**3. Mesenchymal stem cells**

the reduction of costs, with evidence also showing that with fenestrated acellular dermal matrices, the incidence of capsular contractures, infections and seroma formation can be

Complications associated with acellular dermal matrices depend on the type of the acellular dermal matrix used and also the particular procedure it is used for. Breast reconstruction with acellular dermal matrix can cause increased risk of post-operative infection, skin necrosis and post-operative seroma [15]. The correct patient should be identified in order to ensure the risks that are acceptable. Caution needs to be used with obese patients (BMI > 30), simultaneous axillary clearance and smoking history. Radiation will affect any reconstructed breast; however, acellular dermal matrices have, in fact, been shown to reducing the severity of cap-

The use of acellular dermal matrices in abdominal wall reconstruction offers an alternative to a permanent prosthetic mesh and has been in use since mid 2000s [16]. Although some surgeons prefer acellular dermal matrices for abdominal wall reconstructions, concerns have previously been raised regarding the long-term outcomes of acellular dermal matrices as compared to synthetic meshes, with the main worry being the durability. A recent study, however, showed that hernia recurrence rates with acellular dermal matrices were comparable to those done with synthetic mesh—in particular, it was also seen that xenograft acellular dermal matrices led to even lower recurrence rates than human allografts [17]. The question of cost, however, arises again, and synthetic meshes are in fact cheaper than acellular dermal matrices [17].

Outcomes with acellular dermal matrices in breast surgery have already been mentioned (and there is extensive literature for this subject, including a systematic review), but favourable reports have been published on outcomes in pelvic, abdominal, chest wall reconstruction, dural repair, hand surgery, urethral reconstruction and gingival graft procedures, too [6]. Butler et al. successfully used AlloDerm in the reconstruction of large and complex pelvic, chest and abdominal wall defects [18]; however, further studies would be needed in the use of acellular dermal matrices for dural repair (Chaplin et al. successfully used XenoDerm in a porcine model and called it "a nearly ideal dural replacement") [19]. Kim et al. also successfully used acellular dermal matrix for a recurrent first dorsal web space defect, showing excellent cosmetic and functional results [20]. Aichelmann-Reidy et al. showed that acellular dermal matrix could also be a useful substitute in root coverage procedures [21]. Controversies, however, still exist and some studies have shown increased infection rates with ADM-based

Significant costs are also involved when using acellular dermal matrices and remain a topical issue in all fields of surgery. Some situations where costs may be unacceptable have already been considered, for example, with some general surgeons preferencing synthetic meshes in abdominal wall reconstruction due to decreased costs [17]. However, in cases where acellular dermal matrix allows for a two-stage procedure (e.g., implant-based reconstruction with tissue expanders placed during the primary procedure) to be converted into a single-stage procedure (i.e., implant-based ADM reconstruction without the need of tissue expanders), significant savings will be made and this needs to be considered on an indi-

reconstruction as compared to non-ADM-based reconstruction [22].

decreased [5].

52 Tissue Regeneration

sular contracture [15].

vidual patient basis.

The exciting field of stem cell therapies has rapidly evolved in order to provide a potential alternative treatment for tissue repair and to enable the regeneration of injured organs. New developments are continually arising from this promising topic of research.

Stem cells are unique in that they are undifferentiated cells that can renew themselves throughout the entire lifespan of an organism. They develop from one common precursor and have the ability to differentiate into multiple cell types with specific functions (see **Figure 4**). Stem cells are characterised by their ability to self-renew over prolonged periods of time [23]. Stem cells that have the potential to repair surgical wounds include mesenchymal stem cells (MSC), embryonic stem cells (ESC) and induced pluripotent stem cells (iPS) [24].

The most commonly utilised stem cells are MSCs, which are derived from adult patients. Autologous mesenchymal stem cells are present in almost all adult tissues including the dermis, periosteum and adipose tissue, solid organs, such as the liver, lungs and spleen and within bone marrow and blood, including from the peripheries, menstruation and the umbilical cord [25].

