For in vitro differentiation of bone marrow-derived mesenchymal stem cells/mesenchymal stromal cells into osteoblasts by 2-dimensional cell culture a variety of protocols have been used and evaluated in the past. Especially the external phosphate source used to induce mineralization varies considerably both in respect to chemical composition and concentration. In light of the recent findings that inorganic phosphate directs gene expression of genes crucial for bone development, the need for a standardized phosphate source in in vitro differentiation becomes apparent. We show that chemical composition (inorganic versus organic phosphate origin) and concentration of phosphate supplementation exert a severe impact on the results of gene expression for the genes commonly used as markers for osteoblast formation as well as on the composition of the mineral formed. Specifically, the intensity of gene expression does not necessarily correlate with a high quality mineralized matrix. Our study demonstrates advantages of using inorganic phosphate instead of β-glycerophosphate and propose colorimetric quantification methods for calcium and phosphate ions as cost- and time-effective alternatives to X-ray diffraction and Fourier-transform infrared spectroscopy for determination of the calcium phosphate ratio and concentration of mineral matrix formed under in vitro-conditions. We critically discuss the different assays used to assess in vitro bone formation in respect to specificity and provide a detailed in vitro protocol that could help to avoid contradictory results due to variances in experimental design.

True tendon regeneration in human patients remains a vision of musculoskeletal therapies. In comparison to other mesenchymal lineages the biology of tenogenic differentiation is barely understood. Specifically, easy and efficient protocols are lacking that might enable tendon cell and tissue differentiation based on adult (stem) cell sources. In the murine mesenchymal progenitor cell line C3H10T½, overexpression of the growth factor bone morphogenetic protein 2 (BMP2) and a constitutively active transcription factor, Smad8 L+MH2, mediates tendon cell differentiation in vitro and the formation of tendon-like tissue in vivo. We hypothesized that during this differentiation secreted factors involved in extracellular matrix formation exert a major impact on tendon development. Gene expression analyses revealed four genes encoding secreted factors that are notably upregulated: periostin, C-type lectin domain family 3 (member b), RNase A4, and follistatin-like 1. These factors have not previously been implicated in tendon biology. Among these, periostin showed a specific expression in tenocytes of adult mouse Achilles tendon and in chondrocytes within the nonmineralized fibrocartilage zone of the enthesis with the calcaneus. Overexpression of periostin alone or in combination with constitutively active BMP receptor type in human mesenchymal stem cells and subsequent implantation into ectopic sites in mice demonstrated a reproducible moderate tenogenic capacity that has not been described before. Therefore, periostin may belong to the factors contributing to the development of tenogenic tissue.

Musculoskeletal dysfunctions are highly prevalent due to increasing life expectancy. Consequently, novel solutions to optimize treatment of patients are required. The current major research focus is to develop innovative concepts for single tissues. However, interest is also emerging to generate applications for tissue transitions where highly divergent properties need to work together, as in bone-cartilage or bone-tendon transitions. Finding medical solutions for dysfunctions of such tissue transitions presents an added challenge, both in research and in clinics. This review aims to provide an overview of the anatomical structure of healthy adult entheses and their development during embryogenesis. Subsequently, important scientific progress in restoration of damaged entheses is presented. With respect to enthesis dysfunction, the review further focuses on inflammation. Although molecular, cellular and tissue mechanisms during inflammation are well understood, tissue regeneration in context of inflammation still presents an unmet clinical need and goes along with unresolved biological questions. Furthermore, this review gives particular attention to the potential role of a signaling mediator protein, transforming growth factor beta-activated kinase-1 (TAK1), which is at the node of regenerative and inflammatory signaling and is one example for a less regarded aspect and potential important link between tissue regeneration and inflammation.

Introduction: Multipotent stromal cells (MSCs) are among the key candidates in regenerative medicine. However variety of MSC sources and general heterogeneity lead to controversial data in functional characterization. Furthermore, despite intensive usage as preclinical animal model, little is known about MSCs of the common marmoset monkey. Methods: MSCs derived from placental amnion and bone marrow samples from human and common marmoset were characterized in parallel over 12 passages to monitor similarities and significant differences (p ≤ 0.05, Student’s t-test) in MSC markers and major histocompatibility complex (MHC) class I expression by immunohistochemistry, flow cytometry, real-time PCR, metabolic activity test, with special focus on pluripotency associated genes. Results: Human and non-human primate MSCs were characterized for expression of MSC markers and capability of differentiation into mesenchymal lineages. MSCs could be cultured more than 100 days (26 passages), but metabolic activity was significantly enhanced in amnion vs. bone marrow MSCs. Interestingly, MHC class I expression is significantly reduced in amnion MSCs until passage 6 in human and marmoset, but not in bone marrow cells. For MSC markers, CD73 and CD105 levels remain unchanged in amnion MSCs and slightly decline in bone marrow at late passages; CD166 is significantly higher expressed in human MSCs, CD106 significantly lower vs. marmoset. All cultured MSCs showed pluripotency marker expression like Oct-4A at passage 3 significantly decreasing over time (passages 6–12) while Nanog expression was highest in human bone marrow MSCs. Furthermore, human MSCs demonstrated the highest Sox2 levels vs. marmoset, whereas the marmoset exhibited significantly higher Lin28A values. Bisulfite sequencing of the Oct-4 promoter region displayed fewer methylations of CpG islands in the marmoset vs. human. Conclusions: Little is known about MSC characteristics from the preclinical animal model common marmoset vs. human during long term culture. Studied human and common marmoset samples share many similar features such as most MSC markers and reduced MHC class I expression in amnion cells vs. bone marrow. Furthermore, pluripotency markers indicate in both species a subpopulation of MSCs with true ‘stemness’, which could explain their high proliferation capacity, though possessing differences between human and marmoset in Lin28A and Sox2 expression.

The repair of tendon injuries still presents a major clinical challenge to orthopedic medicine. Tendons, like some other tissues, are poorly vascularized and heal slowly. In addition, healing often leads to the formation of fibrous tissue and scar tissue which lack flexibility and biomechanical properties. So the treatment of tendon injuries is challenging. We give an overview of the structure and composition of tendons, pathological states of tendon and natural healing, as well as therapeutic options. We focus in particular on biomaterials that have been specifically developed or suggested for the successful repair of tendon injuries. In addition, we also review factor- and cell-dependent strategies to heal tendon and ligament disorders. Although brief, we hope that this review will be helpful, particularly for those readers who are new to the field of tendon tissue engineering.