The effect of intralesional injection of bone marrow derived mesenchymal stem cells and bone marrow supernatant on collagen fibril size in a surgical model of equine superficial digital flexor tendonitis.
Authors: Caniglia C J, Schramme M C, Smith R K
Journal: Equine veterinary journal
Summary
# Editorial Summary: Bone Marrow Stem Cells and Equine Tendon Repair When superficial digital flexor tendons heal after injury, the newly formed collagen lacks the structural sophistication of native tissue—notably smaller fibril diameters and a simplified, unimodal size distribution rather than the complex bimodal pattern seen in healthy tendon. Caniglia and colleagues investigated whether intralesional injection of autologous bone marrow-derived mesenchymal stem cells (BMDMSCs) could shift this repair process towards true regeneration by examining collagen ultrastructure at 16 weeks post-injury in an experimental surgical defect model; they measured fibril diameters using transmission electron microscopy and calculated mass average diameter (MAD), collagen fibril index (CFI) and area-dependent diameter (ADD) across six horses. Surprisingly, treated and control lesions showed no significant differences in any of these parameters, with both displaying persistently reduced fibril diameter and unimodal distributions compared to normal tissue. This negative result challenges the prevailing hypothesis that BMDMSCs promote regenerative rather than fibrotic healing, suggesting their therapeutic benefit—if present—operates through alternative biological mechanisms such as anti-inflammatory signalling or improved vascularisation rather than direct enhancement of matrix organisation. The finding warrants reconsideration of how we evaluate stem cell efficacy in tendon injury and highlights the importance of moving beyond single timepoint assessments to understand the temporal dynamics of repair.
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Practical Takeaways
- •Intralesional bone marrow-derived mesenchymal stem cell therapy did not improve collagen structure in this experimental model, suggesting the mechanism of stem cell benefit in tendon healing may operate through pathways other than promoting regenerative matrix formation
- •At 16 weeks post-injury, repair tissue in both treated and untreated tendons retained abnormal collagen ultrastructure with smaller fibrils and altered distribution compared to normal tendon, indicating that structural remodelling extends beyond the timeframe examined
- •Further research is needed to identify the actual mechanisms by which mesenchymal stem cells may benefit tendon healing before recommending this treatment as a standard clinical intervention
Key Findings
- •Intralesional injection of autologous bone marrow-derived mesenchymal stem cells had no significant effect on collagen fibril diameter in healing SDFT tissue at 16 weeks post-injury
- •Normal tendon regions showed significantly higher mass average diameter (MAD) and collagen fibril index (CFI) values with bimodal fibril size distribution compared to injured regions
- •Both treated and control injured regions displayed significantly lower MAD and CFI values with unimodal fibril distribution, with no measurable differences between groups
- •Promotion of matrix regeneration over fibrotic repair may not be the mechanism by which mesenchymal stem cells improve tendon healing