Mesenchymal stem cells are multipotent and have ability to self-renew and proliferate over generations without significant loss of their characteristics [11]. They do not present with the ethical and legal concerns associated with embryonic stem cells and unlike committed chondrocytes isolated from hyaline cartilage, they retain their transdifferentiation capability. MSCs were first described in the 1960s by Friendenstein et al. [12]. They demonstrated that rare clonogenic cells isolated from adult rodent bone marrow (BM) aspirates [13] rapidly proliferated in vitro giving rise to distinct colonies, i.e. fibroblastic colony-forming units (CFU-F). These colonies, derived from a single precursor cell, can differentiate into all the different cell lineages of the limb mesoderm (osteoblasts, chodrocytes, adipocytes etc.) [14]. This property gives the potential to generate virtually billions of ex vivo-expanded MSCs from relatively small samples of BM aspirates.

Tissue engineering is a multidisciplinary area of research. It aims to regenerate tissue by the implantation of cells grown ex vivo, or by stimulating the cells to grow into an implanted matrix [15]. The ex vivo expansion of these cells into all types of mesenchymal tissue is paramount to the success of cell-based tissue engineering. MSC-based therapy can be used for tissue regeneration by both site-specific (localised) and systemic applications.

Its basic principle involves the use of living cells with a natural or synthetic support, or scaffold, to produce a three-dimensional (3D) living tissue construct that is implantable and can facilitate a faster rate of tissue repair or fill recipient tissue site. To engineer the new tissue, donor cells are first isolated, harvested and seeded onto the scaffold. The next step is the stimulation of cell proliferation and then the maintenance of cellular differentiation. Finally, the living tissue is implanted into the patient in order to obtain the 3D construct containing the newly seeded living cells in vivo. MSCs are a useful cell source in tissue engineering due to their innate capacity for regeneration and their ability to be easily isolated and expanded using culture techniques.

An ideal tissue engineering scaffold should have some key properties such as being osteoconductive if filling bone site. It should be fully biodegradable, porous, biomechanically stable and non-immunogenic to the host site [16]. Natural biomaterials facilitate better cell attachment, differentiation and function, however, they may vary from batch-to-batch and have difficulties in scaling up by ex vivo expansion of the MSCs. Synthetic biomaterials, on the other hand (of which polyactides are the most widely used), can be engineered according to precise specifications but may not always interact with cells in the desired way. Hence, a biomimetic material which can combine the advantages of both natural and synthetic biomaterials is an ideal scaffold for cell-based tissue engineering.

In a study by Yoon et al. [17], a fully absorbable osteogenic poly (DL-lactide-co-glycolide) (PLGA) scaffold seeded with MSCs was used to repair intercalated bone defects in vivo. Biomaterials can be rendered osteoinductive in several ways and, in this case, the scaffold was loaded with calcitriol, which is shown to induce mineralisation of the bone tissue [18]. The appropriate level of calcitriol in the pores is crucial to support proliferation and differentiation of the cells into osteoblasts. Calcitriol-loaded PLGA scaffolds seeded with/without MSCs were implanted in an animal model of a bone defect. Radiograph assessments showed that only the calcitriol-loaded groups showed bony union at 9 weeks. Also, RT-PCR and histological studies confirmed the development of new osteoid matrix and direct calcium deposits contributing to successful regeneration of the bony defects. At 20 weeks, complete bony union was achieved. The MSCs not only enhanced osteoconduction of the implanted scaffold via angiogenic activation of the host cells, but also underwent differentiation. The PLGA scaffold also degrades completely in 8 weeks without leaving residual metabolites due to its high porosity [19].

However, whether the MSCs themselves differentiated into osteoblasts directly or they promoted angiogenesis of the scaffold remains uncertain. Nevertheless, this fully absorbable synthetic bone graft biomaterial along with marrow stem cells does provide a hopeful allogenic graft substitute to repair large bone defects.

