Objective To investigate the feasibility of tendon engineering in vitro using tenocytes and polyglycolic acids (PGA). Methods Tenocytes were isolated by tissue explant method and expanded in vitro. Cells of the second passage were collected and seeded onto PGA scaffolds made from PGA unwoven fibers at the density of 20×10 6cells/ml. At 1 week postseeding,the constructs were divided into three groups as follows: cell-scaffold constructs under constant tension generated by a U-shaped spring as the experimental group (n=5), cell-scaffold constructs under no tension as control group 1(n=4), cell-free scaffolds under constant tension as control group 2 (n=3). Samples were harvested at 2, 4 and 6 weeks for histological and immunohistochemical (IHC) examinations. Transmission electron microscopy (TEM) and mechanical test were performed to evaluate the constructs of 6 weeks. Results At 2 weeks, the constructs were mainly composed of undegraded PGA fibers. Gross and histological examination revealed no difference between the groups. At 4 weeks, neo-tendon was visible through gross observation in experimental group and control group 1. Histology and immunohistochemistry revealed the formation of collagen fibers. While in control group 2, PGA fibers were mostly degraded. At 6 weeks, the constructs were much thinner in experimental group than those in control group 1(1.44±0.13mm vs 2.55±0.18mm in diameter). TEM showed periodical strata of collagen fibers in the constructs from experimental group and control group 1. However, histology in experimental group revealed longitudinal alignment of collagen fibers, which more resembled natural tendon than neotendon formed in control group 1. Besides, the maximum load to failure(Newton/mm 2) was greater in experimental group than that in control group 1 (1.107±0.327 vs 0.294±0.138, P＜0.05). ConclusionIts possible to engineer tendon substitutes in vitro. Cyclic strain generated by a bioreactor may be the optimal mechanical stimulation and is currently under investigation.