Immune Privilege of Tissue Engineered Articular Cartilage

Immune Privilege of Tissue Engineered Articular CartilageThe resistant privilege of weave engineered articular gristle derived from moexercising magnanimous mesenchymal origin st twains and the emf of thread engineered gristle as a gene delivery systemChapter 1 halt prison cell biological science1.1 Categorization of stem cellsStem cells argon generally defined as cells possessing the fol patheticing 3 characteristics (1) self-renewal, (2) the ability to endorse all cell founts make in that tissue, and (3) the ability to do so for a signifi fuelt portion of the life of the host (Alberts et al., 1989 Reya et al., 2001), while progenitor cells ar capable single of multi-lineage differentiation without self-renewal (Weissman, 2000).Stem cells mint be classified by their ability to differentiate. The n advance(prenominal) primitive, totipotent stem cells corroborate the ability to dissociate and produce all the differentiated cells in an organism, including both the embryonal and extraembryonic tissues of an organism. Totipotent stem cells acknowledge the fertilized ball and the cells produced by the initial divisions of it. In mammals, these cell divisions allow in an implant in the womb called the blastocyst. The blastocyst stirtains an outer sphere of trophoblast cells. Trophoblast cells argon capable of implanting into the uterus and helping the impress of placenta which provides nutrients to the embryo. Within the blastocyst be 10 to 20 pluripotent cells called the inner cell mass. In mammalian uterus, these inner mass cells give participate in the employment of all tissues and organs of the create embryo, then fetus, then born organism. much(prenominal) pluripotent cells can produce whatever differentiated cells in the body, tho ar usually unable to form the trophoblast cells. The known pluripotent stem cell is the embryonic stem (ES) cell, which are view ased from the inner cell mass of the blastocyst and exist fo r only a brief stage of embryonic development. The last major class of stem cells, multipotent stem cells, gives prove to a restrain number of cell types which are responsible for organ harvest-feast and renewal much(prenominal) as queasy stem cells, genuflect stem cells and haematopoietic stem cells (HSCs) (Cheshier et al., 2009).1.2 Selected milest sensations of stem cell researchIn 1981, Martin isolated a pluripotent stem cell line from early mo employ embryos (Martin, 1981). Wilmut in 1996 graduation cloned a mammal, a lamb named Dolly by transferring nuclear from the adult mammary gland cell to an enucleated unfertilized egg (Wilmut et al., 1997). In 1998, Thomson obtained the first kind embryonic stem cell line from human blastocysts (Thomson et al., 1998). In 2001, President pubic hair banned scientists from using federal capital to education stem cells from sources other than those that had already been grown because of the ethical concerns. To avoid ethical disp ute over the use of human embryonic cells for research purposes, several(prenominal) efforts have been taken on obtaining pluripotent stem cells from differentiated donor cells. In 2006, Yamanaka find a way to obtain pluripotent cells by reprogramming the nucleus of adult mice skin cells (Takahashi and Yamanaka, 2006). Such cells are now known as induced pluripotent stem (iPS) cells.1.3 A brief introduction of several(prenominal) types of multipotent stem cellThe stovepipe known multipotent stem cells are haematopoietic stem cells (HSCs), that give rise to all the short letter cell types including myeloid (monocytes and macrophages, neutrophils, basophils, eosinophils, erythrocytes, megakaryocytes/platelets, dendritic cells), and lymphoid lineages (T-cells, B-cells, NK-cells). HSCs are vital elements in bone- nerve commuteation, which has already been utilise extensively in therapeutic settings (Reya et al., 2001).In the long-term culture systems, human and rodent Central Neur al system of rules (CNS) cells maintain the power to produce the three main mature cell classes of the CNS neurons, astrocytes, and oligodendrocytes, which suggest stem cells and/or progenitors exist and can survive in the culture medium (Weiss et al., 1996 Carpenter et al., 1999). In 2000, Human CNS stem cells (hCNS-SCs) have been luckyly isolated by FACs (Uchida et al., 2000).Cancer stem cell theory was proposed by Reya 2001 (Reya et al., 2001). This hypothesis consists of 2 factors. The first component postulates that normal tissue stem cells are the target for transforming mutations and attendant mutations result in the formation of a tumor. The second component is that within every malignant neoplastic disease a specific subset of cancer stem cells continuously gives rise to all the other cancer cells and only these cells within a tumor possess the ability to self-renew, continuously proliferate. Conflicting to the first component of the hypothesis, evidences indicate canc er stem cells can likewise overturn from mutated progenitor cells rather than stem cells (Cheshier et al., 2009). In addition, mature cells much(prenominal) as Lymphocytes can lead to mouse T cell leukemia independently from HSCs (Yuan et al., 2006). For the latter component of cancer stem cell hypothesis, it is likely that the cancer stem cell hypothesis is applicable to some tumors scarce non to others. In hematopoietic and some solid malignancies, only 1 in 100 to 1 in 10 000 primary tumor cells are capable of reproducing the tumor in vivo, such as human breast cancer, human neuroepithelial tumors, head and neck squamous cell carcinomas, and colon cancer. But in melanoma, close 1 in 4 cells possess the ability of proliferation and developing into cancer (Cheshier et al., 2009). Cancer stem cells and CNS stem cells were reviewed by Cheshier et al. (Cheshier et al., 2009).1.4 Mesenchymal stem cells (MSCs) and their differentiation potentialBone marrow is composed of 2 main s ystems of cell, hematopoietic cells and the supporting stromal cells (Bianco et al., 2001). MSCs reside within the marrow, maintain a level of self-renewal, and give rise to progenitor cells that can differentiate into various lineages of tissue, including chondrocytes, osteoblasts, adipocytes, fibroblasts, marrow stroma, and other tissues of mesenchymal origin. The traditional opinion about the multipotent differentiation potential of MSCs was challenged by gain studies. Interestingly, MSCs reside in a diverse host of tissues end-to-end the adult organism and possess the ability to regenerate cell types specific for local tissues e.g. adipose, periosteum, synovial membrane, muscle, dermis, pericytes, blood, bone marrow, and or so recently trabecular bone, reviewed by Tuan et al. (Tuan et al., 2003). Furthermore, in 2002, Jiang et al. reported a rare cell within human bone marrow mesenchymal stem cell cultures that can be expanded extensively without frank senescence. This cell population can differentiate, non only into mesenchymal cells, but similarly cells with visceral mesoderm, neuroectoderm and endoderm characteristics in vitro. to the mellowedest degree somatic cell types could be derived subsequently this population of cells was injected into an early blastocyst (Jiang et al., 2002). These studies suggest mesenchymal stem cells maintained pluripotent properties.Chapter 2 Features of Articular Cartilage2.1 Introduction knock gristle formed highly sophisticate structure during the evolutionary development. There have been considerable research interests related to the gristle cells, chondrocytes. In the last decades these studies do cartilage the first and very boffo tissue engineering science sermon (Brittberg et al. 1994).2.2 Categorization of cartilage tissuesCartilage tissue is categorised in three major types by different biochemical com rigs and structures of their extracellular hyaloplasm (ECM). Elastic cartilage has a minute conc entration of proteoglycans (PGs), and a relatively high proportion of elastin fibres. It exists in the