Response to mechanical loading in healing tendons

........................................................................................... 7 Populärvetenskaplig sammanfattning ............................................. 9 List of papers .................................................................................. 11 Abbrevations .................................................................................. 13 Introduction ................................................................................... 15 Bundles, cross-links and a few cells ........................................................................................16 Tendons heal by three steps .................................................................................................... 17 The strength of the tendon comes from parallel collagen fibres .............................................19 Forces are transferred to the cells though matrix-cytoskeleton connections .........................21 Tendons are dynamic tissues and adapt to loading and unloading ....................................... 24 Aims of the thesis .......................................................................... 35 General ................................................................................................................................... 35 Specific ................................................................................................................................... 35 Materials and methods ................................................................... 37 Study designs .......................................................................................................................... 37 Achilles tendon transection model (all studies) ......................................................................41 Unloading and reloading .........................................................................................................41 Mechanical testing (study I-III, V and VI) ............................................................................. 42 Gene expression analyses (study III-VI) ................................................................................ 43 Histology (study II) ................................................................................................................ 45 Immunohistochemistry (Study V) ......................................................................................... 45 Alkaline phosphatase and luciferase reporter gene assays (study V) .................................... 46 Statistics ................................................................................................................................. 47 Results in brief .............................................................................. 49 Unloading by botulinium toxin reduced the strength of the healing tendon dramatically... 49 ...but short loading episodes increased the strength of the unloaded tendon calluses.......... 50 The loading episodes increased the amount of bleeding in the callus tissue ......................... 52 Healing tendons with continuous loading had less expression of inflammatory genes and more of ECM and tendon specific genes than unloaded ........................................................ 53 Similar but different effect on gene expression by one single loading episode ..................... 54 The gene expression is regulated up to 24 hours after one single loading episode ............... 55 Unloading by botulinium toxin for 5 days did not have much impact on intact tendons ..... 56 The difference between the healing and the intact tendons, however, comprised more than just strength............................................................................................................................ 56 Myostatin stimulates proliferation ......................................................................................... 57 Discussion ..................................................................................... 59 Rest between loadings might allow the tendon callus to contract ......................................... 59 Loading generates more matrix but not necessarily of better quality ................................... 60 What is optimal loading? ........................................................................................................61 Inflammation: good or bad? ................................................................................................... 63 All research has limitations .................................................................................................... 65 Conclusions and future research ................................................... 69 What’s next? ........................................................................................................................... 70 Take-home message: ............................................................................................................... 71 Acknowledgements ......................................................................... 73 References ...................................................................................... 75