Musculoskeletal regeneration requires growing development of stem cell technology. The supply of necessary cells derived from sterile sources in physiological conditions has provided us with an unlimited and sustainable pattern in the field of cell replacement treatment and other applications (Deshpande et al., 2013). The stem cell therapy contributes to proliferative sustainability, multidirectional differentiation and anoxia endurance (Wang et al., 2016). Different tissues have been used as stem cells sources. Doctors have succeeded in making progress in many cells, such as bone marrow mesenchymal stem cells, adipose-derived stem cells and peripheral blood stem cells (Jeevanantham et al., 2012; Delewi et al., 2013). For instance, adipose-tissue derived stem cells (ADSCs) have vast therapeutic potential. On occasion, they have been applied clinically, for example, in maxillary reconstruction, critical limb ischemia or insulin-dependent diabetes mellitus. Besides, its self-renewing and differentiation in different cellular lines, including tendon and bone tissue is very important in many aspects (Ahmad et al., 2012). In vivo studies showed that mesenchymal stem cells (MSCs) can self-renew or differentiate into multiple lineages and regenerate bone, cartilage, muscle, blood, and cardiac tissue (Longo et al., 2013). They found that the transplanted cells improved the function of injured organs by attracting the host's repair cells to the damaged site by releasing certain factors (i.e., VEGF) that indirectly helped in the repair process. However, the exact source of these host repair cells was not fully elucidated. Besides, researchers have concentrated on the part of MSCs as an important trauma-related factor in vasculogenesis and osteogenesis (Gawlitta et al., 2012; Askarinam et al., 2013). In other words, MSC is directed to the damaged site and functions as a regulator for vasculogenesis and osteogenesis (Levi et al., 2011). Moreover, scientists believe that stem cells have huge potential in the course of cell recombination. Human stem cells can proliferate infinitely in vitro, thereby providing samples with reproducible characteristics (Thangarajah et al., 2009).

Although the therapeutic effect of stem cell is very promising, its low efficiency has compromised its potential applications in many aspects (Kang et al., 2012). People tried their best to use priming for improving the therapeutic efficacy of stem cell, but failed due to setbacks like low efficient improvement and an immune deficiency of certain proteins. Bone marrow has been identified as an excellent source of MSCs and can be accessed readily by surgeons intraoperatively. Nevertheless, scientists also attempted to obtain MSCs from fatty tissue (Angeline and Rodeo, 2012). During the last decades, scientists used a variety of different vehicles for the local delivery of MSCs, including type I collagen gels, collagen sponges and fibrin, to the Achilles, patellar, or RC tendons in animal models (Majewski et al., 2009).

In addition to stem cell therapy, blood derived products such as autologous conditioned serum and the latest orthopedic panacea, platelet rich plasma (PRP) has also been widely researched on its plentiful applications in various fields. Bone defects in the oral cavity and maxillofacial region are the major fields (Mehta, 2010); Soon later its applications extended across many fields including periodontal (Moghe et al., 2012; Morikuni et al., 2013) and oral and maxillofacial surgery (Albanese et al., 2013; Daif, 2013), aesthetic plastic surgery (Cervelli et al., 2009), spinal surgery (Okamoto et al., 2012), cardiac bypass surgery (Gravlee, 1994) and treatment of soft-tissue wounds or ulcers (Akhundov et al., 2012; Jiritano et al., 2013). The early introduction of PRP into clinical practice was suggested by Ferrari et al. (1987). PRP is an autologous platelet concentrate obtained from fresh whole blood by centrifugation. To be applied at the surgical site, PRP must be activated to induce platelet degranulation and fibrin polymerization, thus obtaining a clot usually called platelet gel (PG). PRP is naturally heterogeneous for various factors that exist in PRP preparation protocols including: (1) the initial quantity of platelets, (2) the applications of anticoagulants, (3) the use of leukocytes, and (4) the inclusions of activators resulting in different biological outcomes. Apart from naturally autologous state, and no risk of pathogen transmission or immunological rejection, PRP contains the appropriate proportion of certain factors necessary for wound healings (Marques et al., 2015) (referred to in Figure 1).