epiglottis, small laryngeal, the external ear, auditory tube, and the small bronchi, where it is generally required to resist bending forces. Fibrocartilage also possesses a small concentration of PGs, but far little(prenominal) elastins. The meniscus in the knee fit articulation is made of fibrocartilage. Hyaline is the al some widespread cartilage in the human body. It is resistant to compressive or tensile forces collectable to its special type II collagen chain mesh filled with a high concentration of PGs. Hyaline cartilage can be found in the nose, trachea, bronchi, and synovial enounces. In the latter case, it is termed as articular cartilage (Schulz and Bader, 2007).2.3 Compositions of articular cartilageChondrocytes contribute to only 1%- 5% of the tissue volume the remain 95%-99% being extracellular hyaloplasm (ECM). Chondrocytes sense and synthesize all necessary ECM components (Mollenhauer, 2008 Schulz and Bader, 2007). The ECM of articular cartilage consists of about 60-85% water and fade away electrolytes. The solid framework is composed of collagens (10-20%), PGs (3-10%), noncollagenous proteins and glycoproteins. In articular cartilage, 95% of collagen in the ECM is type II collagen fibrils. The rest other types are collagen type IX and XI and a small carve up of types terce, VI, XII and XIV. Normal articular cartilage does not present type I collagen, which is concerned with fibrous tissue. irrelevant Type I and Type III collagens which form thick fibres and thin bres respectively, Type II collagen present in hyaline and elastic cartilages does not form bres. It forms very thin brils which are habituated as a loose mesh that strongly interacts with the ground substance. Type II collagen provides tensile stiffness and effectiveness to articular cartilage and constrains the s rise uping talent generated by highly negatively char ged glycosaminoglycans (GAGs) of the proteoglycans (PGs). The majority (50-85%) of the PG content in articular cartilage were presented by self-aggrandizing molecule aggrecan. It consists of a protein backbone, the core protein, to which unbranching GAGs side chains of chondroitin sulphate (CS) and keratan sulfate (KS) are covalently attached (Figure 1.1). The composition of articular cartilage was extensively reviewed by Schulz and Bader (Schulz and Bader, 2007).Figure 1. Illustration of the extracellular intercellular substance (ECM) organization of articular cartilage (Left) and the schematic sketches (Right) of the most relevant polysaccharides of proteoglycans (PGs) in articular cartilage. The PGs consist of a edge of hyaluronic harsh (HA), to which a core protein is non-covalently attached. On the core protein, glycosaminoglycans (GAGs) such as keratan sulphate (KS) and chondroitin sulfate (CS) are covalently bound in a bottle tangle fashion (Modified from Schulz and Bader , 2007 and Mow and Wang, 1999).2.4 Low capacity of self- bear upon in articular cartilageThe aneural and avascular nature of articular cartilage, coupled with its low cellularity, contribute to both the limited rate and incomplete nature of the darn process following damage (Heywood et al., 2004). The low mitotic potential of chondrocytes in vivo also contributes to its poor ability to undergo self-repair (Kuroda et al., 2007). some researchers believe that cartilage lesions less than 3mm in diameter self-repair with normal hyaline-like cartilage (Revell and Athanasiou, 2009 Schulz and Bader, 2007). In animal studies, full thickness cartilage defects, extending into the subchondral bone, have been reported to repair with the formation of fibrous tissue, which contains relatively low amount of type II collagen and aggrecan, but a relatively high concentration of type I collagen which is not present in normal adult articular cartilage and accordingly exhibits impaired automatic pro perties (Hjertquist et al., 1971).2.5 Metabolism of articular cartilageJoint cartilage is supplied with nutrients and group O by the synovial fluid distribution facilitated by compressive cyclic loading during word movements which acts as a pumping function (Mollenhauer, 2008). Within synovial interchangeables, oxygen fork up to articular chondrocytes is very limited, from 7.5% at the picayune zone down to 1% oxygen tension at the deep zone. It is supposed to be even move on decreased under diseased conditions, such as degenerative arthritis (OA) or rheumatoid arthritis (RA). The metabolism of chondrocytes is largely glycolytic. Oxygen-dependent vim generated by oxidative phosphorylation is just a minor contributor to the overall energy in chondrocytes. Nevertheless, changes in O2 tension have profound effects on cell metabolism, phenotype, gene pictureion, and morphology, as well as response to, and production of, cytokines (Pfander and Gelse, 2007 Gibson et al., 2008). Th e most important component of this hypoxic response is mediated by transcription factor hypoxia-inducible factor-1 (HIF-1), which is present in most hypoxia inducible genes (Pfander and Gelse, 2007 Gibson et al., 2008). Moreover, the matrix turnover in articular cartilage is extremely slow. Proteoglycan turnover is up to 25 eld. Collagen half-life is estimated to range from several decades up to 400 years (Mollenhauer, 2008).Chapter 3 degenerative joint disease (OA)3.1 PrevalenceOsteoarthritis (OA) is the most mutual form of arthritis. More than 40 million US American citizens (approximately 15% of the overall population of the USA) suffer from arthritis (Schulz and Bader, 2007). OA can occur in any joint but is most common in certain joints of the hand, knee, foot and hip joint. OA is the most common reason for do hip- and knee-joint relief (Wieland et al., 2005). Among US adults 30 years of age or older, symptomatic disease in the knee occurs in approximately 6% and symptoma tic hip degenerative joint disease in roughly 3% (Felson and Zhang, 1998).3.2 The symptoms and diagnosisThe symptoms of OA include pain, stiffness and loss of function. OA can be monitored by radiography, magnetic resonance imaging (MRI), and arthroscopy, but radiographs are still considered the gold bill (Wieland et al., 2005).3.3 The pathology of OAThe pathologic characteristics of OA are the slowly developing degenerative breakdown of cartilage the pathological changes in the bone, including osteophyte formation and thicken of the subchondral plate the changes in the synovium such as inflammatory infiltrates ligaments, which are often lax and bridging muscle, which becomes weak. Many people with pathologic and radiographic evidence of degenerative joint disease have no symptoms (Martel-Pelletier, 1999 Felson et al., 2000).A protease family of matrix metalloproteases (MMP) is responsible for the initial occurrence of cartilage matrix digestion. Of this family, collagenases, the stromelysins and the gelatinases are identified as being elevated railway in OA. Another group of MMP is place at the cell membrane surface and is thus named membrane type MMP (MT-MMP) (Martel-Pelletier, 1999).Proinflamatory cytokines such as interleukin (IL)-1, Tumor gangrene factor (TNF)-, IL-6, leukemic inhibitor factor (LIF) and IL-17 are first produced by the synovial membrane and then diffuse into the cartilage through the synovial fluid, where they activate the chondrocytes to produce pro-inflammatory cytokines. These proinflamatory cytokines are considered responsible for the catabolic pathological process (Martel-Pelletier, 1999).In OA cartilage, an increase level of an inducible form of nitric oxide synthase (iNOS) leads to a large amount of nitric oxide (NO) production (Pelletier et al., 2001). NO can inhibit the synthetic thinking of cartilage matrix macromolecules such as aggrecans and can enhance MMP activity (Taskiran et al., 1994 Murrell et al., 1995). It is well stablished that proinflammatory cytokines such as IL-1 act as the key mediators of cartilage breakdown and stimulate the release of inflammatory products (NO) and prostaglandin (PG)E2, via induction of iNOS and cyclo-oxygenase (COX)-2 