ABSTRACT
The myotendinous junction (MTJ) is a weak link in the musculoskeletal system. Here, we isolated the tips of single myofibres from healthy (non-injured) human hamstring muscles for confocal microscopy (n=6) and undertook RNAscope in situ hybridisation (n=6) to gain insight into the profiles of cells and myonuclei in this region, in a fibre type manner. A marked presence of mononuclear cells was observed coating the myofibre tips (confirmed by serial block face scanning electron microscopy and cryosection immunofluorescence), with higher numbers for type I (median 29; range 16–63) than type II (16; 9–23) myofibres (P<0.05). The number of these cells expressing COL22A1 was comparable between fibre types. Myonuclear number and density gradually increased from the myofibre proper towards the tip for both fibre types (P<0.05). COL22A1 was expressed by similar proportions of myonuclei in type I (median 26%; range 13–56) and type II (19%; 3–67) myofibre tips. 70% of the COL22A1-positive nuclei in the MTJ region were myonuclei, and the remaining 30% were MTJ cells. This insight refines our fundamental understanding of the human MTJ at the cell and structural levels.
Footnotes
Author contributions
Conceptualization: C.H., A.M., C.-Y.C.Y., M. Kjaer, A.L.M.; Data curation: C.H., A.L.M.; Formal analysis: C.H., A.M., C.-Y.C.Y., P.S., A.L.M.; Funding acquisition: A.L.M.; Investigation: C.H., A.L.M.; Methodology: C.H., A.M., C.-Y.C.Y., A.L.M.; Project administration: A.L.M.; Resources: M.R.K., M. Kjaer, A.G.v.K.; Supervision: C.-Y.C.Y., A.G.v.K., A.L.M.; Visualization: C.H., A.L.M.; Writing – original draft: C.H., A.L.M.; Writing – review & editing: A.M., C.-Y.C.Y., P.S., M.R.K., M. Kjaer, M. Koch, A.G.v.K., A.L.M.
Diversity & Inclusion
In this study, similar numbers of males and female individuals were included for analysis.
Funding
This work was supported by the Lundbeck Foundation (Grant ID: R344-2020-254 to A.L.M.), Novo Nordisk Fonden (Grant ID: NNF20OC0064829 to A.L.M.), Independent Research Fund Denmark (Grant ID: 10.46540/3101-00063B to A.L.M.).
Data availability
The custom software for the 3D model ‘Automated-muscle-fiber-analysis’ can be found through the public github repository: https://github.com/DBI-INFRA/Automated-muscle-fiber-analysis.
