A Histological observation of inflammatory cells in the rabbit Achilles tendon due to overuse injury

Abstract

The presence or absence of inflammatory cells in chronic Achilles tendinopathy has been a controversial subject in previous studies. Macrophages, T lymphocytes, and neutrophils have previously been detected in injured human Achilles tendons, whereas other authors have reported that there is no evidence for their occurrence. This controversy may stem from the fact that human Achilles tendon overuse injuries usually develop gradually over time, and the time course of inflammation in response to overuse has been difficult to establish in clinical populations. The aim of my study was to examine the presence of inflammatory cells in the Achilles tendon of rabbits that were subjected to repetitive mechanical loading of defined durations. Twenty-Four New Zealand male rabbits were subjected to repetitive mechanical loading of the Achilles tendon and grouped into four groups in this study, according to the exercise time period for each group: 0, 1, 3, and 6 weeks. Achilles tendons were harvested at the end of each time period. Achilles tendons sections were stained with Hematoxylin and Eosin to examine the histological changes. Both Neutrophils and T-lymphocytes were detected by Immunohistochemistry. Macrophages were detected using the Prussian blue staining. A very small number of inflammatory cells were detected in some tissue sections in the control group. Tissue sections from exercised groups 1, 3, and 6 weeks respectively, showed some qualitative changes in tendon morphology. Collagen bundles were disorganized, and hyalinized patches and spaces between collagen fibers were observed. Tenocyte nuclei were rounder and basophilic, and there was an increase in their numbers with loss of parallel alignment. Macrophages, T-lymphocytes, and neutrophils were detected in tendon sections, specifically in the paratenon. Statistically both lymphocytes and macrophages were significantly higher than control at 6 weeks. While the number of macrophages in the control was lower than the 6 weeks group, there was no significant difference between 1 week and 3 weeks. However, no lymphocytes were found at week 3. Neutrophils in all groups showed no significant difference. The evidence of inflammation was not evenly distributed, as some tissue sections from the same groups showed no evidence of inflammatory cells.