有待商榷;而生物材料、基因治疗及干细胞治疗等促进肌腱愈合的方法还停留在分子和动物研究阶段,未来还需要进行临床转化。

明胶三维微球装载人脐带间充质干细胞修复慢性肌腱病

背景:肌腱组织因缺少血管而修复困难,如何促进肌腱的恢复和提高肌腱损伤后干细胞治疗的效果,一直是临床及科研的研究热点和重点。目的:将人脐带间充质干细胞与明胶微载体支架结合构建组织工程化干细胞,通过体外实验与大鼠体内实验观察明胶微载体培养的人脐带间充质干细胞对肌腱病的治疗效果及作用机制。方法:(1)体外细胞实验:将人脐带间充质干细胞接种于三维明胶微载体后观察细胞活性以及存活情况,以常规培养的人脐带间充质干细胞作为对照;(2)动物体内实验:将成年SD大鼠随机分为正常组、肌腱病组、2D组(肌腱病+常规培养人脐带间充质干细胞)、3D组(肌腱病+明胶微载体三维培养的人脐带间充质干细胞),每组6只,治疗4周后进行动物行为学检测以及跟腱病理组织形态观察。结果与结论:(1)体外细胞实验:接种于明胶微载体的人脐带间充质干细胞存活率高,且随着时间延长细胞增殖速率增加;与对照组相比,三维明胶微载体培养的细胞活性更好;(2)动物体内实验:治疗4周后,与肌腱病组比较,3D组大鼠下肢功能恢复良好及组织病理学评分显著改善,而2D组也可一定程度改善肌腱病损伤,但效果不及3D组;(3)结果表明,三维明胶微载体培养的人脐带间充质干细胞可以促进肌腱损伤组织修复再生,且修复效果优于常规培养人脐带间充质干细胞。

影响肌腱干细胞分化的因素

背景:肌腱病是一种肌肉骨骼疾病,以疼痛和活动能力下降为特征,伴有胶原蛋白紊乱和血管增生的病理变化。肌腱病常易发生在运动员、体力劳动者和老年人身上。肌腱病的机制之一是“失败的愈合反应”,而导致失败愈合反应的部分原因是肌腱干细胞的错误分化。目的:通过阅读相关文献,介绍肌腱干细胞的特性,总结影响肌腱干细胞向肌腱细胞分化的因素以及导致肌腱干细胞错误分化(分化为脂肪细胞、骨细胞和软骨细胞)的因素,同时阐述肌腱干细胞在临床中的应用局限。方法:检索PubMed及Web of Science数据库中相关文献,检索词为“Tendon Stem/Progenitor Cells,Tendinopathy,Tendon injury,differentiation”,通过阅读筛选出相关文献,最终纳入109篇文献进行结果分析。结果与结论:①肌腱干细胞是可以自发分化为肌腱的一种干细胞,它具有自我更新、克隆和多向分化的能力,不同的外部条件作用于肌腱干细胞可以导致其向不同方向分化。调节肌腱干细胞命运的具体因素并不确定。当肌腱中的干细胞更新和分化出现异常时,会导致肌腱愈合失败,进而导致肌腱病。②衰老、细胞外基质成分的变化、过度的机械刺激、前列腺素E2和白细胞介素6以及白细胞介素10和一些系统性疾病可能对调控肌腱干细胞的错误分化有重要意义。③促进肌腱干细胞向腱细胞分化的可能有利因素有:一些生长因子和细胞因子、适度的机械刺激和细胞外基质的地形、低氧张力、药物以及某些转录基因和蛋白。④目前最为理想的治疗手段则是对内源性肌腱干细胞进行调节,或者外源性肌腱干细胞刺激内源性肌腱干细胞的增殖分化。⑤未来研究进一步了解调节肌腱干细胞错误分化的因素,可深入了解肌腱病的发病机制并找到治疗靶点,阐述诱导肌腱干细胞向肌腱分化则可促进其在组织工程中的应用。

间充质干细胞治疗跟腱病机制研究进展

跟腱病(AT)无论是通过保守疗法还是手术疗法,都无法完全恢复跟腱的自然成分、组织构造和机械性能。间充质干细胞(MSCs)能够向肌腱组织方向分化,并通过发挥其免疫调控、细胞外基质重塑和血管生成的功能,促进肌腱的愈合。在肌腱的再生和修复过程中,MSCs起着关键的作用,并且具有广阔的应用潜力。现对MSCs在治疗AT中的作用机制进行阐述,以期为AT的研究和治疗提供思路。

