**2. Musculoskeletal injuries**

#### **2.1. Osteoarticular disorders and regeneration physiology**

The articular cartilage (AC) is a thin connective tissue layer that covers the bone extremities of the joint [4]. The AC presents a notable matrix structure organization [6], a limited number of chondrocytes [7], and a rich water content [8]. The most important biomechanical functions of the AC include weight bearing and a smooth distribution of forces to the adjacent subchondral bone, providing nonfrictional motion of joints [4, 8]. The AC is divided into three layers. The most superficial one is thin, with a smooth surface. In this layer, the collagen fibers are aligned parallel to the tissue surface. In the middle, the articular cartilage is constituted by larger collagen fibers, with a nonparallel organization structure. The deep zone has a parallel alignment of the collagen fibers, vertically to the tissue surface [8]. The unique matrix structure, rich in collagen fibers, proteoglycans, and interstitial fiber, provides a viscoelastic environment that allows the AC to support its biomechanical functions [8]. It has been well established that the AC has limited self-healing capacities [3–5, 7–15] due to its intrinsic characteristics, namely its avascular nature, limited number of resident stem cells, and unique matrix organization [4]. Partial defects on mature cartilage do not heal spontaneously. On the other hand, complete defects are associated with the formation of fibrocartilage, which presents inferior mechanical characteristics [4]. Injuries affecting both, AC and subchondral bone, named osteochondral lesions, often evolve to secondary osteoarthritis (OA) [8, 9]. OA is a syndrome, characterized by AC degeneration, matrix loss, fissure formation, culminating with defects on the cartilage surface, and impacting on surrounding articular tissues, such as the subchondral bone, joint ligaments, the synovial membrane (SM), and periarticular muscle tissue [16], culminating with joint dysfunction and severe pain [9]. OA is one of the most frequent diseases affecting individuals worldwide, thus representing a major impact on the society's health [17]. Many other diseases culminate in OA, if not diagnosed early and treated, such as osteochondritis dissecans, affecting specially teenagers and young adults [18].

Chondrocytes are highly specialized cells, responsible for the production and maintenance of healthy cartilage matrix [19, 20]. However, these cells are particularly differentiated, with poor migration and proliferation abilities; thus, treatment represents a problematic challenge [17]. Several surgical treatment approaches have been developed in the past years. However, they all have inherent problems, impacting on patients' long-term healing process [21]. Surgical procedures that stimulate the bone marrow (BM), such as abrasion, distraction, drilling, and microfractures, are said to promote chondrogenesis phenomena, by inducing the BM mesenchymal stem cells (MSCs) from the subchondral bone. However, in most cases, these techniques lead to the formation of fibrocartilaginous tissue, instead of hyaline cartilage, probably due to an overloading of the BM and a small number of MSCs available, and the repaired cartilage often degenerates in the long term [3–5, 11]. Alternative regenerative approaches, regarding cartilage tissue engineering, are being developed, in order to overcome these disadvantages. Mosaicplasty is characterized by the transplantation of various small autologous osteochondral grafts to the injured joint site [13]. This procedure, however, is not suitable for OA patients or suffering from rheumatoid arthritis (RA), as chondrocytes in these patients have different biological properties [14]. This procedure promotes a shortterm relief on patient's symptoms but fails to repair the damaged tissue and hyaline cartilage [4]. RA is a systemic autoimmune disease, characterized by a continuous inflammation phenomenon, a result of an intrinsic imbalance, culminating in a major synovial hyperplasia, bone, and cartilage damage [15]. Treatments involving artificial prosthesis are quite invasive and lifetime limited [9], as well as the mosaicplasty treatment technique is invasive and causes damages to the donor site [13] and fails to restore functional, as well as, phenotypically stable hyaline cartilage [4].

healing processes [2–5]. Different treatment techniques have been developed in the past years,

**Figure 2.** Synovial membrane-derived MSCs obtained from enzymatic digestion and explants technique; images of

**Figure 1.** Synovial membrane-derived MSC can be obtained from different species. From the left to the right: canine, equine, and human. Images of cultured cells were obtained from the work developed within our research group.

The purpose of this chapter is to review on the available literature regarding synovial membrane-derived MSC therapies applied to musculoskeletal disorders, both in human and veterinary medicine. **Figure 1** illustrates synovial membrane-derived MSC from three different

MSCs can be obtained from the synovial membrane tissue through two different procedures:

We will address the musculoskeletal injuries and intrinsic repair mechanism and MSC sources applicable for its treatment, focusing on the advantages of synovial membrane-derived MSCs.