enzymes (Chowdhury et al., 2008).3.4 Risk factorsOsteoarthritis is considered to be a systemic disease although severe joint injury whitethorn be sufficient to cause osteoarthritis. There are several systemic risk factors related to OA. (1) Age Osteoarthritis increases with ages, the incidence and preponderance of disease increased 2- to 10-fold from 30 to 65 years of age and increased further thereafter in a community- tushd survey (Oliveria et al., 1995). (2) hormonal status and bone density women winning estrogen have a decreased prevalence of radiographic osteoarthritis (Nevitt et al., 1996). Before 50 years of age, the prevalence of osteoarthritis in most joints is higher(prenominal) in men than in women. After about age 50 years, women are more often touch with hand, foot, and knee osteoarthritis than men. In most studies, hip osteoarthritis is more frequent in men (van Saase et al., 1989). Evidence suggests an inverse relationship between osteoarthritis and osteoporosis (Felson et al., 2000). (3) Nutritional factors evidence indicates that continuous scene to oxidants contributes to the development of many common age-related diseases, including osteoarthritis. McAlindon et al. reported a threefold reduction in risk for progressive radiographic osteoarthritis was observed in persons in the middle and highest tertile of vitamin C intake compared with those whose intake was in the lowest tertile (McAlindon et al., 1996a). Vitamin D intake was observed associated with the progression of OA although not associated with risk for new-onset radiographic osteoarthritis (McAlindon et al., 1996b passage et al., 1999). (4) Genetics genetic factors account for at least 50% of cases of osteoarthritis in the hands and hips and a littler percentage in the knees (Spector et al., 1996). Candidate genes for common forms of osteoarthritis include the vitamin D receptor gene, insulin-like growing factor I genes, cartilage oligomeric protein genes, and the HLA region (Felson et al., 2000).Local mechanical factors include the body weight and the pathological alterations of the mechanical environment of the joint. Persons who are overweight have a high prevalence of knee osteoarthritis (Felson et al., 1997). OA is also considered to be related to alterations in joint mechanical environments such as knee laxity, the displacement or rotation of the tibia with respect to the femur proprioception, the witting and unconscious perception of joint position and movement knee alignment , knee position in reference to the hip and ankle (Felson et al., 2000).In addition, joint dysplasias, fractures of articular surfaces, and tears of menisci and ligaments that increase joint instability precede the development of osteoarthri tis in a high percentage of affected joints. Risk factors for posttraumatic osteoarthritis include high body mass, high level of activity, residual joint instability or malalignment, and persistent articular surface incongruity (Buckwalter et al., 1997 Honkonen 1995).3.5 TreatmentsThe medicine treatment of OA was dominated by COX2 inhibitors (Flower 2003). The other medicines include glucosamine, chondroitin (McAlindon et al., 2000), and hyaluronic acid (Lo et al., 2003). In addition, both aerobic walking and muscle strengthening exercise reduce pain and impediment from osteoarthritis (Roddy et al., 2005).Articular cartilage lesions, both of traumatic or pathological origin, do not heal spontaneously and often undergo progressive retrogression towards osteoarthritis (OA). The most frequently used treatments include the arranged joint replacement, mosaicplasty, marrow stimulation, and autologous condrocyte implantation (ACI) (Steinwachs et al., 2008). Total joint replacement is mo st commonly performed in people over 60 years of age. (NHS 2006 Brittberg et al., 1994) Mosaicplasty is an autologous osteochondral transplantation system through which cylindrical periosteum grafts are taken from periphery of the patellofemoral bailiwick which bears less weight, and transplanted to defective subjects. This transplantation can be done with various diameters of grafts (Haklar et al., 2008 NHS, 2006). Marrow stimulation methods include arthroscopic operating theatre to smooth the surface of the damaged cartilage area microfracture, drilling, abrasion. All marrow stimulation methods base on the penetration of the subchondral bone plate at the bottom of the cartilage defect. The outflowing bone marrow blood contains the mesenchymal stem cells which are stabilised by the clot formation in the defect. These pluripotent stem cells which are able to differentiate into fibrochondrocytes, result in fibrocartilage repair with variable amounts of type I, II and III collag en (Steinwachs et al., 2008). The ACI tissue engineering treatment will be discussed in the next chapter.Chapter 4 Tissue engineering and autologous chondrocyte implantation (ACI)4.1 Overview of tissue engineering technologiesTissue engineering is defined as the application of the principles and methods of engineering and the life sciences toward the fundamental understanding of structure-function relationships in normal and pathological mammalian tissues and the development of biological substitutes to restore, maintain, or rectify tissue function (Langer and Vacanti, 1993). Three factors are considered as the principles of tissue engineering, including the utilization of biocompatible and mechanically suitable holds, an reserve cell source, and bioactive molecules to promote the differentiation and maturation of the cell type of interest (Song et al., 2004).Potential applications of tissue engineering are involved in the following fields skin, cartilage, bone, cardiovascular di seases, organs (e.g. liver, pancreas, bladder, trachea and breast), central noisome system (e.g. spinal cord), and miscellaneous (e.g. soft tissue, ligaments). Although research is being carried out in all these fields, only few products have already entered the marketplace. The most successful products up to now are tissue engineered skin which is mainly used for wound mop up, autologous chondrocyte implantation (ACI), and artificial bone graft (Hsing et al., 2003).4.2 autologous chondrocyte implantation (ACI)In 1984, a study in rabbits reported successful treatment of focal patellar defects with the use of ACI. One year after transplantation, impertinently formed cartilage-like tissue typically covered about 70 percent of the defect (Grande et al. 1989). In 1987, Brittberg firstly performed ACI in 23 people with deep cartilage defects in the knee. ACI is described as the following influence cartilage cells are taken from a minor load-bearing(prenominal) area on the upper med ian femoral condyle of the damaged knee via an arthroscopic mathematical function, cultivated for four to six weeks in a laboratory and then, in return surgery, introduced back into the damaged area as a liquid or mesh-like transplant at last, a periosteal flap sutured in place to secure the transplant (Figure 2 Brittberg et al., 1994).Genzyme Biosurgery with its product Carticel was the first friendship which introduced ACI into market and is the market leader in USA. Carticel is a classic ACI procedure using the periosteal cover (Hsing et al., 2008). instantly the periosteum is often replaced by an artificial resorbable cover such as collagen I/III and hyaluronan membrane, such as ChondroGide or Restore (De Puy, Warzaw, Indiana) (Gooding et al., 2006 Jones and Peterson, 2006). Another new method uses chondrocytes cultured on a tri-dimensional (3D), perishable sustain. This kind of scaffold, cut to the required size, is fixed into the lesion by anchoring stitches or its stick y nature. The 3D cell seeded scaffold eliminates the using of cover, thus simplifies the surgery procedure, saves the surgery time, and opens up the possibility of an arthroscopic surgery instead of the open surgery which causes more tissue damage. HYALOGRAFT from Italy is one of the European market leaders. It is a cartilage substitute made of