Gap junction protein expression and cellularity: comparison of immature and adult equine digital tendons

Abstract

Injury to the energy-storing superficial digital flexor tendon is common in equine athletes and is age-related. Tenocytes in the superficial digital flexor tendon of adult horses appear to have limited ability to respond adaptively to exercise or prevent the accumulation of strain-induced microdamage. It has been suggested that conditioning exercise should be introduced during the growth period, when tenocytes may be more responsive to increased quantities or intensities of mechanical strain. Tenocytes are linked into networks by gap junctions that allow coordination of synthetic activity and facilitate strain-induced collagen synthesis. We hypothesised that there are reductions in cellular expression of the gap junction proteins connexin (Cx) 43 and 32 during maturation and ageing of the superficial digital flexor tendon that do not occur in the non-injury-prone common digital extensor tendon. Cryosections from the superficial digital flexor tendon and common digital extensor tendon of 5 fetuses, 5 foals (1-6 months), 5 young adults (2-7 years) and 5 old horses (18-33 years) were immunofluorescently labelled and quantitative confocal laser microscopy was performed. Expression of Cx43 and Cx32 protein per tenocyte was significantly higher in the fetal group compared with all other age groups in both tendons. The density of tenocytes was found to be highest in immature tissue. Higher levels of cellularity and connexin protein expression in immature tendons are likely to relate to requirements for tissue remodelling and growth. However, if further studies demonstrate that this correlates with greater gap junctional communication efficiency and synthetic responsiveness to mechanical strain in immature compared with adult tendons, it could support the concept of early introduction of controlled exercise as a means of increasing resistance to later injury.

Tissue-Specific Ageing of Rat Tendon-Derived Progenitor Cells

“…Corroborative of work published by other groups [14,15,18], we found a decrease in cell progenitor density in mature patellar, Achilles and tail tendons as compared to respective young tendons (Table 1). This difference reached statistical significance for both Achilles and tail tendons, with the tail tendon having the lowest cells/mg density (Table 1).…”

“…It has, however, been previously reported that tendon cell density is reduced with age. For example, in equine digital tendons, there is a progressive loss in tenocyte density (expressed as cells/mm 2 ) as horses age from foal (1-6 months) to young adult (2-7 years) to mature (18-33 years) [14]. It was similarly found that fibroblast number is reduced with age in rabbit Achilles tendons [15].…”

“…Key focal adhesion proteins were differentially localized in young and old tendon fibroblasts and aged tenocytes displayed reduced cell motility [3]. The expression of connexin 43 and 32 proteins per equine tenocyte was much higher in foetal tendon samples than in tendon samples from foals, young adults, or old horses [14]. In addition, tendon stem/progenitor cells from aged/degenerated human Achilles tendon biopsies were shown to exhibit delayed wound closure, decelerated motion and notable transcriptomal shifts [20].…”

Isolation and Characterization of 2 New Human Rotator Cuff and Long Head of Biceps Tendon Cells Possessing Stem Cell–Like Self-Renewal and Multipotential Differentiation Capacity

“…Our results show an increased Cx43 gene expression in BMSCs and SS and LHB tendon stem cells compared with DF. According to a previous study, which showed Cx43 mRNA upregulation in immature tendons, 30 we can hypothesize that tendon stem cells are immature cells having a high Cx43 expression to provide a sustained gap-junction communication that is necessary for the differentiation process.…”

Achilles Tendon Injuries in Elite Athletes: Lessons in Pathophysiology from Their Equine Counterparts

“…(A) The planar, zig-zag waveform followed by in-register collagen fibrils can be seen at the fascicular (and whole tendon) level. significantly more tenocytes (tendon fibroblasts; Figure 3) per unit area than the CDET (Stanley et al 2007). Although both tendons reduce collagen turnover with age, the CDET consistently maintains the higher level (Birch, Worboys, et al 2008); in adult tendons, the level of procollagen type I aminopropeptide (cleaved during collagen synthesis) is higher per square millimeter in the less cellular CDET (Young et al 2009).…”