There has been great enthusiasm within the literature regarding the potential use of stem cells in tissue regeneration over the last decade. The initial focus of research surrounded the clinical applications of embryonic stem cells. However, over the past decade, there has been a move within the scientific community to research the potential applications of mesenchymal

**Figure 4.** Skeletal regeneration by mesenchymal stem cells: what else (courtesy of Andrades et al. [26]).

stem cells. In comparison to embryonic-derived stem cells, there are less ethical concerns surrounding their cultivation and utilisation [27].

Traditionally MSCs were derived from adult bone marrow using a bone marrow aspirate. However, lately there has been mounting interested in harvesting MSCs from adipose tissue, these are known as adipose-derived stem cells (ASCs). ASCs are of value as they are abundant in supply and easily accessible by means of an excised solid block of tissue or through liposuction techniques [27]. The International Society for Cellular Therapy instituted the following criteria to identify human mesenchymal stem cells (hMSCs) (see **Table 2**) [28].

Adipose tissue is a highly complex tissue derived from mesodermal origin [28]. Its main functions include energy storage, insulation, protection from mechanical injury, endocrine properties and now as a source of multipotent stem cells [29]. It can be classified as brown and white adipose tissue. Thermogenic brown tissue is responsible for energy expenditure and is mostly found in the foetus and new born babies [29]. White adipose tissue is located subcutaneously and intraabdominally and is responsible for energy storage and insulation (**Figure 5**). White tissue tends to be in abundance and thus renders it a viable long-term option for supply of stem cells [25].

Additionally, subcutaneous adipose tissue can be classified as superficial or deep tissue. The differential potential of ASCs may be altered depending on the location of the harvest. Taranto et el. demonstrated varying stem cell properties within subcutaneous tissues dependant on their location. Adipose tissue yielded from superficial tissues demonstrated increased multipotency [31]. One study has shown that ASCs derived from superficial tissues displayed a slightly higher osteogenic potential than from the deep layer [32]. Previous reports suggest that the yield of ASCs is 100–500 times higher in comparison to bone marrow-derived stem cells [30, 32].

Throughout the literature, there are a number of methods described for the cultivation of MSCs. Naderi et al. describes the isolation and cultivation techniques to obtain ASCs [33]. The adipose tissue is chopped and digested by collagenase and centrifuged in the laboratory. Isolated stem cells are cultivated and subsequently differentiated into a variety of different cell lineages. During pre-clinical trials, ASCs have proven to be very stable under cell culture conditions with a normal haploid karyotype remaining following 100 duplications [34]. ASCs can successfully be cryopreserved whilst maintaining their viability therefore ASCs could be potentially stored prior to use [35].

An extensive volume of research investigating the role and mechanism of action of MSCs in wound healing has been undertaken. Motegi et al. and Fromm-Dornieden et al. recently summarised this into two main categories [28, 34]. These include promoting wound healing through: (I) paracrine actions with nearby cells through the release of growth factors and cytokines and (II) differentiation of cells into resident cells to create a scaffolding to encourage healing. The paracrine mechanisms enable numerous growth factors to be secreted including basic fibroblast growth factor (bFGF), hepatocyte growth factor (HGF), keratinocytes growth factor (KGF), transforming growth factor beta (TGF-β) and vascular endothelial growth factor (VEGF), which in turn promotes angiogenesis and therefore wound healing [29, 33]. These growth factors are thought to have anti-inflammatory actions, enhancing wound healing by dampening down inflammation at the wound site [29]. Macrophage recruitment is increased. Macrophages are classified as classically activated (M1) and alternatively activated (M2). M2 macrophages have an important role in the progression of wound healing and it is thought that MSCs increase macrophage polarisation in wounds and therefore enhance wound healing [36]. Endothelial cell recruitment is also increased [36]. MSCs have the ability to differentiate into the resident site-specific cells, including, fibroblasts, myofibroblasts, keratinocytes and pericytes [36].

