Intra-Fascicular Chondroid-like Bodies in the Ageing Equine Superficial Digital Flexor Tendon comprise Glycosaminoglycans and Type II Collagen.

Abstract

The superficial digital flexor tendon (SDFT) is considered functionally equivalent to the human Achilles tendon. Circular chondroid depositions scattered amongst the fascicles of the equine SDFT are rarely reported. The purpose of this study was the detailed characterisation of intra-fascicular chondroid-like bodies (ICBs) in the equine SDFT, and the assessment of the effect of ageing on the presence and distribution of these structures. Ultra-high field magnetic resonance imaging (9.4 T) series of SDFT samples of young (1-9 years) and aged (17-25 years) horses were obtained, and 3D reconstruction of ICBs was performed. Morphological evaluation of the ICBs included histology, immunohistochemistry and transmission electron microscopy. The number, size and position of ICBs was determined and compared between age groups. There was a significant difference (P = 0.008) in the ICB count between young and old horses with ICBs present in varying number (13 - 467; median = 47, mean = 132.6), size and distribution in the SDFT of aged horses only. There were significantly more ICBs in the tendon periphery when compared to the tendon core region (P = 0.010). Histological characterisation identified distinctive cells associated with increased glycosaminoglycan and type II collagen extracellular matrix content. Ageing and repetitive strain frequently cause tendon micro-damage prior to the development of clinical tendinopathy. Documentation of the presence and distribution of ICBs is a first step towards improving our understanding of the impact of these structures on the viscoelastic properties, and ultimately their effect on the risk of age-related tendinopathy in energy-storing tendons. This article is protected by copyright. All rights reserved.