生物3D打印仿生支架促进肩袖损伤后的愈合

背景:大多数肩袖损伤发生于冈上肌腱,由于肌腱组织缺少血管以及肩袖复杂的解剖结构,临床治疗效果非常有限。组织工程技术和干细胞生物学的快速发展为提高肌腱修复质量带来了新的希望。目的:通过生物3D打印技术制备人脐带间充质干细胞/甲基丙烯酸酐化明胶复合支架,观察该支架修复肩袖损伤的效果。方法:(1)体外细胞实验:制备明胶微载体,将人脐带间充质干细胞接种于明胶微载体表面构建组织工程化干细胞。制备甲基丙烯酸酐化明胶水凝胶打印墨水,使用甲基丙烯酸酐化明胶水凝胶打印墨水重悬组织工程化干细胞,放入3D打印机的生物墨水容器中进行打印,蓝光照射固化5 min后即得人脐带间充质干细胞/甲基丙烯酸酐化明胶复合支架。通过死活染色、CCK-8实验检测支架内人脐带间充质干细胞的活性。(2)动物体内实验:采用随机区组设计方法将24只SD大鼠随机4组,每组6只:正常组不进行任何处理,肩袖损伤组、单纯支架组、细胞支架组建立冈上肌腱撕裂的肩袖损伤模型,单纯支架组、细胞支架组造模后分别将甲基丙烯酸酐化明胶支架、人脐带间充质干细胞/甲基丙烯酸酐化明胶复合支架植入肌腱损伤处。术后4周,分别进行大鼠行为学测试及肩袖冈上肌腱组织学病理组织形态观察。结果与结论:(1)体外细胞实验:死活染色结果显示,明胶微载体可减轻3D打印过程对人脐带间充质干细胞造成的损伤,随着培养时间的延长,支架内的人脐带间充质干细胞存活率升高;CCK-8实验结果显示,随着培养时间的延长,支架内的人脐带间充质干细胞活性无明显变化。(2)动物体内实验:行为学测试结果显示,相较于肩袖损伤组、单纯支架组,细胞支架组大鼠四肢运动功能明显改善;肩袖冈上肌腱苏木精-伊红与Masson染色结果显示,相较于肩袖损伤组、单纯支架组,细胞支架组肌纤维排列较规律,无明显的炎性细胞浸润,胶原容积百分比降低;免疫荧光染色结果显示,相较于肩袖损伤组、单纯支架组,细胞支架组肩袖冈上肌腱内白细胞介素6、肿瘤坏死因子α蛋白表达明显降低。(3)结果表明,生物3D打印的人脐带间充质干细胞/甲基丙烯酸酐化明胶复合支架可促进肩袖损伤组织修复再生。

富血小板血浆源性外泌体对肌腱干/祖细胞增殖及迁移能力的影响

目的探究富血小板血浆源性外泌体(PRP-Exos)对肌腱干/祖细胞(TSPC)增殖及迁移能力的影响。方法采用聚合沉淀结合超速离心法提取PRP-Exos,采用透射电镜和纳米颗粒跟踪分析技术鉴定其形态、浓度及粒径,采用蛋白免疫印迹法检测外泌体表面标志蛋白及血小板膜糖蛋白的表达水平。提取并培养大鼠TSPC,采用流式细胞术及免疫荧光染色检测TSPC表面标志分子的表达情况。CCK-8法及EdU实验评估PRP-Exos对TSPC增殖情况的影响。细胞划痕实验及Transwell实验评估PRP-Exos对TSPC迁移情况的影响。结果提取的PRP-Exos呈典型茶托状结构,浓度为4.9×10^(11)个/mL,平均粒径为(132.2±56.8)nm,PRP-Exos表面表达CD9、CD63及CD41。提取的TSPC表达CD44蛋白。PRP-Exos可被TSPC摄取,共培养48 h时,浓度为50、100μg/mL的PRP-Exos可显著促进TSPC的增殖(P均<0.001),且两组之间差异无统计学意义(P=0.283);共培养24 h后,浓度为50μg/mL的PRP-Exos可显著促进TSPC的迁移(P<0.001)。结论体外培养条件下,PRP-Exos对大鼠TSPC的增殖及迁移具有显著促进作用。

Does erroneous differentiation of tendon-derived stem cells contribute to the pathogenesis of calcifying tendinopathy？