autologous chondrocytes delivered on a biocompatible 3D matrix, entirely composed of a derivative of hyaluronic acid (Marcacci et al. 2005).4.3 Clinical results of ACIBrittberg canvas the long-term durability of ACI-treated patients, 61 patients were followed for at least five years up to 11 years post-surgery (mean 7.4 years). After two years, 50 out of 61 patients were graded good-excellent. At the five to 11 years fulfil, 51 of the 61 were graded good-excellent (Brittberg et al., 2003). Since 1997 the year FDA approved ACI, this method has been astray performed in more than 20,000 patients all over the world. It has been reported to be effective in relieving clinical symptoms, such as pain and function (Wakitani et al., 2008).In a randomised controlled study, Knutsen et al. studied 80 patients who needed local cartilage repair with lesions on the femoral condyles of 2-10 cm2. There were no signicant differences in clinical results at 5 years follow-up (Knutsen et al., 2007). In another randomised controlled study that compared mosaicplasty with ACI, there was no significant difference in the number of patients who had an excellent or good clinical outcome at 1 year (69% 29/42 and 88% 51/58, respectively). In the subgroup of patients who had repairs to lesions of the medial femoral condyle, significantly more patients who had ACI had an excellent or good outcome (88% 21/24) compared with those who had mosaicplasty (72% 21/29) (p Clinical results of ACI were reviewed by Gikas 2009 (Gikas et al., 2009). Generally speaking, the outcomes of ACI treatment have been encouraging. However, most randomised controlled studi es showed no significant difference between ACI and traditional treatments.4.4 Limitations of ACIMicrofracture is a very simple and low-cost procedure whereas ACI costs about $10 000 per patient. If ACI is not found to be more effective for improving articular cartilage repair than microfracture, the procedure will not be continued (Wakitani et al., 2008).There are several possible reasons to be blamed for the limitations of the traditional ACI procedure. The cell source in ACI is the cartilage tissue derived via an arthroscopic procedure from the low load-bearing area on the upper medial femoral condyle of the damaged knee. However, Wiseman et al. found the chondrocytes isolated from the low loaded area of the knee joint respond to mechanical stimulations in a distinct manner with the chondrocytes from the high loaded area, which suggests the traditional cell source of ACI may not provide profuse mechanical response and may further lead to the insufficient mechanical properties of the repaired tissue (Wiseman et al. 2003).As cultured in monolayer, chondrocytes undergo a process of dedifferentiation and give birth a more broblast-like morphology, which is accompanied by an increase in proliferation and an altered phenotype. Type II collagen, the major protein produced by chondrocytes in articular cartilage, are down-regulated in the culture, while collagen types I and III are increased (Glowacki et al., 1983 Stocks et al., 2002 Benya et al., 1978). The agregating proteoglycan aggrecan of articular cartilage, is down-regulated during dedifferentiation and replaced by proteoglycans not specic to cartilage, such as versican (Glowacki et al., 1983 Stocks et al., 2002). Therefore, monolayer cultured chondrocytes do not express the origninal phenotype, and their ability to regenerate damaged cartilage tissue is impaired. Upon implantation, dedifferentiated cells may form a brous tissue expressing collagen type I that does not have appropriate mechanical propertie s, which may lead to abjection and failure of the repair tissue (Brodkin et al., 2004). Chondrocytes grown in conditions that support their round shape, such as plating in high-density monolayer (Watt, 1988) and seeding in 3D structure (Benya and Shaffer, 1982) can maintain their differentiated phenotype much longer compared to cells spread in monolayer cultures.Although ACI can still be considered to be one of commonly form of repair of cartilage defects, it does have a number of scientific limitations. Some of those can be resolved using more comp tissue engineered strategies which incorporates cells, scaffold materials and potentially biochemical, biomechanical and/or physical stimulation in a controlled bioreactor environment.4.5 Tissue engineering strategies for ACIChondrocytes derived from the low load bearing area of the knee joint respond in a distinct manner with the chondrocytes from the high loaded area. Chondrocytes cultured in monolayer have a dedifferentiation phenom enon as described above. In addition, the limitation of the transplant volume is ceaselessly a major problem in autograft to be overcome (Kitaoka et al., 2001 Vinatier et. al, 2009). Accordingly, potential cell sources are widely studied for the future improvement of ACI approach, which will be discussed in Chapter 4.Seeding in 3D structures (Benya and Shaffer, 1982) can maintain chondrocytes differentiated phenotype. Ideally, cell scaffolds for tissue engineering should edge several design criteria (1) The surface should earmark cell adhension and growth, (2) neither the polymer nor its degradation products should provoke inflammation or toxicity when implanted in vivo, (3) the material should be reproducibly processable into three dimensional structures, (4) the porosity should be at least 90% in order to provide a high surface area for cell-polymer interactions, sufficient space of extracellular matrix regeneration, and minimal diffusional constraints during in vitro culture, (5) the scaffold should resorb once it has served its purpose of providing a template for the regenerating tissue, since foreign materials carry a permanent risk of inflammation, and (6) the scaffold degradation rate should be adjustable to match the rate of tissue regeneration by the cell type of interest (Freed et al., 1994).Synthetic materials such as poly (glycolic acid) (PGA), poly (lactic acid) (PLA), and poly (lactic-co-glycolic acid) (PLGA) have been investigated for use as cartilage tissue engineering scaffolds (Cima et al., 1991 Vacanti et al., 1991). Both, in vitro and in vivo studies have demonstrated these scaffold maintained the chondrocyte phenotype and the production of cartilage extracellular matrix (ECM) (Barnewitz et al., 2006 Kaps et al., 2006). Moreover, PLGA is used as a scaffold material for matrix-based autologous chondrocyte transplantation clinically (Ossendorf et al., 2007).Natural materials have also been investigated in the application of tissue engineer ing scaffolds in ACI. Collagen-based biomaterials are widely used in todays clinical practice (for example, haemostasis and cosmetic surgery). Collagen is also be commonly used as main components in tissue engineered skin products. Several commercial ACI products have used collagenous membraneas as the replacement for the periosteum to close the defect, such as ChondroGide or Restore (De Puy, Warzaw, Indiana) (Cicuttini et al., 1996 Jones and Peterson, 2006). The combination of type I collagen with GAG in scaffolds had a positive effect on chondrocyte phenotype (van Susante et al., 2001). Hyaluronic acid is a non-sulphated GAG that makes up a large proportion of cartilage extracellular matrix (Schulz and Bader, 2007). Matrices composed of hyaluronan have been frequently used as carriers for chondrocytes. Facchini et al. conrms the hyaluronan derivative scaffold Hyaff 11 as a suitable scaffold both for chondrocytes and mesenchymal stem cells for the treatment of articular cartilage d efects in their study (Facchini et al., 2006). Sugar-based cancel polymers such as chitosan, alginate and agarose can be formulated as hydrogels and in some cases sponges or pads. Although these materials are extensively used in in vitro research, their division in in vivo cartilage reconstruction is still limited (Stoop, 2008).Growth factors are proved to be able to promote the formation of new cartilage tissue in both explants and engineered