Stem Cell-based Tissue Engineering Approaches for Musculoskeletal Regeneration
Stem Cell-based Tissue Engineering Approaches for Musculoskeletal Regeneration
Alireza Farahnak,1,*
1. Department of Biology, Science and Art, Yazd
Introduction: The fields of regenerative medicine and tissue engineering have grown dramatically since their early inception in the 1960s and 1970s. Early attempts simply sought to transplant somatic cells into a lesion area but typically led to little or no success. The development of biomaterial scaffolds further advanced tissue engineering by allowing for the creation of biomimetic environments that enhanced cell maintenance and differentiation. While somatic cells, such as osteoblasts and chondrocytes, were among the first cell sources to be used in various tissue engineering applications, the prospects of tissue engineering were given new momentum with the addition of stem cells to the pool of cell choices. Adult tissue-derived stem cells, including mesenchymal stem cells (MSCs), became the backbone for cell therapies due to their expansion and multipotent potential, and demonstrated success in clinical applications. Further, stem cells may be more advantageous than somatic cells due to their tendency to favor anabolism instead of catabolism, whereas somatic cells are more so poised to maintain tissue homeostasis. Additionally, the isolation of somatic cells can induce donor site morbidity, and somatic cells limit the potential of allogeneic cell therapy due to their immunogenicity. The isolation of embryonic stem cells (ESCs) by Evans and Kaufman in 1981 from mouse embryos and by Thomson from human embryos in 1998 further stimulated the field by providing a cell source with seemingly infinite expansibility. Tissue engineering approaches are now conceivably able to target and derive almost any cell in the body. Stem cell-based research has exploded in recent years, attracting a great deal of scientific and public attention. An overarching goal of stem cell-based research is to understand how tissues/organs are formed and diseases develop, and in so doing, develop more effective therapies to treat diseases that are otherwise difficult to cure by current medical procedures. The isolation of ESCs is considered one of the major milestones fueling this movement, as it has provided a reliable tool to study tissue/organ formation and pathology and thus paved the way for fields like regenerative medicine and tissue engineering to emerge.
Methods: 1. In vitro Differentiation of Stem Cells: Researchers can direct the differentiation of stem cells (such as mesenchymal stem cells or induced pluripotent stem cells) into specific lineages such as osteoblasts (bone cells) or chondrocytes (cartilage cells) under defined laboratory conditions. This can be achieved through manipulating the culture conditions, including growth factors, extracellular matrix (ECM) components, and mechanical stimuli.
2. Scaffold-Based Approaches: Scaffolds made of biocompatible materials (e.g., hydrogels, polymers, ceramics) can be used to support stem cell attachment, proliferation, and differentiation. These scaffolds can be engineered to mimic the native ECM of musculoskeletal tissues, providing the necessary mechanical and biochemical cues for tissue formation.
3. Combination with Growth Factors and Cytokines: The incorporation of bioactive molecules, such as growth factors (e.g., bone morphogenetic proteins, transforming growth factor-beta) can enhance stem cell differentiation and promote tissue repair. These factors can be incorporated into scaffolds or delivered in a sustained manner at the site of injury.
4. Gene Therapy: Engineering stem cells to express specific genes that promote proliferation and differentiation can enhance their therapeutic potential. This can involve the use of viral or non-viral vectors to deliver the genes of interest.
5. Physical Stimuli: Applying mechanical or electrical stimuli to stem cells can influence their behavior and promote differentiation toward musculoskeletal lineages. Techniques such as bioreactor systems can provide dynamic culture conditions that simulate the in vivo environment.
6. Decellularized Matrices: Decellularized tissues derived from donors can serve as scaffolds that retain native ECM components while removing cells. These matrices can provide a natural microenvironment for stem cells, facilitating tissue regeneration when transplanted into the body.
7. Combination of Stem Cells with Other Therapies: Combining stem cell therapy with other treatment modalities, such as physical rehabilitation or pharmacological interventions, may improve outcomes in musculoskeletal regeneration.
Results: Stem cell-based tissue engineering offers promising solutions for musculoskeletal regeneration, addressing issues such as bone fractures, cartilage damage, and tendon injuries. Mesenchymal stem cells (MSCs), embryonic stem cells (ESCs), and induced pluripotent stem cells (iPSCs) are key players due to their ability to differentiate into bone, cartilage, and muscle tissues. Scaffold-based systems, growth factor delivery, and gene editing are central approaches that enhance tissue repair. These strategies show potential in bone regeneration, cartilage repair, tendon healing, and muscle regeneration. While challenges like immune rejection and scalability remain, advances in biomaterials and gene editing hold promise for future clinical applications.
Conclusion: Multiple factors regulate the self-renewal and differentiation of relevant stem cell types into musculoskeletal lineages, and elucidation of environmental cues directing appropriate cell activities has greatly advanced the field of tissue engineering. However, for in vitro tissue engineering products to become a clinical reality, studies investigating the combined effect of multiple environmental cues will need to be conducted. For example, studies investigating the role of GFs during a specific stage of the tissue engineering process are typically carried out under normoxic conditions, and it is entirely possible that the observed effects from these studies would not persist under hypoxic conditions, which are more physiologically relevant. More importantly, however, researchers will need to find conditions that can improve the resulting phenotype of differentiated musculoskeletal cell types. For chondrogenesis, we still need to determine how to reproducibly repress the hypertrophic and fibrocartilaginous characteristics of chondrocytes derived from MSCs, and for ESCs, we likely need to expand upon the three-step differentiation protocol from Oldershaw and colleagues to further enhance the chondrogenic differentiation program. For osteogenesis, MSCs appear quite adept at differentiating into osteoblasts in vitro, but the challenge will be engineering a vascularized tissue of physiologically relevant architecture. To this end, the most promising avenue may be exploiting the ability of hypertrophic chondrocytes to recapitulate endochondral ossification when implanted in vivo.