Hydroxyapatite (HA) has been used extensively in orthopaedics, particularly with promoting osteoconduction of prosthesis. Maracci et al. [3] investigated the use of HA-coated ceramic scaffolds seeded with MSCs to repair long bone defects. The MSCs were expanded in vitro and seeded onto the scaffolds. These were placed in the long bone defects of four patients. The patients were followed up over 6-7 years and evaluated using radiographs at fixed intervals. The results showed integration of implants with host bone, new bone formation inside bioceramic pores and vascular ingrowth. However these outcomes were all progressive. Seven months after implant surgery, complete fusion and full functional recovery of the limbs was achieved. This was less than half the recovery time compared to the traditional bone allograft.

Future strategies in this field would be to use more ideal costructs like PLGA which provide the initial support but is resorbed at a similar rate to new bone deposition in the scaffold. It should also provide more stable fixation and site-specific formation of new bone as shown in animal studies using porous calcium phosphate-based scaffolds [20, 21]. Ultimately, there is a need for randomised controlled trials after performing more animal studies to apply this to a clinical setting. In addition, the cost benefit ratio will also need to be considered after sufficient trials have been carried out to validate the use of this technology over traditional methods of bone reconstruction [2].

Bone volume is determined by the relative rates of bone formation and resorption. Growth factors are protein signalling agents that act locally to stimulate formation and proliferation of osteoblasts [22]. The multilineage potential of MSCs occurs under guided differentiation to obtain the required cell type. This can only be achieved under the modulation of appropriate growth factors [23]. Once damaged, chondrocytes, especially from articular cartilage have only a limited capacity in self-repair [24]. The use of growth factors extends to regeneration of muscle, tendon, ligament and connective tissue. Hence, growth factors are used to retain MSCs’ proliferative capacity and differentiation potential, which is lost during their ex vivo expansion. However, delivering sufficient quantities of growth factor locally in a sustained manner has proven difficult until the advent of gene transfer technology. Combining this gene-based delivery system with tissue bio-engineering is a potential approach to achieving more biologically active product at the site of the defect than that delivered by exogenous recombinant proteins [25].

There are a number of growth factors used for regenerative applications in orthopaedics. Some of these will be discussed here.

Mesenchymal stem cells, that are the most commonly used type of cells can be easily isolated and cultured in vitro. Harnessing these osteoprogenitor cells that have the ability to differentiate into all the different cell lineages of the limb mesoderm gives them the potential to generate virtually billions of ex vivo-expanded MSCs that can be implanted at the site of the defect, hence a relatively non-invasive technique. However, being able to isolate a demonstrably pure, multipotent and self-renewing stem cell sub fraction from bone marrow stroma, or in fact any other tissue source continues. Future research should aim to focus on purification of these stem cells in addition to trying to fully understand the potential of these primitive cells so they can be harnessed to their full potential to enhance tissue repair.

Osteoconductive 3D constructs seeded with MSCs is a highly porous, biodegradable and biomechanically stable scaffold which does not evoke an immunogenic host cell response. These bio-engineered graft substitutes make an ideal alternative to the traditional bone/cartilage grafts, showing no donor site morbidity and being completely resorbed from the host after a period of time. Future strategies would involve designing a scaffold which provides stable fixation and maximised strength along with site-specific formation of new skeletal tissue.

Growth factors contribute angiogenic and regenerative potential along with mechanical stability in the differentiated tissue. Harnessing this capacity of growth factors as extracellular signalling agents can be used for the differentiation of bone and other tissues to mimic the natural development process [55]. Gene-based delivery systems have aided the delivery of sustained quantities of these growth factors but further exploration of associated risks and benefits as a source for cell-based tissue regeneration in longitudinal animal studies is required.

Ultimately, there is a need for randomised controlled trials on human populations to apply these findings to a clinical setting. In addition, the cost benefit ratio will also need to be considered after sufficient trials have been carried out to validate the use of this technology over traditional methods of tissue reconstruction.

None declared.