constructs. Insulin-like growth factor-I (IGF-I), transforming growth factor-1 (TGF-1) increases, basic fibroblast growth factor (bFGF) can stimulate cell proliferation and/or biosynthesis in chondrocytes which wereImmune Privilege of Tissue Engineered Articular CartilageImmune Privilege of Tissue Engineered Articular CartilageThe immune privilege of tissue engineered articular cartilage derived from mouse adult mesenchymal stem cells and the potential of tissue engineered cartilage as a gene delivery methodChapter 1 Stem cell biology1.1 Cat egorization of stem cellsStem cells are generally defined as cells possessing the following 3 characteristics (1) self-renewal, (2) the ability to produce all cell types made in that tissue, and (3) the ability to do so for a significant portion of the life of the host (Alberts et al., 1989 Reya et al., 2001), while progenitor cells are capable only of multi-lineage differentiation without self-renewal (Weissman, 2000).Stem cells can be classified by their ability to differentiate. The most primitive, totipotent stem cells have the ability to divide and produce all the differentiated cells in an organism, including both the embryonic and extraembryonic tissues of an organism. Totipotent stem cells include the fertilized egg and the cells produced by the initial divisions of it. In mammals, these cell divisions result in an implant in the uterus called the blastocyst. The blastocyst contains an outer sphere of trophoblast cells. Trophoblast cells are capable of implanting into the ut erus and helping the form of placenta which provides nutrients to the embryo. Within the blastocyst are 10 to 20 pluripotent cells called the inner cell mass. In mammalian uterus, these inner mass cells will participate in the production of all tissues and organs of the developing embryo, then fetus, then born organism. Such pluripotent cells can produce any differentiated cells in the body, but are usually unable to form the trophoblast cells. The best-known pluripotent stem cell is the embryonic stem (ES) cell, which are obtained from the inner cell mass of the blastocyst and exist for only a brief stage of embryonic development. The last major class of stem cells, multipotent stem cells, gives rise to a limited number of cell types which are responsible for organ growth and renewal such as neural stem cells, skin stem cells and haematopoietic stem cells (HSCs) (Cheshier et al., 2009).1.2 Selected milestones of stem cell researchIn 1981, Martin isolated a pluripotent stem cell lin e from early mouse embryos (Martin, 1981). Wilmut in 1996 first cloned a mammal, a lamb named Dolly by transferring nuclear from the adult mammary gland cell to an enucleated unfertilized egg (Wilmut et al., 1997). In 1998, Thomson obtained the first human embryonic stem cell line from human blastocysts (Thomson et al., 1998). In 2001, President Bush banned scientists from using federal funds to study stem cells from sources other than those that had already been grown because of the ethical concerns. To avoid ethical dispute over the use of human embryonic cells for research purposes, many efforts have been taken on obtaining pluripotent stem cells from differentiated donor cells. In 2006, Yamanaka find a way to obtain pluripotent cells by reprogramming the nucleus of adult mice skin cells (Takahashi and Yamanaka, 2006). Such cells are now known as induced pluripotent stem (iPS) cells.1.3 A brief introduction of several types of multipotent stem cellThe best known multipotent stem cells are haematopoietic stem cells (HSCs), that give rise to all the blood cell types including myeloid (monocytes and macrophages, neutrophils, basophils, eosinophils, erythrocytes, megakaryocytes/platelets, dendritic cells), and lymphoid lineages (T-cells, B-cells, NK-cells). HSCs are vital elements in bone-marrow transplantation, which has already been used extensively in therapeutic settings (Reya et al., 2001).In the long-term culture systems, human and rodent Central Neural System (CNS) cells maintain the capacity to produce the three main mature cell classes of the CNS neurons, astrocytes, and oligodendrocytes, which suggest stem cells and/or progenitors exist and can survive in the culture medium (Weiss et al., 1996 Carpenter et al., 1999). In 2000, Human CNS stem cells (hCNS-SCs) have been successfully isolated by FACs (Uchida et al., 2000).Cancer stem cell hypothesis was proposed by Reya 2001 (Reya et al., 2001). This hypothesis consists of 2 components. The first compone nt postulates that normal tissue stem cells are the target for transforming mutations and successive mutations result in the formation of a tumor. The second component is that within every cancer a specific subset of cancer stem cells continuously gives rise to all the other cancer cells and only these cells within a tumor possess the ability to self-renew, continuously proliferate. Conflicting to the first component of the hypothesis, evidences indicate cancer stem cells can also arise from mutated progenitor cells rather than stem cells (Cheshier et al., 2009). In addition, mature cells such as Lymphocytes can lead to mouse T cell leukemia independently from HSCs (Yuan et al., 2006). For the latter component of cancer stem cell hypothesis, it is likely that the cancer stem cell hypothesis is applicable to some tumors but not to others. In hematopoietic and some solid malignancies, only 1 in 100 to 1 in 10 000 primary tumor cells are capable of reproducing the tumor in vivo, such a s human breast cancer, human neuroepithelial tumors, head and neck squamous cell carcinomas, and colon cancer. But in melanoma, nearly 1 in 4 cells possessed the ability of proliferation and developing into cancer (Cheshier et al., 2009). Cancer stem cells and CNS stem cells were reviewed by Cheshier et al. (Cheshier et al., 2009).1.4 Mesenchymal stem cells (MSCs) and their differentiation potentialBone marrow is composed of two main systems of cell, hematopoietic cells and the supporting stromal cells (Bianco et al., 2001). MSCs reside within the marrow, maintain a level of self-renewal, and give rise to progenitor cells that can differentiate into various lineages of tissue, including chondrocytes, osteoblasts, adipocytes, fibroblasts, marrow stroma, and other tissues of mesenchymal origin. The traditional opinion about the multipotent differentiation potential of MSCs was challenged by further studies. Interestingly, MSCs reside in a diverse host of tissues throughout the adult o rganism and possess the ability to regenerate cell types specific for local tissues e.g. adipose, periosteum, synovial membrane, muscle, dermis, pericytes, blood, bone marrow, and most recently trabecular bone, reviewed by Tuan et al. (Tuan et al., 2003). Furthermore, in 2002, Jiang et al. reported a rare cell within human bone marrow mesenchymal stem cell cultures that can be expanded extensively without obvious senescence. This cell population can differentiate, not only into mesenchymal cells, but also cells with visceral mesoderm, neuroectoderm and endoderm characteristics in vitro. Most somatic cell types could be derived after this population of cells was injected into an early blastocyst (Jiang et al., 2002). These studies suggest mesenchymal stem cells maintained pluripotent properties.Chapter 2 Features of Articular Cartilage2.1 IntroductionJoint cartilage formed highly sophisticated structure during the evolutionary development. There have been considerable research intere sts related to the cartilage cells, chondrocytes. In the last decades these studies made cartilage the first and very successful tissue engineering treatment (Brittberg et al. 1994).2.2 Categorization of cartilage tissuesCartilage tissue is categorised in three major types by different biochemical compositions and structures of their extracellular matrix (ECM). Elastic cartilage has a small concentration of proteoglycans (PGs), and a relatively high proportion of elastin fibres. It exists in the epiglottis, small