Scaffold Biomaterials in Tissue Regeneration in Surgery http://dx.doi.org/10.5772/intechopen.73657 55

**Figure 5.** Distribution of brown and white adipose tissue within the human body (courtesy of Kocan et al. [29]).

Cells known as pericytes, with similar features to mesenchymal cells, have been discovered within the blood vessels in multiple organs. Crisan et al. described that certain perivascular cells isolated from various organs, including the skin, showed differentiation into multiple lineages

**<sup>1.</sup>**Proliferation *in vitro* as plastic-adherent cells.

**<sup>2.</sup>**Positive expression of CD105, CD73, CD90 and negative expression of the haematopoietic cell surface markers CD45, CD34, and CD14, CD11b and CD79α, or CD19 and HLA-DR.

**<sup>3.</sup>**Differentiation into osteoblasts, adipocytes and chondrocytes in culture conditions *in vitro.*

**Table 2.** Adapted from 'Mesenchymal stem cells: The roles and functions in cutaneous wound healing and tumour growth' [27].

stem cells. In comparison to embryonic-derived stem cells, there are less ethical concerns sur-

Traditionally MSCs were derived from adult bone marrow using a bone marrow aspirate. However, lately there has been mounting interested in harvesting MSCs from adipose tissue, these are known as adipose-derived stem cells (ASCs). ASCs are of value as they are abundant in supply and easily accessible by means of an excised solid block of tissue or through liposuction techniques [27]. The International Society for Cellular Therapy instituted the following

Adipose tissue is a highly complex tissue derived from mesodermal origin [28]. Its main functions include energy storage, insulation, protection from mechanical injury, endocrine properties and now as a source of multipotent stem cells [29]. It can be classified as brown and white adipose tissue. Thermogenic brown tissue is responsible for energy expenditure and is mostly found in the foetus and new born babies [29]. White adipose tissue is located subcutaneously and intraabdominally and is responsible for energy storage and insulation (**Figure 5**). White tissue tends to be in abundance and thus renders it a viable long-term option for supply of stem cells [25].

Additionally, subcutaneous adipose tissue can be classified as superficial or deep tissue. The differential potential of ASCs may be altered depending on the location of the harvest. Taranto et el. demonstrated varying stem cell properties within subcutaneous tissues dependant on their location. Adipose tissue yielded from superficial tissues demonstrated increased multipotency [31]. One study has shown that ASCs derived from superficial tissues displayed a slightly higher osteogenic potential than from the deep layer [32]. Previous reports suggest that the yield of ASCs is 100–500 times higher in comparison to bone marrow-derived stem cells [30, 32]. Throughout the literature, there are a number of methods described for the cultivation of MSCs. Naderi et al. describes the isolation and cultivation techniques to obtain ASCs [33]. The adipose tissue is chopped and digested by collagenase and centrifuged in the laboratory. Isolated stem cells are cultivated and subsequently differentiated into a variety of different cell lineages. During pre-clinical trials, ASCs have proven to be very stable under cell culture conditions with a normal haploid karyotype remaining following 100 duplications [34]. ASCs can successfully be cryopreserved whilst maintaining their viability therefore ASCs could be

An extensive volume of research investigating the role and mechanism of action of MSCs in wound healing has been undertaken. Motegi et al. and Fromm-Dornieden et al. recently summarised this into two main categories [28, 34]. These include promoting wound healing through: (I) paracrine actions with nearby cells through the release of growth factors and cytokines and (II) differentiation of cells into resident cells to create a scaffolding to encourage healing. The

**2.**Positive expression of CD105, CD73, CD90 and negative expression of the haematopoietic cell surface markers

**Table 2.** Adapted from 'Mesenchymal stem cells: The roles and functions in cutaneous wound healing and tumour

criteria to identify human mesenchymal stem cells (hMSCs) (see **Table 2**) [28].

rounding their cultivation and utilisation [27].

54 Tissue Regeneration

potentially stored prior to use [35].

**1.**Proliferation *in vitro* as plastic-adherent cells.

growth' [27].

CD45, CD34, and CD14, CD11b and CD79α, or CD19 and HLA-DR.