The biological characteristics of PRP rely on the concentration in platelets. Proper preparations can help PRP secrete many growth factors (GFs) at high concentrations, including transforming growth factor-β, platelet-derived growth factor, insulin-like growth factor, vascular endothelial growth factor and epidermal growth factor. Therefore, standardization for PRP is very important. But variation in PRP concentration usually leads to decreased and unstable repairing effect in tissue regeneration. There are a few dozens of protocols and purification systems for PRP isolation in the world. Different methods result in different PRP contents and concentrations as well as variant clinical outcomes. Most researches indicated that five times content of normal platelets can contribute to effective regeneration by PRP. And higher concentration does not induce a better result (Marx, 2004). The best concentration of PRP has not been decided yet. But different purification facilities and methods will result in different biological characteristics and variance in clinical outcomes (Yu et al., 2011). Kreuz and colleagues used different isolation kits and centrifugation for PRP preparation. The results turned out that Dr.PRP Kit could most effectively promote mesenchymal progenitor cell proliferation, compared with other methods. Moreover, some protocols could even exhibit a higher chondrogenic differentiation potential than others (Kreuz et al., 2015). Masoudi and colleagues also mentioned different modes of PRP preparation affected other aspects, like extended viability under low temperature and increased platelet yields with longer centrifugation period (Masoudi et al., 2016). Scientists have expected to combine PRP and stem cell in the field of tissue regeneration, especially bone, cartilage and tendon repair for a long time. However, the relationship between PRP preparation and stem cell activity is very controversial. There are a few important applications of PRP and stem cells in musculoskeletal regeneration in recent years. Rubio-Azpeitia and colleagues used platelet-rich plasma (PRP) three dimensional printing scaffold to induce mesenchymal stem cells differentiation, and found increased expression of collagen 1 and 3 in this model. Besides, other articular genes were upregulated as well in mesenchymal stem cells stimulated by PRP (Rubio-Azpeitia et al., 2017). Tang and colleagues experimented on rabbit derived PRP for chondrogenic induction of adiposed stem cells. Collagen II was significantly increased accompanied by aggrecan expression (Tang et al., 2015). However, Huber and colleagues discovered thrombin was effective in helping production of PRP, with an easier and faster style in comparison with other activation and purification method. However, they tested GFs like PDGF-AA, VEGF and VEGF, and found this kind of isolation method had no positive effects in promoting GFs secretion (Cares et al., 2016). PRP is vital for advancing proliferation of bone precursor cells in different animals, including rabbits and goats (Lin et al., 2013; Kim et al., 2014). In spite of rich potentials of direct applications of PRP for wound healings, combining PRP with bone marrow mesenchymal stem cells (BMSCs) for tissue regeneration applications becomes a rising interest for a long time. PRP is a significant supplement for PRP-gel delivery carrier for cell transplantation. In addition, it is mentioned that PRP can contribute to the proliferation of BMSCs and their differentiation into osteoblasts (Montgomery et al., 2011). The GFs in PRP have many functions, including accelerating angiogenesis and promoting collagen formation, thus leading to wound healing and tissue formation. Hence, PRP is thought to be an ideal origin of GFs for many applications.

According to the aspects discussed above, GFs originate from PRP and cooperate with stem cells in bone healing and soft tissue repair. This review is mainly aimed at the interaction of PRP derived growth factors with stem cells in various differentiation into mature somatic cells in musculoskeletal regeneration.

Growth factors derived from PRP can contribute to tissue regeneration, by assisting cell migration, proliferation, differentiation and extra-cellular matrix synthesis (Gulotta et al., 2011; Isaac et al., 2012). However, the varying GF concentrations may have different biologic effects, resulting in the fact that individual differences in GF levels should be considered for reliable interpretation of the biologic functions and standardized application of PRP. Numerous GFs, including the basic fibroblast growth factor (bFGF), platelet-derived growth factor (PDGF), and transforming growth factor-β (TGF-β), which are released from PRP, can be detected during the early phase of wound healing (referred to in Table 1). Many of these GFs have a unique temporal expression profile and are thought to play an important role in the musculoskeletal regeneration. Generally, GFs are delivered locally to the bone and soft tissue regeneration site with tissue-engineered scaffolds, covered sutures, or dissolved in a fibrin-like encapsulation (Losi et al., 2013). Successful strategies to biologically augment tissue repair will require appropriate combination of GFs, responding host cells to the growth factor signaling and an optimal delivery method (Gulotta et al., 2011).

Stem cells have become more and more important in regenerative medicine and can be isolated easily and reproducibly from human tissues. They have many advantages such as easy and long-term proliferation, multi-lineage potential and tolerance toward hypoxic environments. The application of stem cells from various tissues in diverse diseases and anti-aging treatments is very promising in regenerative medicine. PRP is most often considered as a bio-aggregate of GFs and used to supplement biomaterials. It can also be prepared with fibrinogen, which can be easily activated to form fibrin gel with thrombin or calcium. Soon later its applications extended across many fields including periodontal and oral and maxillofacial surgery, treatment of soft-tissue wounds and so forth. Further investigation will be focused mainly on the exact signaling pathways in the interaction between genetic molecules mediated or released from GFs derived from PRP and their effects in modification, migration, and differentiation of stem cells. Primary questions lie in: (1) how to isolate certain GFs that influence a specific stem cell. We know that PRP derived growth factors can induce stem cell differentiation, proliferation and adhesion. However, a few GFs are upregulated at the same time. Therefore, it is hard to confirm the exact effecting growth factor in the process. (2) how to control and prolong the effective time of each growth factor for the best use. Normally, GFs are active under certain conditions for a short period of time. Proteins can be inactivated by many physical and chemical cues. It brings some difficulty to clinical scenario applications. (3) how to successfully prepare a desired PRP with a specific isolation protocol. There are many isolation methods with different facilities under varies temperature, humidity, and blood origin. It is hard to reach a consensus for the best PRP purification protocol. More emphasis will be laid on this aspect in preclinical and clinical researches. Once we can elucidate the underlying mechanism completely, we will illuminate the road ahead and create more miracles in musculoskeletal regeneration.

YQ, QH, WC, JS, XZ, YO, CF, and WY participated in its design, searched databases, extracted and assessed studies and helped to draft the manuscript. WY conceived the initial idea and the conceptualization, participated in the data extraction and analysis, and revised the manuscript. All authors read and approved the final manuscript.