laryngeal, the external ear, auditory tube, and the small bronchi, where it is generally required to resist bending forces. Fibrocartilage also possesses a small concentration of PGs, but far less elastins. The meniscus in the knee joint is made of fibrocartilage. Hyaline is the most widespread cartilage in the human body. It is resistant to compressive or tensile forces due to its special type II collagen fibril mesh filled with a high concentration of PGs. Hyaline cartila ge can be found in the nose, trachea, bronchi, and synovial joints. In the latter case, it is termed as articular cartilage (Schulz and Bader, 2007).2.3 Compositions of articular cartilageChondrocytes contribute to only 1%- 5% of the tissue volume the remaining 95%-99% being extracellular matrix (ECM). Chondrocytes sense and synthesize all necessary ECM components (Mollenhauer, 2008 Schulz and Bader, 2007). The ECM of articular cartilage consists of about 60-85% water and dissolved electrolytes. The solid framework is composed of collagens (10-20%), PGs (3-10%), noncollagenous proteins and glycoproteins. In articular cartilage, 95% of collagen in the ECM is type II collagen fibrils. The rest other types are collagen type IX and XI and a small fraction of types III, VI, XII and XIV. Normal articular cartilage does not present type I collagen, which is concerned with fibrous tissue. Unlike Type I and Type III collagens which form thick fibres and thin bres respectively, Type II collag en present in hyaline and elastic cartilages does not form bres. It forms very thin brils which are disposed as a loose mesh that strongly interacts with the ground substance. Type II collagen provides tensile stiffness and strength to articular cartilage and constrains the swelling capacity generated by highly negatively charged glycosaminoglycans (GAGs) of the proteoglycans (PGs). The majority (50-85%) of the PG content in articular cartilage were presented by large molecule aggrecan. It consists of a protein backbone, the core protein, to which unbranched GAGs side chains of chondroitin sulphate (CS) and keratan sulfate (KS) are covalently attached (Figure 1.1). The composition of articular cartilage was extensively reviewed by Schulz and Bader (Schulz and Bader, 2007).Figure 1. Illustration of the extracellular matrix (ECM) organization of articular cartilage (Left) and the schematic sketches (Right) of the most relevant polysaccharides of proteoglycans (PGs) in articular cartil age. The PGs consist of a strand of hyaluronic acid (HA), to which a core protein is non-covalently attached. On the core protein, glycosaminoglycans (GAGs) such as keratan sulphate (KS) and chondroitin sulfate (CS) are covalently bound in a bottle brush fashion (Modified from Schulz and Bader, 2007 and Mow and Wang, 1999).2.4 Low capacity of self-repair in articular cartilageThe aneural and avascular nature of articular cartilage, coupled with its low cellularity, contribute to both the limited rate and incomplete nature of the repair process following damage (Heywood et al., 2004). The low mitotic potential of chondrocytes in vivo also contributes to its poor ability to undergo self-repair (Kuroda et al., 2007). Some researchers believe that cartilage lesions less than 3mm in diameter self-repair with normal hyaline-like cartilage (Revell and Athanasiou, 2009 Schulz and Bader, 2007). In animal studies, full thickness cartilage defects, extending into the subchondral bone, have bee n reported to heal with the formation of fibrous tissue, which contains relatively low amount of type II collagen and aggrecan, but a relatively high concentration of type I collagen which is not present in normal adult articular cartilage and accordingly exhibits impaired mechanical properties (Hjertquist et al., 1971).2.5 Metabolism of articular cartilageJoint cartilage is supplied with nutrients and oxygen by the synovial fluid diffusion facilitated by compressive cyclic loading during joint movements which acts as a pumping function (Mollenhauer, 2008). Within synovial joints, oxygen supply to articular chondrocytes is very limited, from 7.5% at the superficial zone down to 1% oxygen tension at the deep zone. It is supposed to be even further decreased under pathological conditions, such as osteoarthritis (OA) or rheumatoid arthritis (RA). The metabolism of chondrocytes is largely glycolytic. Oxygen-dependent energy generated by oxidative phosphorylation is just a minor contribu tor to the overall energy in chondrocytes. Nevertheless, changes in O2 tension have profound effects on cell metabolism, phenotype, gene expression, and morphology, as well as response to, and production of, cytokines (Pfander and Gelse, 2007 Gibson et al., 2008). The most important component of this hypoxic response is mediated by transcription factor hypoxia-inducible factor-1 (HIF-1), which is present in most hypoxia inducible genes (Pfander and Gelse, 2007 Gibson et al., 2008). Moreover, the matrix turnover in articular cartilage is extremely slow. Proteoglycan turnover is up to 25 years. Collagen half-life is estimated to range from several decades up to 400 years (Mollenhauer, 2008).Chapter 3 Osteoarthritis (OA)3.1 PrevalenceOsteoarthritis (OA) is the most common form of arthritis. More than 40 million US American citizens (approximately 15% of the overall population of the USA) suffer from arthritis (Schulz and Bader, 2007). OA can occur in any joint but is most common in cer tain joints of the hand, knee, foot and hip. OA is the most common reason for total hip- and knee-joint replacement (Wieland et al., 2005). Among US adults 30 years of age or older, symptomatic disease in the knee occurs in approximately 6% and symptomatic hip osteoarthritis in roughly 3% (Felson and Zhang, 1998).3.2 The symptoms and diagnosisThe symptoms of OA include pain, stiffness and loss of function. OA can be monitored by radiography, magnetic resonance imaging (MRI), and arthroscopy, but radiographs are still considered the gold standard (Wieland et al., 2005).3.3 The pathology of OAThe pathologic characteristics of OA are the slowly developing degenerative breakdown of cartilage the pathological changes in the bone, including osteophyte formation and thickening of the subchondral plate the changes in the synovium such as inflammatory infiltrates ligaments, which are often lax and bridging muscle, which becomes weak. Many people with pathologic and radiographic evidence of o steoarthritis have no symptoms (Martel-Pelletier, 1999 Felson et al., 2000).A protease family of matrix metalloproteases (MMP) is responsible for the initial occurrence of cartilage matrix digestion. Of this family, collagenases, the stromelysins and the gelatinases are identified as being elevated in OA. Another group of MMP is localized at the cell membrane surface and is thus named membrane type MMP (MT-MMP) (Martel-Pelletier, 1999).Proinflamatory cytokines such as interleukin (IL)-1, Tumor necrosis factor (TNF)-, IL-6, leukemic inhibitor factor (LIF) and IL-17 are first produced by the synovial membrane and then diffuse into the cartilage through the synovial fluid, where they activate the chondrocytes to produce proinflammatory cytokines. These proinflamatory cytokines are considered responsible for the catabolic pathological process (Martel-Pelletier, 1999).In OA cartilage, an increased level of an inducible form of nitric oxide synthase (iNOS) leads to a large amount of nitri c oxide (NO) production (Pelletier et al., 2001). NO can inhibit the synthesis of cartilage matrix macromolecules such as aggrecans and can enhance MMP activity (Taskiran et al., 1994 Murrell et al., 1995). It is well stablished that proinflammatory cytokines such as IL-1 act as the key mediators of cartilage breakdown and stimulate the release of inflammatory products (NO) and prostaglandin (PG)E2, via induction of iNOS and cyclo-oxygenase (COX)-2 enzymes (Chowdhury et al., 2008).3.4 Risk factorsOsteoarthritis is considered to be a systemic disease although severe joint injury may be sufficient