**Regenerative Medicine: A New Paradigm in Bone Regeneration**

Orlando Chaparro and Itali Linero

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/62523

#### **Abstract**

[127] Salter E, Goh B, Hung B, Hutton D, Ghone N, Grayson WL. Bone tissue engineering

[128] Carver SE, Heath CA. Semi-continuous perfusion system for delivering intermittent physiological pressure to regenerating cartilage. Tissue Eng. 1999;5(1):1–11.

[129] Altman GH, Horan RL, Martin I, Farhadi J, Stark PRH, Volloch V, et al. Cell differen‐

[130] Goldstein AS, Juarez TM, Helmke CD, Gustin MC, Mikos AG. Effect of convection on osteoblastic cell growth and function in biodegradable polymer foam scaffolds.

[131] Cartmell SH, Porter BD, García AJ, Guldberg RE. Effects of medium perfusion rate on cell-seeded three-dimensional bone constructs in vitro. Tissue Eng. 2003;9(6):1197–203.

[132] Freed LE, Guilak F, Guo XE, Gray ML, Tranquillo R, Holmes JW, et al. Advanced tools for tissue engineering: scaffolds, bioreactors, and signaling. Tissue Eng. 2006;12(12):

[133] NASA/Marshall Space Flight Center: Free Download & Streaming: Internet Archive.

[134] Granet C, Laroche N, Vico L, Alexandre C, Lafage-Proust MH. Rotating-wall vessels, promising bioreactors for osteoblastic cell culture: comparison with other 3D condi‐

[135] Nishi M, Matsumoto R, Dong J, Uemura T. Engineered bone tissue associated with vascularization utilizing a rotating wall vessel bioreactor. J Biomed Mater Res - Part A.

[136] Yeatts AB, Fisher JP. Bone tissue engineering bioreactors: dynamic culture and the

[137] Yeatts AB, Choquette DT, Fisher JP. Bioreactors to influence stem cell fate: augmenta‐ tion of mesenchymal stem cell signaling pathways via dynamic culture systems.

[138] Dahlin RL, Meretoja V V., Ni M, Kasper FK, Mikos AG. Design of a high-throughput flow perfusion bioreactor system for tissue engineering. Tissue Eng Part C Methods.

[139] Mikos. Flow perfusion culture of mesenchymal stem cells for bone tissue engineering.

[140] Hoffmann W. Novel perfused compression bioreactor system as an in vitro model to

[141] Chua CK, Yeong WY. Bioprinting: Principles and Applications. World Scientific

bioreactors: a role in the clinic? Tissue Eng Part B Rev. 2012;18(1):62–75.

tiation by mechanical stress. FASEB J. 2002;16(2):270–2.

Biomaterials. 2001;22(11):1279–88.

Bioreactor rotating wall vessel.

2013;101 A(2):421–7.

2012;18(10):817–20.

Publishing Co. Pte. Ltd.; 2015.

tions. Med Biol Eng Comput. 1998;36(4):513–9.

influence of shear stress. Bone. 2011. 171–81.

Biochim Biophys Acta. 2013;1830(2):2470–80.

Stem Book. Harvard Stem Cell Institute; 2008; pp. 1–18.

investigate fracture healing. Front Bioeng Biotechnol. 2015;3:10.

3285–305.

252 Advanced Techniques in Bone Regeneration

Bone defects are the cause of functional disability and the restoration of skeletal function remains an important challenge on orthopedics, neurosurgery and oral and maxillofa‐ cial surgery. Because of the limitations of the currently used techniques for the reconstruction of bone defects and the difficulties for the implementation of new therapeutic strategies, a new paradigm in the field of reconstructive surgery has arisen, leading to tissue engineering and regenerative medicine. Mesenchymal stem cells (MSC) have emerged as a promising alternative for the treatment of bone lesions. It was postulated that the therapeutic action was the result of proliferation and differentia‐ tion of MSCs, replacing injured tissue. However, recent studies have shown that MSCs secrete a number of trophic factors that have a strong effect during repair and tissue regeneration. This represents a shift from a paradigm centered on MSC proliferation and differentiation to a new paradigm in which the MSCs exert their beneficial effect by the secretion of paracrine factors that induce endogenous repair mechanisms. This chapter will bring together basic and clinical aspects, focused on novel findings on MSC paracrine effect and the development of new therapeutic strategies based on growth factors, cytokines and signaling molecules involved in bone regeneration.

