Journal of Research and Practice on the Musculoskeletal System JOURNAL OF RESEARCH AND PRACTICE ON THE MUSCULOSKELETAL SYSTEM Mini Review Stem cells in fracture healing Efstathios G. Athanasakis “Evangelismos” General Hospital of Athens, Athens, Greece Abstract Bone fractures are the most common injuries seen in emergency departments worldwide. Fracture healing is a complex physiological process. Existing research has proved that Mesenchymal stem cells (MSCs) assist and augment fracture healing. This review is examining the importance of MSCs in bone healing. A literature search was performed using PubMed, and relevant articles were retrieved. Keywords: Stem cells, Fracture healing, Nonunion Introduction have a demonstrated use in the treatment of hematopoietic and bone marrow disorders8-10. Fracture healing Adult stem cells (somatic stem cells), reside in specific Bone fractures are the most common injuries seen in tissues and their main role is thought to be that of maintenance emergency departments worldwide. Bone tissue has a and repair of tissues11. Hematopoietic stem cells (HSCs) can remarkable capacity of scar-free repair following a fracture, self-renew continuously in the marrow and differentiate into through fracture healing, a process that recapitulates the full complement of cell types found in blood. Thus they many aspects of embryonic skeletal development1,2. The play the role of premier adult stem cells12. However, most general pattern of secondary (indirect) bone healing, which orthopaedic research has been focused on mesenchymal is the most common form of fracture healing3, based on stem cells (MSCs). Mesenchymal stem cells(MSCs) are endochondral ossification, consists of haematoma formation, stromal cells that have the ability to self-renew and also inflammation, angiogenesis, cartilage formation (with exhibit multilineage differentiation. MSCs have been retrieved subsequent calcification, cartilage removal and then bone from a variety of tissues like periosteum, muscle, blood, formation) and bone remodeling4. Primary (direct) healing blood vessels, synovium, and fat13. They can be obtained based on intramembranous ossification, with no periosteal from a patient and cultured in vitro and can thus act as reaction or visible callus formation, doesn’t occur commonly autografts, diminishing the concerns of immunogenicity that in the natural process of bone healing and is common in ORIF. exist with other graft sources14. The principles of isolation, proliferation, and differentiation describe the general goals Stem cells of stem cell research and provide the basis of the potential of stem cell use in orthopaedics. The sources of MCs are varied. Stem cells can be defined as undifferentiated cells that The ideal source should be easily accessible, harvesting can proliferate and have the capacity both to self-renew and should be noninvasive and cells should be rapidly expandable differentiate to one or more different cell lineages. Stem cells can be classified according to their source. They can be isolated from fetuses, or adult tissue, but the range of cell types to which they can differentiate varies. Embryonic stem cells (ESCs) result from the isolation and The author has no conflict of interest. cultivation of cells from the inner cell mass of the blastocyst, Corresponding author: Efstathios G. Athanasakis, MD, “Evangelismos” General Hospital of Athens, 45-47 Ipsilantou that forms at approximately 5 days after fertilization5. They Str., 10676, Athens, Greece are pluripotent, capable of differentiating into cells derived [email protected] from all three germ layers6. E-mail: Fetal stem cells can be isolated from fetal blood, bone Edited by: Konstantinos Stathopoulos marrow, liver, and kidney7. There is a high concentration of Accepted 25 June 2019 Published under Creative Common License CC BY-NC-SA 4.0 (Attribution-Non Commercial-ShareAlike) Common License CC BY-NC-SA 4.0 (Attribution-Non Creative Published under fetal stem cells in cord blood. HSCs isolated from cord blood www.jrpms.eu 10.22540/JRPMS-04-020 JRPMS | March 2020 | Vol. 4, No. 1 | 20-23 20 Stem cells in fracture healing by in vitro culture15. Autologous bone marrow contains MSCs colleagues report that the role of stem cells for successful that can be directly or percutaneously injected into the injury bone regeneration is critical (diamond concept). Osteogenic site. However, it is an invasive procedure and often yields cells, osteoconductive scaffolds growth factors and the a few cells. Adipose tissue is probably the richest source mechanical environment is the diamond model for treating in human body and may provide an abundant population of fracture healing, nonunions and large bone defects21,22. Such MSCs16. The techniques that