to cause osteoarthritis. There are several systemic risk factors related to OA. (1) Age Osteoarthritis increases with ages, the incidence and prevalence of disease increased 2- to 10-fold from 30 to 65 years of age and increased further thereafter in a community-based survey (Oliveria et al., 1995). (2) Hormonal status and bone density women taking estrogen have a decreased prevalence of rad iographic osteoarthritis (Nevitt et al., 1996). Before 50 years of age, the prevalence of osteoarthritis in most joints is higher in men than in women. After about age 50 years, women are more often affected with hand, foot, and knee osteoarthritis than men. In most studies, hip osteoarthritis is more frequent in men (van Saase et al., 1989). Evidence suggests an inverse relationship between osteoarthritis and osteoporosis (Felson et al., 2000). (3) Nutritional factors evidence indicates that continuous exposure to oxidants contributes to the development of many common age-related diseases, including osteoarthritis. McAlindon et al. reported a threefold reduction in risk for progressive radiographic osteoarthritis was observed in persons in the middle and highest tertile of vitamin C intake compared with those whose intake was in the lowest tertile (McAlindon et al., 1996a). Vitamin D intake was observed associated with the progression of OA although not associated with risk for new -onset radiographic osteoarthritis (McAlindon et al., 1996b Lane et al., 1999). (4) Genetics genetic factors account for at least 50% of cases of osteoarthritis in the hands and hips and a smaller percentage in the knees (Spector et al., 1996). Candidate genes for common forms of osteoarthritis include the vitamin D receptor gene, insulin-like growth factor I genes, cartilage oligomeric protein genes, and the HLA region (Felson et al., 2000).Local mechanical factors include the body weight and the pathological alterations of the mechanical environment of the joint. Persons who are overweight have a high prevalence of knee osteoarthritis (Felson et al., 1997). OA is also considered to be related to alterations in joint mechanical environments such as knee laxity, the displacement or rotation of the tibia with respect to the femur proprioception, the conscious and unconscious perception of joint position and movement knee alignment , knee position in reference to the hip and ankle (Fe lson et al., 2000).In addition, joint dysplasias, fractures of articular surfaces, and tears of menisci and ligaments that increase joint instability precede the development of osteoarthritis in a high percentage of affected joints. Risk factors for posttraumatic osteoarthritis include high body mass, high level of activity, residual joint instability or malalignment, and persistent articular surface incongruity (Buckwalter et al., 1997 Honkonen 1995).3.5 TreatmentsThe medicine treatment of OA was dominated by COX2 inhibitors (Flower 2003). The other medicines include glucosamine, chondroitin (McAlindon et al., 2000), and hyaluronic acid (Lo et al., 2003). In addition, both aerobic walking and muscle strengthening exercise reduce pain and disability from osteoarthritis (Roddy et al., 2005).Articular cartilage lesions, both of traumatic or pathological origin, do not heal spontaneously and often undergo progressive degeneration towards osteoarthritis (OA). The most frequently used tr eatments include the artificial joint replacement, mosaicplasty, marrow stimulation, and autologous condrocyte implantation (ACI) (Steinwachs et al., 2008). Total joint replacement is most commonly performed in people over 60 years of age. (NHS 2006 Brittberg et al., 1994) Mosaicplasty is an autologous osteochondral transplantation method through which cylindrical periosteum grafts are taken from periphery of the patellofemoral area which bears less weight, and transplanted to defective areas. This transplantation can be done with various diameters of grafts (Haklar et al., 2008 NHS, 2006). Marrow stimulation methods include arthroscopic surgery to smooth the surface of the damaged cartilage area microfracture, drilling, abrasion. All marrow stimulation methods base on the penetration of the subchondral bone plate at the bottom of the cartilage defect. The outflowing bone marrow blood contains the mesenchymal stem cells which are stabilised by the clot formation in the defect. These pluripotent stem cells which are able to differentiate into fibrochondrocytes, result in fibrocartilage repair with varying amounts of type I, II and III collagen (Steinwachs et al., 2008). The ACI tissue engineering treatment will be discussed in the next chapter.Chapter 4 Tissue engineering and autologous chondrocyte implantation (ACI)4.1 Overview of tissue engineering technologiesTissue engineering is defined as the application of the principles and methods of engineering and the life sciences toward the fundamental understanding of structure-function relationships in normal and pathological mammalian tissues and the development of biological substitutes to restore, maintain, or improve tissue function (Langer and Vacanti, 1993). Three factors are considered as the principles of tissue engineering, including the utilization of biocompatible and mechanically suitable scaffolds, an appropriate cell source, and bioactive molecules to promote the differentiation and maturation of th e cell type of interest (Song et al., 2004).Potential applications of tissue engineering are involved in the following fields skin, cartilage, bone, cardiovascular diseases, organs (e.g. liver, pancreas, bladder, trachea and breast), central nervous system (e.g. spinal cord), and miscellaneous (e.g. soft tissue, ligaments). Although research is being carried out in all these fields, only few products have already entered the market. The most successful products up to now are tissue engineered skin which is mainly used for wound cover, autologous chondrocyte implantation (ACI), and artificial bone graft (Hsing et al., 2003).4.2 Autologous chondrocyte implantation (ACI)In 1984, a study in rabbits reported successful treatment of focal patellar defects with the use of ACI. One year after transplantation, newly formed cartilage-like tissue typically covered about 70 percent of the defect (Grande et al. 1989). In 1987, Brittberg firstly performed ACI in 23 people with deep cartilage defe cts in the knee. ACI is described as the following procedure cartilage cells are taken from a minor load-bearing area on the upper medial femoral condyle of the damaged knee via an arthroscopic procedure, cultivated for four to six weeks in a laboratory and then, in open surgery, introduced back into the damaged area as a liquid or mesh-like transplant at last, a periosteal flap sutured in place to secure the transplant (Figure 2 Brittberg et al., 1994).Genzyme Biosurgery with its product Carticel was the first company which introduced ACI into market and is the market leader in USA. Carticel is a classic ACI procedure using the periosteal cover (Hsing et al., 2008). Today the periosteum is often replaced by an artificial resorbable cover such as collagen I/III and hyaluronan membrane, such as ChondroGide or Restore (De Puy, Warzaw, Indiana) (Gooding et al., 2006 Jones and Peterson, 2006). Another new method uses chondrocytes cultured on a tri-dimensional (3D), biodegradable scaffol d. This kind of scaffold, cut to the required size, is fixed into the lesion by anchoring stitches or its sticky nature. The 3D cell seeded scaffold eliminates the using of cover, thus simplifies the surgery procedure, saves the surgery time, and opens up the possibility of an arthroscopic surgery instead of the open surgery which causes more tissue damage. HYALOGRAFT from Italy is one of the European market leaders. It is a cartilage substitute made of autologous chondrocytes delivered on a biocompatible 3D matrix, entirely composed of a derivative of hyaluronic acid (Marcacci et al. 2005).4.3 Clinical results of ACIBrittberg studied the long-term durability of ACI-treated