The SAGE Encyclopedia of Stem Cell Research. Группа авторов
is high and, even after treatment, there is no restoration of full function. The use of stem cells to restore function of injured tendons and ligaments will help immensely in veterinary medicine and will aid in the treatment of injured animals. There is considerable interest in the use of stem cells in veterinary medicine and stem cells derived from animals are isolated and cultured for use in treatment.
The injection of stem cells directly into the site of injury may increase the rate of repair, but it requires the injection of specific growth factors to encourage the stem cells to follow required growth and development. This has brought into focus tissue engineering, which is the process of developing functional tissue that can be introduced directly at the injury site. There is an ever-increasing need to create better treatment procedures, and now stem cells are being genetically modified to make them function better.
Genetically Modified Stem Cells for Tissue Injury
Growth factors that are required for the growth and differentiation of stem cells or for the development of tissue-engineered matrix are expensive and need to be injected periodically. To avoid this, stem cells are being genetically modified so that they produce the growth factors on their own and would not require injection of additional growth factors. This type of tissue-engineered matrix can be mass produced and adopted for commercial use.
Amrita Duraiswamy
Sri Ramachandra Medical College
See Also: Cartilage, Tendons, and Ligaments: Cell Types Composing the Tissue; Cartilage, Tendons, and Ligaments: Development and Regeneration Potential; Cartilage, Tendons, and Ligaments: Existing or Potential Regenerative Medicine Strategies; Cartilage, Tendons, and Ligaments: Stem and Progenitor Cells in Adults.
Further Readings
Sugimoto, Y., et al. “Scx+/Sox9+ Progenitors Contribute to the Establishment of the Junction Between Cartilage and Tendon/Ligament.” Development, v.140/11 (2013).
Walsh, William R. Repair and Regeneration of Ligaments, Tendons, and Joint Capsule. New York: Springer-Verlag, 2005.
Zhang, J. and James H. C. Wang. “Human Tendon Stem Cells Better Maintain Their Stemness in Hypoxic Culture Conditions.” PLoS One, v.8/4 (2013).
Cartilage, Tendons, and Ligaments: Development and Regeneration Potential
Cartilage, Tendons, and Ligaments: Development and Regeneration Potential
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Cartilage, Tendons, and Ligaments: Development and Regeneration Potential
Fibrous connective tissues such as tendons and ligaments attach muscles to bones and bones to bones, respectively. This implies a gradual variation in composition, structure, and mechanical strength that ensures dissipation of stress and load on a joint. Injuries to tendons and ligaments are fairly common in sports or coupled with other orthopedic injuries, but restoration to original condition is almost never achieved and reinjuries are expected even after healing. It has been observed that natural healing after injuries to joints occurs through formation of fibrovascular scar tissue and not by regeneration of the original graded tissues, making the site susceptible to reinjury. Cartilage, on the other hand, is a firm and tough connective tissue that acts as a cushion between bones. Injury or gradual erosion of the cartilage can lead to painful joints, affecting the quality of life as it has limited capacity for healing.
Tissue engineering has emerged as a viable technique for repair and regeneration of damaged connective tissues. It utilizes a scaffold of a suitable material and shape, seeded with precursor cells and loaded with appropriate factors promoting growth and differentiation. The outer ear, a small piece of trachea as well as a patella, of cartilaginous and osteochondral origin respectively, have been successfully grown in vitro using this technique.
Interface tissue engineering (ITE) goes a step further to synthesize tissue for complex interfaces such as ligament to bone, tendon to bone, and cartilage to bone. The interface is actually only 100 μm to 1 mm in length but ITE necessitates thorough understanding of the cell composition, maturation of various cell types, as well as the structure–function relationships at the interface site. It is expected that the unique nature of the site would need the presence of specific transcription factors/proteins and its function would also be regulated by the extent of load bearing by the associated muscles.
The scaffolds constitute the most crucial component in the regeneration of connective tissues. Ideally, they should be biodegradable, porous, nontoxic and nonantigenic, flexible, and support nutrient and waste transport. They provide the framework for adhesion, growth, and differentiation of cells and facilitate cellular interactions. Scaffolds were made of a single component, calcium phosphate, 30 years ago but a number of novel materials have been tested for composite scaffolds over the last decade. These include synthetic microfibers such as porous polyethylene (PPE), poly-L lactic acid (PLLA), polylactide-co-glycolide (PLGA), and polyurethane, as well as biological materials such as collagen and silk. There are several studies reporting both the advantages and disadvantages of collagen-based scaffolds. Nanofibers form excellent scaffolds owing to their biocompatibility and possible variations in fiber diameter and alignment. The interaction of clay with biomolecules is also being studied to explore its suitability in tissue engineering.
Located in the knee and performed arthroscopically, an anterior cruciate ligament reconstruction surgery removes the torn ligament and replaces it with a tissue graft of either autografts or allografts. (Wikimedia Commons/Phalinn Ooi)
Scaffolds that mimic in vivo conditions at the interfaces should compulsorily be stratified and should attempt to replicate the gradation in structure and function found in native tissues. A triphasic scaffold that has been used for the regeneration of the anterior cruciate ligament (ACL) interface with bone involves three distinct, yet continuous phases: Phase A comprising PLGA 10:90 mesh was designed for fibroblast culture, Phase B made up of PLGA microspheres is for fibrochondrocyte culture and fibrocartilage formation, and Phase C composed of PLGA and bioactive glass (BG) composite microspheres was designed for bone formation. Each of the phases was seeded with the relevant cell types—fibroblasts, articular chondrocytes, and osteoblasts—and the matrix heterogeneity and density were evaluated over a period of eight weeks. Spatially separated but extensive tissue infiltration and matrix deposition with continuity was observed in Phases A and C. Migration of cells to Phase B and its vascularization were observed too. The PLGA-BG phase is instrumental in promoting mineralization and calcification of the cartilage and bone-like matrices in Phase B and C, respectively. The mechanical properties were also observed to improve with higher cell density. This graft has also been successfully grown in an athymic rat model. Nanofibers of PLGA and hydroxyapatite (HA) nanoparticles have also been used to assemble the triphasic scaffold with similar results.
Further work in the field needs to focus on optimization of the growth conditions of cells on the scaffold and its in vivo evaluation. The effect of biological, chemical, and mechanical stimulation on regeneration has to be studied under both in vitro and in vivo conditions. Incorporation of biologically active molecules into the nanofiber scaffold is a biomimetic approach to promoting cell growth and differentiation. Scaffolds coated with type 1 collagen and laminin showed enhanced proliferation of cells. Plasma treatment of the scaffold also promoted cell division reproducibly. Another successful and innovative approach has involved the fabrication of a scaffold by electrospinning basic fibroblast growth factor (bFGF), releasing PLGA nanofibers onto knitted silk microfibers. Two transcription factors, SOX-9 and scleraxis (Scx), have been identified to promote chondrogenesis and tenogenesis at the tendon/ligament to bone interface, and their incorporation into the scaffold structure is being planned.
After a multi-tissue graft has been successfully generated, its biological fixation or functional integration with each other as well as the host environment should