ARE THERE DISTINCT SUBTYPES OF TENOCYTES WHICH RELATE TO AGE AND TENDON FUNCTION?

Abstract

The energy-storing human Achilles tendon and equine superficial digital flexor tendon (SDFT) show no adaptation to exercise unlike muscle and bone, and are prone to injury. Injury involves microdamage accumulation until there is sufficient weakening for rupture to occur during normal athletic activity. Anatomically opposing positional tendons, such as the common digital extensor tendon (CDET) in the horse rarely suffer exercise–induced injury. Tenocytes maintain the extra-cellular matrix, but in energy-storing tendons they appear unable to adequately repair microdamage as it occurs. Tenocytes have been classified subjectively into 3 subtypes on the basis of histological nuclear morphology. Long, thin type 1 cells are thought to be less synthetically active than cigar-shaped type 2 cells, but their exact morphology and relative proportions in different tendon sites and ages has not been clearly defined. We hypothesised that tenocytes are separable into morphologically distinct subtypes, reflecting differences in age and functional requirements within and between specific tendons. Samples were taken from tensional and compressed regions of the SDFT and CDET of 5 neonates, 5 foals (1–6 m), 5 young adults (2–6 y) and 5 old horses (18–33 y) Cell nuclei were counted and measured in digital images from histological sections by computerised image analysis. Total tenocyte densities and proportions of the 3 subtypes were calculated for each age group, as were nuclear length:width ratios. Length:width ratio distributions for all horses were evaluated using a normality test followed by a paired t-test. There was a significantly higher total cellularity in the SDFT than the CDET, with a higher proportion of type 1 tenocytes in the CDET. With age, total cellularity decreased in all tendon sites and an increase in the proportion of type 1 tenocytes was observed in tensional regions. Foal and neonatal tendons contained significantly higher proportions of type 2 tenocytes than older tendons. The morphology of the two main subtypes in all age groups was significantly different; type 1 tenocytes had a higher nuclear length:width ratio (mean ± SD = 9.6 ± 2.5) than type 2 (mean ± SD =4.7 ±1.1) (p< 0.001). We were able to objectively separate tenocytes into 3 distinct subtypes based on nuclear length:width ratio measurements. There were significant differences in proportions of subtypes with tendon site and age. The positional tendon had significantly lower cellularity and a higher proportion of type 1 tenocytes; these cells may be less functionally active but sufficient to maintain the matrix in a tendon which is not subjected to high levels of strain. The SDFT continues to grow up to 2 years of age and is subjected to high strains, explaining the need for relatively higher proportions of type 2 cells. There is however an age-related increase in type 1 cells in both tendons which may explain an inability of the adult energy-storing tendon to adapt to exercise and to repair microdamage. Understanding the stimulus for age-related changes in tenocyte subtype proportions in tendons with different functions may help us understand the pathogenesis of exercise-induced tendon injury and to develop more appropriate training regimens.