patients, 61 patients were followed for at least five years up to 11 years post-surgery (mean 7.4 years). After two years, 50 out of 61 patients were graded good-excellent. At the five to 11 years follow-up, 51 of the 61 were graded good-excellent (Brittberg et al., 2003). Since 1997 the year FDA approved ACI, th is method has been widely performed in more than 20,000 patients all over the world. It has been reported to be effective in relieving clinical symptoms, such as pain and function (Wakitani et al., 2008).In a randomised controlled study, Knutsen et al. studied 80 patients who needed local cartilage repair with lesions on the femoral condyles of 2-10 cm2. There were no signicant differences in clinical results at 5 years follow-up (Knutsen et al., 2007). In another randomised controlled study that compared mosaicplasty with ACI, there was no significant difference in the number of patients who had an excellent or good clinical outcome at 1 year (69% 29/42 and 88% 51/58, respectively). In the subgroup of patients who had repairs to lesions of the medial femoral condyle, significantly more patients who had ACI had an excellent or good outcome (88% 21/24) compared with those who had mosaicplasty (72% 21/29) (p Clinical results of ACI were reviewed by Gikas 2009 (Gikas et al., 2009). Gen erally speaking, the outcomes of ACI treatment have been encouraging. However, most randomised controlled studies showed no significant difference between ACI and traditional treatments.4.4 Limitations of ACIMicrofracture is a very simple and low-cost procedure whereas ACI costs about $10 000 per patient. If ACI is not found to be more effective for improving articular cartilage repair than microfracture, the procedure will not be continued (Wakitani et al., 2008).There are several possible reasons to be blamed for the limitations of the traditional ACI procedure. The cell source in ACI is the cartilage tissue derived via an arthroscopic procedure from the low load-bearing area on the upper medial femoral condyle of the damaged knee. However, Wiseman et al. found the chondrocytes isolated from the low loaded area of the knee joint respond to mechanical stimulations in a distinct manner with the chondrocytes from the high loaded area, which suggests the traditional cell source of ACI may not provide enough mechanical response and may further lead to the insufficient mechanical properties of the repaired tissue (Wiseman et al. 2003).As cultured in monolayer, chondrocytes undergo a process of dedifferentiation and adopt a more broblast-like morphology, which is accompanied by an increase in proliferation and an altered phenotype. Type II collagen, the major protein produced by chondrocytes in articular cartilage, are down-regulated in the culture, while collagen types I and III are increased (Glowacki et al., 1983 Stocks et al., 2002 Benya et al., 1978). The agregating proteoglycan aggrecan of articular cartilage, is down-regulated during dedifferentiation and replaced by proteoglycans not specic to cartilage, such as versican (Glowacki et al., 1983 Stocks et al., 2002). Therefore, monolayer cultured chondrocytes do not express the origninal phenotype, and their ability to regenerate damaged cartilage tissue is impaired. Upon implantation, dedifferentiated cells may form a brous tissue expressing collagen type I that does not have appropriate mechanical properties, which may lead to degradation and failure of the repair tissue (Brodkin et al., 2004). Chondrocytes grown in conditions that support their round shape, such as plating in high-density monolayer (Watt, 1988) and seeding in 3D structure (Benya and Shaffer, 1982) can maintain their differentiated phenotype much longer compared to cells spread in monolayer cultures.Although ACI can still be considered to be one of commonly form of repair of cartilage defects, it does have a number of scientific limitations. Some of those can be resolved using more comprehensive tissue engineered strategies which incorporates cells, scaffold materials and potentially biochemical, biomechanical and/or physical stimulation in a controlled bioreactor environment.4.5 Tissue engineering strategies for ACIChondrocytes derived from the low load bearing area of the knee joint respond in a distinct manner with the chondrocytes from the high loaded area. Chondrocytes cultured in monolayer have a dedifferentiation phenomenon as described above. In addition, the limitation of the transplant volume is always a major problem in autograft to be overcome (Kitaoka et al., 2001 Vinatier et. al, 2009). Accordingly, potential cell sources are widely studied for the future improvement of ACI approach, which will be discussed in Chapter 4.Seeding in 3D structures (Benya and Shaffer, 1982) can maintain chondrocytes differentiated phenotype. Ideally, cell scaffolds for tissue engineering should meet several design criteria (1) The surface should permit cell adhension and growth, (2) neither the polymer nor its degradation products should provoke inflammation or toxicity when implanted in vivo, (3) the material should be reproducibly processable into three dimensional structures, (4) the porosity should be at least 90% in order to provide a high surface area for cell-polymer interactions, sufficient spa ce of extracellular matrix regeneration, and minimal diffusional constraints during in vitro culture, (5) the scaffold should resorb once it has served its purpose of providing a template for the regenerating tissue, since foreign materials carry a permanent risk of inflammation, and (6) the scaffold degradation rate should be adjustable to match the rate of tissue regeneration by the cell type of interest (Freed et al., 1994).Synthetic materials such as poly (glycolic acid) (PGA), poly (lactic acid) (PLA), and poly (lactic-co-glycolic acid) (PLGA) have been investigated for use as cartilage tissue engineering scaffolds (Cima et al., 1991 Vacanti et al., 1991). Both, in vitro and in vivo studies have demonstrated these scaffold maintained the chondrocyte phenotype and the production of cartilage extracellular matrix (ECM) (Barnewitz et al., 2006 Kaps et al., 2006). Moreover, PLGA is used as a scaffold material for matrix-based autologous chondrocyte transplantation clinically (Ossen dorf et al., 2007).Natural materials have also been investigated in the application of tissue engineering scaffolds in ACI. Collagen-based biomaterials are widely used in todays clinical practice (for example, haemostasis and cosmetic surgery). Collagen is also be commonly used as main components in tissue engineered skin products. Several commercial ACI products have used collagenous membraneas as the replacement for the periosteum to close the defect, such as ChondroGide or Restore (De Puy, Warzaw, Indiana) (Cicuttini et al., 1996 Jones and Peterson, 2006). The combination of type I collagen with GAG in scaffolds had a positive effect on chondrocyte phenotype (van Susante et al., 2001). Hyaluronic acid is a non-sulphated GAG that makes up a large proportion of cartilage extracellular matrix (Schulz and Bader, 2007). Matrices composed of hyaluronan have been frequently used as carriers for chondrocytes. Facchini et al. conrms the hyaluronan derivative scaffold Hyaff 11 as a suitabl e scaffold both for chondrocytes and mesenchymal stem cells for the treatment of articular cartilage defects in their study (Facchini et al., 2006). Sugar-based natural polymers such as chitosan, alginate and agarose can be formulated as hydrogels and in some cases sponges or pads. Although these materials are extensively used in in vitro research, their role in in vivo cartilage reconstruction is still limited (Stoop, 2008).Growth factors are proved to be able to promote the formation of new cartilage tissue in both explants and engineered constructs. Insulin-like growth factor-I (IGF-I), transforming growth factor-1 (TGF-1) increases, basic fibroblast growth factor (bFGF) can stimulate cell proliferation and/or biosynthesis in chondrocytes which were