**3.**Differentiation into osteoblasts, adipocytes and chondrocytes in culture conditions *in vitro.*

**Figure 5.** Distribution of brown and white adipose tissue within the human body (courtesy of Kocan et al. [29]).

paracrine mechanisms enable numerous growth factors to be secreted including basic fibroblast growth factor (bFGF), hepatocyte growth factor (HGF), keratinocytes growth factor (KGF), transforming growth factor beta (TGF-β) and vascular endothelial growth factor (VEGF), which in turn promotes angiogenesis and therefore wound healing [29, 33]. These growth factors are thought to have anti-inflammatory actions, enhancing wound healing by dampening down inflammation at the wound site [29]. Macrophage recruitment is increased. Macrophages are classified as classically activated (M1) and alternatively activated (M2). M2 macrophages have an important role in the progression of wound healing and it is thought that MSCs increase macrophage polarisation in wounds and therefore enhance wound healing [36]. Endothelial cell recruitment is also increased [36]. MSCs have the ability to differentiate into the resident site-specific cells, including, fibroblasts, myofibroblasts, keratinocytes and pericytes [36].

Cells known as pericytes, with similar features to mesenchymal cells, have been discovered within the blood vessels in multiple organs. Crisan et al. described that certain perivascular cells isolated from various organs, including the skin, showed differentiation into multiple lineages both *in vitro* and *in vivo* [37]. This research surmised that it is likely that blood vessel walls may hold a reserve of mesenchymal-like stem cells that are involved in the repair and neovascularisation of wounds. However, the exact mechanisms and significance remains unknown.

In addition to skin wound healing, there have been advances within the role of stem cells in orthopaedics. A recent study focused on the role ASCs in the repair of meniscal injuries. Toratani et al. created meniscal defects in rabbits and injected autologous stem cells from adipose tissue into half of the subjects [43]. ASCs were found to promote meniscus healing in the rabbit model. This paper offers promise for future clinical uses as a potential new treatment

Scaffold Biomaterials in Tissue Regeneration in Surgery http://dx.doi.org/10.5772/intechopen.73657 57

Stem cells could potentially revolutionise the treatment of chronic heart disease. Atherosclerosis is the leading cause of morbidity and mortality in the developed world with risk factors including diabetes, hypertension, smoking and obesity. Researchers have endeavoured to develop a stem cell-based therapy for the treatments of ischaemic heart disease and cardiac failure. Numerous preclinical studies have demonstrated promising therapeutic benefits using ASCs with the improvement in left ventricular function and reduction in infarct size [44]. However, these successful results have yet to be seen in human trials. The difference in results is thought to be due to the source of stem cells. In animal trials, MSCs were harvested from healthy donors; however, in comparison in the clinical trials, the stem cells were collected from the patient with known atherosclerotic disease and potentially other serious co-morbidities [45]. Further research in this field continues to evolve to in order to create a successful therapy.

Other novel tissue regeneration methods have been trialled in both animal and human studies. For example, genetics is an ever-evolving field when it comes to finding ways and methods of aiding tissue regeneration. Animal studies provide a starting point for future discoveries—for example, Kang et al. investigated tissue regeneration enhancer elements (TREEs), providing evidence that these elements trigger gene expression in injury sites [46]. The authors of this particular study felt that these findings could further be extrapolate in the future to assess their regenerative potential in vertebrate organs [46]. Gene regulation to aid tissue regeneration has been investigated in human studies, too. Recent studies by Finkel et al. and Mendell et al. showed promise in motor neurone diseases, specifically spinal muscular atrophy [47, 48]. Finkel et al. modified promoted increased production of the survival motor neurone (SMN) protein with an antisense oligonucleotide drug and showed that infants with spinal muscular atrophy receiving this drug were more likely to be alive and have improved motor function that the control group [47]. Patients in the Mendell et al. study received adeno-associated viral vector infusion containing DNA coding for SMN; these patients again, survived longer, achieved motor milestones better and had improved motor function than historical cohorts [48]. Musculoskeletal tissue regeneration is a great challenge for scientists and lots of studies have looked into potential options, in addition to the two mentioned already. Padilla et al. discuss a variety of techniques, including blood derived biological drug delivery therapies, which have significant potential for tissue regeneration [49]. For example, platelets release hepatocyte growth factor and stromal cell-derived growth factor 1, both known to be involved in wound healing and proliferation [49]. There is a significant need for further randomised trials and systematic reviews to assess if these therapies could be used routinely for the treatment of musculoskeletal

for meniscal injuries subject to further studies.