The articular cartilage (AC) is a thin connective tissue layer that covers the bone extremities of the joint [4]. The AC presents a notable matrix structure organization [6], a limited number of

but until now, no ideal regenerative treatment approach has yet been established [2, 4].

enzymatic digestion and explants technique, both illustrated in **Figure 2**.

isolated cells were obtained from the work developed within our research group.

**2.1. Osteoarticular disorders and regeneration physiology**

species: canine, equine, and human.

78 Tissue Regeneration

**2. Musculoskeletal injuries**

Autologous chondrocyte transplantation (ACI) is a cell-based technique that consists of harvesting chondrocytes from a nonweight bearing joint, first reported by Brittberg et al. [9]. Chondrocytes are expanded *in vitro* in a monolayer culture and then implanted in the lesion site. Despite the small amounts of donor cartilage used, it is necessary to minimize the invasiveness of the technique [13]. During the *in vitro* expansion period, many chondrocytes dedifferentiate and become unsuited to produce stable hyaline cartilage, thus impacting the final clinical outcome [4, 18]. Further, an uneven distribution of the transplanted chondrocytes at the lesion site is very common, as well as the diffusion of the cells from the cartilage defect [8]. To overcome these difficulties, transplantation of tissue-engineered cartilage was developed, evolving *ex-vivo* techniques, however with short-term successful results [18], in part due to intrinsic characteristics of the AC, as its antiadherent properties, which do not facilitate the integration of repaired tissue into the adjacent cartilage tissue [8].

Primary myopathies are characterized by a progressive atrophy of skeletal muscle fibers, thus resulting in deterioration, and compromising movements [2]. As the intrinsic repair ability of the mature skeletal muscle is limited, and pharmacology suppression of the inflammatory and immune response only provides a mild and finite effect, alternative cellular therapies have been developed, aiming at promoting the healing process [1, 2, 22]. Myoblasts would be an obvious choice, due to their role in the muscle repair mechanism. However, they are poorly expandable *in vitro* and undergo senescence quite easily [2, 22]. It is reported that about 90% of the transplanted myoblast cells die within the first hours [25]. Most genetic muscular disorder defects lie in the protein binding between the extracellular matrix and the cytoskeleton of the muscle cell, thus resulting in mechanical stress and continuous contraction movements, leading to muscle degeneration, and consequent tissue loss [2]. The Duchenne muscular dystrophy (DMD) is one of the most common genetic disorders in children. It is characterized by the lack of dystrophin at the muscle fiber sarcolemma. This disorder results in progressive and irreversible muscle degeneration and consequent death [1, 22]. Regarding these genetic disorders, myoblasts exist in small number and are not easily recovered in muscle biopsies [22]. Moreover, in the earlier stages of the disease, SCs divide to form myoblasts that fuse to the existing muscle fibers. However, those SCs transport the exact same genetic defect as the

Synovia-Derived Mesenchymal Stem Cell Application in Musculoskeletal Injuries: A Review

http://dx.doi.org/10.5772/intechopen.74596

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other muscle cells they are replacing. Thus, they will eventually die too [2].

**3. MSC sources applicable for musculoskeletal regeneration**

blood and stroma [29].

in the actual days.

Regenerative medicine approaches regarding stem cell therapies have been developed in the past decades as a promising strategy, focusing primarily on immune/anti-inflammatory modulation [15] and cancer treatment [29]. Furthermore, their potential has been employed in cartilage [30] and skeletal muscle repair [1], the latter in a more immature state of development.

MSCs represent a fair candidate to innovative therapies because of their intrinsic unique abilities. MSCs were first harvested from the bone marrow by Friedenstein in 1976 [29, 31, 32], but now their presence is well established in virtually all postnatal tissues [5, 31–34], being involved in the tissue growth and homeostasis [31]. They have since been isolated from different adult tissues [30–32, 35, 36], such as fat, bone marrow, bone, cartilage [6], periosteum [32], nervous tissues, tendon, ligament, epithelium, SM, lung, peripheral blood, skeletal muscle, and nonadult tissues, such as amniotic fluid, placenta, and umbilical cord

MSCs are plastic-adherent cell, fibroblast-like [29, 36], able to self-renew [32, 37]. They are characterized by an extensive proliferation ability in culture and have the potential to differentiate *in vitro* into different lineages [5, 32, 33, 36], including adipogenic, chondrogenic,

MSCs' unique characteristics explain the interest of application on the development of regenerative cell therapies: ease of isolation, high expansion rates *in vitro*, low immunogenicity, and multipotency [4]. However, MSCs' definition and characterization still represent a challenge

osteogenic [6, 29, 31, 35], myogenic [6, 31, 35, 37], and neurogenic [6, 37].