**Keywords:** mesenchymal stem cells, paracrine effect, bone regeneration, growth fac‐ tors, cytokines

### **1. Introduction**

The regeneration of bone tissue remains an important challenge in the field of orthopedic and maxillofacial surgery. Bone defects produced by trauma, tumors, infectious diseases, biochem‐

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ical disorders, congenital disorders or abnormal skeletal development are the major causes of functional disability, and esthetic and psychological trauma for patients.

One of the goals of treating a bone defect is to restore the normal morphology and function of the affected structure. Specific surgical techniques such as distraction osteogenesis, implanta‐ tion of biomaterials (bone substitutes) and implants of bone grafting have been developed to reach bone regeneration [1, 2]. Demand for bone grafts is considerable and represents the second most common procedure after blood transplants, with more than 2.2 million bone grafts performed annually worldwide in orthopedics and dentistry [3].

Despite advances in bone regeneration and the availability of many treatments, most clinicians and researchers continue to come to the same conclusion: autologous bone grafting remains the "gold standard," compared to other reconstructive procedures [4–9]. Bone from the same patient lacks immunogenicity and contains all the elements necessary to effectively induce tissue regeneration. It has osteoprogenitor cells which go directly to the implant site, cytokines and extracellular matrix [5], providing the three classic elements of an ideal bone graft: osteogenesis, osteoconduction and osteoinduction [5–7, 9, 10]. However, autologous bone grafts have several important limitations, including high risk of morbidity in the donor site [5, 6, 11], with disadvantages in terms of costs, time of surgical procedure, discomfort for the patient and possible complications.

Additionally, many times the volume of tissue available for the procedure is not sufficient to fill or cover a defect, given the limited availability of autologous tissue [4, 10], and the quality of the autograft is highly variable and is influenced by age and metabolic abnormalities of the patient [7]. To overcome these limitations, a variety of exogenous substitutes, including allografts, xenografts and alloplastic materials, have been introduced into clinical practice in the past three decades [4]. However, these substitutes have less osteogenic and osteoinductive properties [6, 12] and a greater possibility of transmission of infectious diseases [6, 8], restrict‐ ing their use [8].

In order to successfully overcome the shortcomings of current approaches for bone regenera‐ tion, tissue engineering emerged as a discipline that provides the necessary tools for bone regeneration and restoration. The presence of cell populations that orchestrate the release of growth factors, the maintenance of a stable matrix and the stimulation of angiogenesis are key factors to successful regeneration of bone tissue, because they play a decisive role in the healing process [13, 14]. The technologies developed recently based on tissue engineering, such as gene therapy, stem cell therapy and the application of osteoinductive growth factors, looking for the control of the dynamics of these elements to enable more predictable bone regeneration surge as a significant promise in clinical practice [15].

Cell-based therapy for the regeneration of bone tissue has been extensively investigated. Several cell types have been used as alternatives for the reconstruction of bone tissue, including osteoblasts, embryonic stem cells, periostium derived-progenitor cells (a specialized cell type that covers bone surfaces and have the potential to differentiate into multiple mesenchymal tissues, including bone) and mesenchymal stem cells, also known as multipotential stromal cells (MSC) [16].

MSC has become one of the best alternatives in cell therapy and specifically in bone regener‐ ation. MSCs can be isolated from virtually all vascularized tissue and they are able to differ‐ entiate into various mesenchymal tissues such as bone, cartilage, muscle, tendon, adipose tissue and hematopoiesis-supporting stroma. However, a growing number of recent reports in the literature have revealed that even if a therapeutic effect can be documented, the implanted MSC cells do not differentiate and do not survive for a long time [17, 18].

The use of MSCs in the treatment of musculoskeletal injuries was initially based on their ability to differentiate into various cell types [1, 7, 8]. The rationale was that after implantation or MSC injection, the cells would be able to colonize the injured site and differentiate into the appro‐ priate lineages. This mechanism has now been challenged by a new paradigm to extend it to an alternative mechanism called **paracrine effect**, where MSCs secrete biologically active molecules which have beneficial effects on the injured tissues [9] by inhibiting fibrosis, apoptosis and inflammation [10, 11] and promoting angiogenesis and tissue regeneration [19– 21].