Calcifying tendinopathy is a tendon disorder with calcium deposits in the mid-substance presented with chronic activity-related pain, tenderness, local edema and various degrees of incapacitation. Most of current treatments are neither effective nor evidence-based because its underlying pathogenesis is poorly understood and treatment is usually symptomatic. Understanding the pathogenesis of calcifying tendlinopathy is essential for its effective evidence-based management. One of the key histopathological features of calcifying tendinopathy is the presence of chondrocyte phenotype which surrounds the calcific deposits, suggesting that the formation of calcific deposits was cellmediated.Although the origin of cells participating in the formation of chondrocyte phenotype and ossification is still unknown, many evidences have suggested that erroneous tendon cell differentiation is involved in the process. Recent studies have shown the presence of stem cells with self-renewal and multi-differentiation potential in human,horse, mouse and rat tendon tissues. We hypothesized that the erroneous differentiation of tendon-derived stem cells （TDSCs） to chondrocytes or osteoblasts leads to chondrometaplasia and ossification and hence weaker tendon, failed healing and pain, in calcifying tendinopathy. We present a hypothetical model on the pathogenesis and evidences to support this hypothesis. Understanding the key role of TDSCs in the pathogenesis of calcifying tendinopathy and the mechanisms contributing to their erroneous differentiation would provide new opportunities for the management of calcifying tendinopathy. The re-direction of the differentiation of resident TDSCs to tenogenic or supplementation of MSCsprogrammed for tenogenic differentiation may be enticing targets for the management of calcifying tendinopathy in e future.

Understanding cellular and molecular mechanisms of pathogenesis of diabetic tendinopathy

There is accumulating evidence of an increased incidence of tendon disorders in people with diabetes mellitus.Diabetic tendinopathy is an important cause of chronic pain,restricted activity,and even tendon rupture in individuals.Tenocytes and tendon stem/progenitor cells(TSPCs)are the dominant cellular components associated with tendon homeostasis,maintenance,remodeling,and repair.Some previous studies have shown alterations in tenocytes and TSPCs in high glucose or diabetic conditions that might cause structural and functional variations in diabetic tendons and even accelerate the development and progression of diabetic tendinopathy.In this review,the biomechanical properties and histopathological changes in diabetic tendons are described.Then,the cellular and molecular alterations in both tenocytes and TSPCs are summarized,and the underlying mechanisms involved are also analyzed.A better understanding of the underlying cellular and molecular pathogenesis of diabetic tendinopathy would provide new insight for the exploration and development of effective therapeutics.

Origin of tendon stem cells in situ

BACKGROUND： Adult stem cells are surveillance repositories capable of supplying a renewable source of progenitors for tissue repair and regeneration to maintain tissue homeostasis throughout life. Many tissue-resident stem cells have been identified in situ, which lays the foundation for studying them in their native microenvironment, i.e. the niche. Within the musculoskeletal system, muscle stem cells have been unequivocally identified in the mouse, which have led to considerable advances in understanding their role in muscle homeostasis and regeneration. On the other hand, for bone and tendon progenitor cells, mesenchymal stem cells have been used as the main in vitro cell model as they can differentiate into osteogenic, chondrogenic and tenogenic fates. Despite considerable efforts and employment of modern tools, the in vivo origins of bone and tendon stem cells remain debated. Tendon regeneration via stem cells is understudied and deserves attention as tendon damage is noted for a bleak, time-consuming recovery and the repaired tendon seldom regains the structural integrity and strength of the native, uninjured state. OBJECTIVE： Here we review the past efforts and recent studies toward defining adult tendon stem cells and understanding tendon regeneration instead of tendon development. The focus is on adult tendon resident cells in situ and the uncertainty of their roles in regeneration. METHODS： A systematic literature search using the Pubmed search engine was conducted encompassing the seminal papers in the tendon field. CONCLUSIONS： Investigation of tendon stem cells in situ is in its infancy mainly due to lack of necessary tools and standardized injury model. We propose a concerted effort toward establishing a comprehensive cell atlas of the tendon, making genetic tools and choosing a reliable injury model for coordinated studies among different laboratories. Increasing our basic understanding should aid future therapeutic innovations to shorten and enhance the tendon repair/regeneration process.

Manifestation of Pathological States of Numerous Diseases in the Largest Organ of the Human Body: (I) Basics and the Diseases of Tendon

We analyze the crucial biochemical and biophysical properties of the basic constituents—connective tissues (CT), and interstitial fluid (IF) constituting the non-cellular part of the fascia. We provide ample evidence that the resident cells and cells in transit in the fascia are continuously interacting with the non-cellular constituents to form an active organ with well-defined functions. We show evidence that pathological states of diseases of internal organs, as well as that of the constituents of the fascia itself, manifest in certain CTIF domains of the fascia. Numerous diseases originate from imbalance of the digestion and synthesis of the native collagen triple helices. Review on the scanning electron microscopy examination of cross-section of tendons indicates that micro-fibrils of collagen I form regular geometrical structures, supporting the hypothesis that the collagen fibrils assemble like liquid crystals. Information on the age of Achilles tendons has been reported, based on dating of the 14C atoms generated from the nuclear bomb tests in 1955-1963. The causes of spontaneous tendon rupture and tendinopathy are analyzed. Plausible clinical measures to treat tendinopathy are briefly discussed, including the application of synthetic mechano-growth factor, glyceryl trinitrate patch (to supply nitric oxide), platelet rich plasma, proteomic profile analysis and microRNA 29a therapy.