#### **2.2. Skeletal muscle injuries and repair mechanisms**

Musculoskeletal disorders also impair the life and well-being of millions of individuals. They are usually characterized by long and incomplete healing processes that culminate into permanent musculoskeletal lesions [1].

Regarding the muscular tissue, in specific, the skeletal muscle, its constitution includes syncytial fibers that are characterized by the presence of a peripheral, postmitotic myonuclei [22]. Under experimental conditions, the skeletal muscle presents notable regeneration ability. Concerning clinical disorders, injuries or ischemia results in considerable tissue loss, that is, generally, not replaced [23]. In an adult, the intrinsic healing capacity of the skeletal muscle tissue relies on the presence of a resident, mononuclear, undifferentiated cell population, known as satellite cells (SCs) [22]. These cells are located between the sarcolemma of myofibers and the basal lamina [1, 24] and have the ability to migrate considerable distances, within the muscle tissue [23]. In a mature, healthy musculoskeletal tissue, SCs are predominantly on a mitotically quiescent state and respond to environmental signaling [22]. It is well established that microenvironmental signals are responsible for gene reprograming and cell phenotype changes [25]. Those signals, resulting from biophysical phenomena, such as growth, injuries, or weight bearing, induce existing SC to proliferate, differentiate, and fuse to existing muscle fibers, thus, mediating postnatal muscle regeneration [22, 26]. These environmental signals comprehend the release of growth factors from the impaired muscle fibers [1, 23], more accurately, myogenic regulatory factors (MRF), which are MyoD, Myo5, myogenin, and MRF4 [22]. The first two growth factors have a more active role during the embryonic development of the skeletal muscle lineage. After division, the SCs become myoblasts, which undergo a terminal differentiation process and fusion to the preexisting muscle fibers. Myogenin is responsible for promoting the terminal differentiation process and fusion [22]. MyoD also plays an important role by promoting the beginning of the proliferation phase of the SC. The absence of MyoD implies a cycle where SCs suffer several division rounds but return to a quiescent state [27]. On the other hand, lack of myogenin causes a severe deficit in the muscle tissue differentiation, resulting in the formation of unfunctional muscle fibers [28]. Thus, satellite cells recapitulate the MRF expression from the embryonic stage, during muscle repair processes. But, when in a quiescent state, SCs do not express detectable levels of MRF [1].

Primary myopathies are characterized by a progressive atrophy of skeletal muscle fibers, thus resulting in deterioration, and compromising movements [2]. As the intrinsic repair ability of the mature skeletal muscle is limited, and pharmacology suppression of the inflammatory and immune response only provides a mild and finite effect, alternative cellular therapies have been developed, aiming at promoting the healing process [1, 2, 22]. Myoblasts would be an obvious choice, due to their role in the muscle repair mechanism. However, they are poorly expandable *in vitro* and undergo senescence quite easily [2, 22]. It is reported that about 90% of the transplanted myoblast cells die within the first hours [25]. Most genetic muscular disorder defects lie in the protein binding between the extracellular matrix and the cytoskeleton of the muscle cell, thus resulting in mechanical stress and continuous contraction movements, leading to muscle degeneration, and consequent tissue loss [2]. The Duchenne muscular dystrophy (DMD) is one of the most common genetic disorders in children. It is characterized by the lack of dystrophin at the muscle fiber sarcolemma. This disorder results in progressive and irreversible muscle degeneration and consequent death [1, 22]. Regarding these genetic disorders, myoblasts exist in small number and are not easily recovered in muscle biopsies [22]. Moreover, in the earlier stages of the disease, SCs divide to form myoblasts that fuse to the existing muscle fibers. However, those SCs transport the exact same genetic defect as the other muscle cells they are replacing. Thus, they will eventually die too [2].