have been described for MSCs’ problems can be treated by bone autografts and allografts isolation and identification include flow cytometry, per that can be combined with MSCs taken from bone marrow plating, cell transfers, and antibody-coated heads. aspirates. Autologous bone marrow is a very rich source of Proliferation is the process by which MScs are plated in MSCs and growth factors and can be injected into the injury vitro (on animal products usually) and grown in monolayers17. site directly23-25. Differentiation is the process by which MSCs under Current evidence has shown that cells with the MSC appropriate in vitro or in vivo conditions become more phenotype reside within human skeletal muscles have the specialized cells e.g. osteoblasts, adipocytes, and capability of in vivo bone formation in combination with chondrocytes. This procedure is induced through hormonal, bone scaffolds26. Moreover, MSCs can be used as a tool to chemical, and mechanical factors. The multipotency and the deliver bone morphogenic protein-2 (BMP-2) by in vivo and extensive proliferative ability of MSCs make them very good ex vivo approaches acting as conduits for gene therapy. candidates for biological cell-based tissue repair approaches Lieberman and colleagues implanted into the quadriceps like fracture healing18. muscle of severe combined immune deficient mice, bone morphogenetic protein-2-producing cells. Thus showed Material and methods that abundant heterotopic bone formation could be induced A review of the literature was undertaken, and articles and large segmental femoral defects in nude rats could be published later than 2000 were included. Searches were successfully healed27. Musgrave and colleagues showed that performed using PubMed in the English language to identify In vivo injection of different cell populations into the triceps both original and review articles concerning the use of MSCs surae musculature of hind limbs of adult immunodeficient in fracture healing. Key words used included stem cells, (SCID) mice had. As a result the secretion of bone MSCs, bone marrow, adipose tissue, treatment, fracture morphogenetic protein-2 and then bone formation28. healing, bone defect, nonunion, pseudarthrosis, delayed Scaffolds seeded with MSCs have very often a clinical union, long bones. The articles were downloaded directly from application in tissue engineering and bone regeneration. publishers or other online resources. We excluded as non- They support and promote the osteogenic differentiation of relevant those papers that didn’t concern the enhancement MSCs. Thus enhancing fracture healing Xue D and colleagues of fracture healing through the use of stem cells. showed that the combination of HMGB1 gelatin sponges with MSC sheets promotes fracture healing in rat with tibial Results fractures that treated surgically. The HMGB1 gelatin sponge Eighteen articles that met our inclusion and exclusion scaffolds helped the expansion of mesenchymal stem cells criteria were incorporated in this review. Stem cells have (MSCs) and promoted the osteogenic differentiation of MSCs 29 applications throughout the field of orthopaedics. Especially and MSC sheets . in fracture healing, they play a very important role. MSC’s Meeson R and his colleagues showed that the can be used for the treatment of fractures by direct injection, administration of VEGF and the CXCR4 antagonist AMD3100 by systemic infusion, as genetically modified cells or through would increase the availability of MSCs and improve fracture the combination of scaffolds and growth factors. Direct healing. They stabilized with an external fixator a 1.5mm infusion may be better than a systemic infusion of MSC’s femoral osteotomy in rats. The bone volume was increased because only a few migrate to the site of fracture. Gao J in animals treated with VEGF and AMD310030. and et al. used bone marrow-derived MSCs from rats that Another interesting finding of Dong P and his colleagues were cultured, labeled with 111 In-oxine and infused to the is that melatonin promotes osteoblastic differentiation of animals and found that these cells were accumulated to the MSCs and fracture healing in rats with femoral fracture via lungs, while only a small percentage was found in the liver NPY/NPY1R signaling31. and spleen after iv or intra-artery infusion19. Schrepfer and Xu L and colleagues using an open femur fracture colleagues using mouse MSCs’ showed that cell trapping model in rats showed that Sox11-modified mesenchymal in lungs could be reduced with IV Sodium nitroprusside stem cells (MSCs) accelerate bone fracture healing. Sox11 pretreatment, increasing MSC passage through the lung regulates MSC differentiation and migration, and Sox11- capillaries, and potentially facilitating