**4. Gene regulation**

Patient selection for harvesting is an important factor because biologic properties can be affected by systemic disease. Adipose tissue that is extracted from patients with diabetes is inferior to adipose tissue that has not been subjected to systemic disease. In tissue exposed to systemic illness, there is loss of cell differentiation ability, increased levels of failed division and apoptosis and an overall reduction in the levels of growth factors secreted [38].

There are few human clinical trials involving the applications of MSCs and even fewer evaluating the utilisation of adipose cells. The current use of ASCs in clinical practice remains limited to trials. A number of animal model studies have demonstrated the promising possibilities of adipose-derived stem cells and there are a number of small pilot clinical trials, which have been published in the literature recently with many new studies emerging frequently. This exciting data gives promise to the potential clinical applications of ASCs and with new information continuing to evolve, the routine use of stem cells in clinical practice remains a tangible prospect in the near future. This section of the chapter provides up to date evidence and a summary of recent studies involving ASCs.

Nie et al. investigated the mechanisms of action of ASCs in wound healing [39]. ASCs were incorporated into full thickness excisional wounds in both diabetic and non-diabetic rats. The study showed that wound healing was accelerated and time taken to close wounds in both groups was shortened. There was increased re-epithelisation and advanced development of granulation tissue within the wound. Enhanced neovascularisation was also shown due to the increased secretion of angiogenic growth factors.

Park et al. recently investigated the role of allogenic ASCs in the treatment of complex perianal fistulas secondary to Crohn's disease [40]. In this small pilot multicentre, clinical trial participants had complex non-healing perianal fistulas, which had not healed by conventional techniques (surgery or infliximab treatment). The initial group of participants received a smaller dose of ASCs than the second group. At 6-month follow-up, 50% of participants had achieved complete closure of the fistula, which was maintained at the final follow-up at 8 months.

A phase one clinical trial demonstrated the effect of autologous-derived adipose stem cells in patients with severe peripheral arterial disease with chronic non-healing ulcer disease. All participants had non-vascularisable critical limb ischaemia with lower limb rest pain or ulcers and a low ankle systolic oxygen pressure. ASCs were injected intramuscularly into the ischaemic limb with no complications recorded. Most participants showed an increase in trans-cutaneous oxygen pressure and improved ulcer healing [41].

Kim et al. studied the effectiveness of stem cell treatment in patients with chronic non-healing wounds following complications of soft tissue nasal fillers [42]. ASCs were harvested from the patient's subcutaneous adipose tissue. Following preparation in the laboratory, the adipose cell containing solution was injected into the dermis and subcutaneous layer around the wound. All participants experienced enhanced wound healing and at 6 months post treatment all wound sizes were reduced. These results lead the authors to propose that stem cells could be considered in the future for routine use as a treatment of complications of filler injections.

In addition to skin wound healing, there have been advances within the role of stem cells in orthopaedics. A recent study focused on the role ASCs in the repair of meniscal injuries. Toratani et al. created meniscal defects in rabbits and injected autologous stem cells from adipose tissue into half of the subjects [43]. ASCs were found to promote meniscus healing in the rabbit model. This paper offers promise for future clinical uses as a potential new treatment for meniscal injuries subject to further studies.

Stem cells could potentially revolutionise the treatment of chronic heart disease. Atherosclerosis is the leading cause of morbidity and mortality in the developed world with risk factors including diabetes, hypertension, smoking and obesity. Researchers have endeavoured to develop a stem cell-based therapy for the treatments of ischaemic heart disease and cardiac failure. Numerous preclinical studies have demonstrated promising therapeutic benefits using ASCs with the improvement in left ventricular function and reduction in infarct size [44]. However, these successful results have yet to be seen in human trials. The difference in results is thought to be due to the source of stem cells. In animal trials, MSCs were harvested from healthy donors; however, in comparison in the clinical trials, the stem cells were collected from the patient with known atherosclerotic disease and potentially other serious co-morbidities [45]. Further research in this field continues to evolve to in order to create a successful therapy.
