**3. Utilisation of mesenchymal stem cells in wound healing**

### **3.1. Definition of mesenchymal stem cells**

MSCs can be defined as a heterogeneous population of cells which are non‐hematopoietic stem cells with the potential capacity to differentiate into various somatic lineages and tissues of both mesenchymal and non‐mesenchymal origin [67]. Song et al. [68] have defined MSCs as a type of stem cell population capable of self‐renewing and differentiating into different cell types with pluripotent potential. On the other hand, MSCs have also been termed marrow stromal cells, or fibroblastoid colony‐forming unit (FCFU) [69], mesenchymal stem cells, multipotent mesenchymal stromal cells or stromal progenitor cells [23]. MSCs can be isolated from different tissues; however, they share the major criteria defining MSCs with minor differences related to their differentiation capacity and cell surface expression profile [67, 70]. These differences have challenged the definition of MSCs. In 2006, the International Society for Cellular Therapy (ISCT) attempted to demystify the nomenclature of MSCs suggesting that the term mesenchymal stem cells should only be referred to the cells which are characterised by the specific criteria, while the nomenclature of multipotent mesenchymal stromal cells should be used to describe the fibroblast‐like plastic‐adherent population irrespective of their source of origin [16, 23, 71]. Three minimal criteria have been agreed to become consensus characteristics shared by human MSCs [16, 23, 72, 73]. These criteria are:


### **3.2. Clinical applications of mesenchymal stem cells**

MSCs have been considered as safe irrespective to therapeutics since there is no critical inverse or side effect of MSCs has been detected on disease conditions [74, 75]. The characteristics of MSCs make them good candidates for regenerative medicine and tissue engineering [8, 25]. The most popular application of MSCs in regenerative medicine is in wound healing and skin regeneration [76]. However, MSCs have other clinical applications including ameliorating tissue damage in nearly all the major organs in the body such as skin regeneration, cardiac therapy, hepatic cirrhosis [23, 77, 78], brain, lung and kidney repair [23, 77]. In addition, MSCs can be used for pancreatic regeneration, neurological defects, limb ischemia, graft‐versus‐host disease (GvHD), rheumatoid arthritis, osteoarthritis (OA) and other bone and cartilage disorders [78]. The availability of MSCs in normal human skin suggests that these cells potentiate vital functions in wound healing and could be a promising solution for the treatment of chronic wounds [8, 25].

### **3.3. Mesenchymal stem cells in skin regeneration**

**3. Utilisation of mesenchymal stem cells in wound healing**

characteristics shared by human MSCs [16, 23, 72, 73]. These criteria are:

**1.** The isolated MSCs should possess plastic adherence ability.

CD79a and HLA‐DR surface molecules.

**3.2. Clinical applications of mesenchymal stem cells**

of chronic wounds [8, 25].

MSCs can be defined as a heterogeneous population of cells which are non‐hematopoietic stem cells with the potential capacity to differentiate into various somatic lineages and tissues of both mesenchymal and non‐mesenchymal origin [67]. Song et al. [68] have defined MSCs as a type of stem cell population capable of self‐renewing and differentiating into different cell types with pluripotent potential. On the other hand, MSCs have also been termed marrow stromal cells, or fibroblastoid colony‐forming unit (FCFU) [69], mesenchymal stem cells, multipotent mesenchymal stromal cells or stromal progenitor cells [23]. MSCs can be isolated from different tissues; however, they share the major criteria defining MSCs with minor differences related to their differentiation capacity and cell surface expression profile [67, 70]. These differences have challenged the definition of MSCs. In 2006, the International Society for Cellular Therapy (ISCT) attempted to demystify the nomenclature of MSCs suggesting that the term mesenchymal stem cells should only be referred to the cells which are characterised by the specific criteria, while the nomenclature of multipotent mesenchymal stromal cells should be used to describe the fibroblast‐like plastic‐adherent population irrespective of their source of origin [16, 23, 71]. Three minimal criteria have been agreed to become consensus

