Mechanisms of collagen fibril alignment in tendon injury: From tendon regeneration to artificial tendon

The process by which collagen fibrils are aligned following tendon injury remains unknown. Therefore, we analyzed the process of tendon regeneration by transmission electron microscopy, using a film model method. In mice, the Achilles tendon of medial head was transected. On day 3, after only the proximal end of the transected tendon was placed on film and kept in vivo, a translucent substance containing granules, called tendon gel, was secreted. On day 5, the granules assembled in a loose (L) layer, and coalesced tightly in a dense (D) layer, forming an L-D-L layered pattern. On day 10, granules showed high electron density in H layers, which developed into D-H-D layers on day 13. The distal end was placed on film to face the proximal end. On day 10, the tendon gel showed a D-H-D layer pattern. On day 11, mechanical stress from muscular constriction changed the tendon gel to aligned collagen fibrils (6±2nm in diameter). Thereafter, the diameter of the fibrils increased. Tendon gel harvested on day 5 or day 10 was pulled manually or by hanging weights (about 0.6MPa). Aligned collagen fibrils (32±7nm in diameter) were created by traction using tendon gel harvested on day 10.