Recent advances in tissue engineering of cartilage, bone, and tendon

Quantitative Constitutive Behaviour and Viscoelastic Properties of Fresh Flexor Tendons

The objective of this study was to obtain detailed high quality experimental data under well-controlled test conditions in order to quantify tendon viscoelastic behaviour and provide an experimental basis for large deformation mathematical modelling. Eighty-six fresh chicken flexor digitorum profundus (FDP) tendons were mechanically tested using an Instron 5565 universal testing machine and a Bioplus bath containing physiological saline solution. At low strain rates (≤ 0.001s−1), no strain rate dependence was found and the value of the elastic modulus was 324±17 MPa. At medium strain rates (0.003s−1 – 0.1s−1), the elastic stiffness increased with increasing strain rate. For example, the values of the elastic modulus were 427±10, 653±21 and 837±11 MPa for strain rates of 0.006s−1, 0.012s−1 and 0.05s−1 respectively. A series of stress relaxation experiments at different levels of strain were conducted. The higher the initial stress level, the faster the stress relaxed. For all stress relaxation tests, the relationship of normalised relaxation function G(t) against log time (ln(t)) was approximately linear (R2 = 0.935 ± 0.028) and this supports Fung's quasi-linear viscoelasticity (QLV) hypothesis. Some preliminary work on bovine flexor tendons using a video extensometer is also reported.

In vitro tendon engineering with avian tenocytes and polyglycolic acids: a preliminary report

Although there are many reports of in vivo tendon engineering using different animal models, only a few studies involve the short-term investigation of in vitro tendon engineering. Our previous study demonstrated that functional tendon tissue could be engineered in vivo in a hen model using tenocytes and polyglycolic acid (PGA) fibers. This current study explored the feasibility of in vitro tendon engineering using the same type of cells and scaffold material. Tenocytes were extracted from the tendons of a hen's foot with enzyme digestion and cultured in DMEM plus 10% FBS. Unwoven PGA fibers were arranged into a cord-like construct and fixed on a U-shape spring, and tenocytes were then seeded on PGA fibers to generate a cell-PGA construct. In experimental group 1, 22 cell-scaffold constructs were fixed on the spring with no tension and collected at weeks 4 (n = 7), 6 (n = 7) and 10 (n = 8); in experimental group 2, five cell-scaffold constructs were fixed on the spring with a constant strain and collected after 6 weeks of culture. In the control group, three cell-free scaffolds were fixed on the spring without tension. The collected engineered tendons were subjected to gross and histological examinations and biomechanical analysis. The results showed that tendon tissue could be generated during in vitro culture. In addition, the tissue structure and mechanical property became more mature and stronger with the increase of culture time. Furthermore, application of constant strain could enhance tissue maturation and improve mechanical property of the in vitro engineered tendon (1.302 +/- 0.404 Mpa with tension vs 0.406 +/- 0.030 Mpa without tension at 6 weeks). Nevertheless, tendon engineered with constant strain appeared much thinner in its diameter than tendon engineered without mechanical loading. Additionally, its collagen fibers were highly compacted when compared to natural tendon structure, suggesting that constant strain may not be the optimal means of mechanical load. Thus, application of dynamic mechanical load with a bioreactor to the construction of tendon tissue will be our next goal in this series of in vitro tendon engineering study.

Repair of Tendon Defect with Dermal Fibroblast Engineered Tendon in a Porcine Model

Harvesting autologous tenocytes for tendon engineering may cause secondary tendon defect at the donor site. Dermal fibroblasts are an easily accessible cell source and do not cause major donor site defect. This study aims to explore the possibility of tendon engineering using dermal fibroblasts. A total of 45 hybrid pigs were randomly divided into three groups: experimental group (n = 15)--repair of tendon defect with a dermal fibroblast engineered tendon; control group 1 (n = 15)--repair of defect with a tenocyte engineered tendon; and control group 2 (n = 15)-repair of defect with a scaffold alone. Both autologous dermal fibroblasts and tenocytes were seeded on polyglycolic acid (PGA) unwoven fibers to form a cell-scaffold construct and cultured in vitro for 7 days before in vivo implantation to repair a defect of flexor digital superficial tendon. Specimens were harvested at weeks 6, 14, and 26 for gross, histological, and mechanical analyses. Microscopy revealed good attachment of both dermal fibroblasts and tenocytes on PGA fibers and matrix production. In vivo results showed that fibroblast and tenocyte engineered tendons were similar to each other in their gross view, histology, and tensile strength. At 6 weeks, parallel collagen alignment was observed at both ends, but not in the middle in histology, with more cellular components than natural tendons. At weeks 14 and 26, both engineered tendons exhibited histology similar to that of natural tendon. Collagens became parallel throughout the tendon structure, and PGA fibers were completely degraded. Interestingly, dermal fibroblast and tenocyte engineered tendons did not express type III collagen at 26 weeks, which remained observable in normal pig skin and control group 2 tissue using polarized microscopy, suggesting a possible phenotype change of implanted dermal fibroblasts. Furthermore, both fibroblast and tenocyte engineered tendons shared similar tensile strength, about 75% of natural tendon strength. At 6 weeks in control group 2, neo-tissue was formed only at the peripheral area by host cells. A cord-like tissue was formed at weeks 14 and 26. However, the formed tissue was histologically disorganized and mechanically weaker than both cell-engineered tendons (p < 0.05). These results suggest that dermal fibroblasts may have the potential as seed cells for tendon engineering.