**2.** More than 95 % of the isolated MSCs must express CD73 (SH3), CD90 and CD105 (HS2), and more than 98 % of the isolated MSCs do not express CD14, D19, CD34, CD45, CD11b,

**3.** The isolated MSCs have the capacity to differentiate into osteoblastic, chondrogenic and

MSCs have been considered as safe irrespective to therapeutics since there is no critical inverse or side effect of MSCs has been detected on disease conditions [74, 75]. The characteristics of MSCs make them good candidates for regenerative medicine and tissue engineering [8, 25]. The most popular application of MSCs in regenerative medicine is in wound healing and skin regeneration [76]. However, MSCs have other clinical applications including ameliorating tissue damage in nearly all the major organs in the body such as skin regeneration, cardiac therapy, hepatic cirrhosis [23, 77, 78], brain, lung and kidney repair [23, 77]. In addition, MSCs can be used for pancreatic regeneration, neurological defects, limb ischemia, graft‐versus‐host disease (GvHD), rheumatoid arthritis, osteoarthritis (OA) and other bone and cartilage disorders [78]. The availability of MSCs in normal human skin suggests that these cells potentiate vital functions in wound healing and could be a promising solution for the treatment

adipogenic lineages under *in vitro* standard differentiation conditions.

**3.1. Definition of mesenchymal stem cells**

106 Wound Healing - New insights into Ancient Challenges

Wounding in mammals evokes two types of biological responses: tissue regeneration and wound repair. Recently, skin regeneration especially cutaneous regeneration via MSCs leads to accelerated wound closure, re‐epithelialisation and angiogenesis [7]. BM‐MSCs transplanted into the injury site expressing keratinocyte‐specific protein (KSP) form glandular structures [79, 80]. One of these successful studies is the induction of BM‐MSCs to acquire phenotypic characteristics of sweat gland cells (SGCs) *in vitro* followed by re‐transplantation of these cells into fresh wounds in five patients and resulted in recovery of functional sweat gland participating in perspiration function during 2 to 12 months follow‐up [81]. Another study focusing on chronic diabetic foot ulcers showed that injection of a biografts consisting of a combination of MSCs and autologous skin fibroblasts resulted in increase of both dermal thickness and vascularity and decreased wound size [82, 83]. Another study has showed that MSCs acquire phenotypic characteristics of epidermal cells or vascular endothelial cells after *in vitro* culture in media supplemented with EGF or VEGF, respectively [82]. MSCs also undergo trans‐differentiation into keratinocytes enabling them to interact with the original epidermal cells suggesting that MSCs can participate directly in tissue regeneration of both dermal and epidermal cells [30]. These characteristics, collectively, reveal the plasticity of MSCs and suggesting them promising therapeutic for the regeneration of skin and consequently wound healing [81].

### **3.4. Modes of action of mesenchymal stem cells in wound healing**

The wound‐healing process requires interaction between cells, extracellular matrix proteins (EMP) and biomolecules such as growth factors, cytokines and chemokines in which MSCs are a pivotal player in the coordination of the repair processes [11, 84]. Differentiation and

**Figure 1.** Potential applications of MSCs in wound healing. MSC therapy contributes to skin wound healing via two mechanisms: (1) Differentiation into skin‐like cells, thereby compensating for the loss cells due to damaged tissue. (2) Promote proliferation and migration of skin cells into the injury site by secreting soluble factors and macrovesicles. MSC secretions represented by MSC‐CM and MSC‐EXOSOME can be either injected onto the wounded skin area or applied on skin wound using biofilm dressings.

paracrine signalling have both been implicated as mechanisms by which MSCs recruit other host cells in all steps of healing process to improve tissue repair [23]. To better understand the role of MSCs in wound healing, we have divided their participation in repair into two major mechanisms: (1) cell‐mediated repair and (2) secretory‐mediated repair (**Figure 1**).
