Nociceptive and sympathetic innervations in the abaxial part of the cranial horn of the equine medial meniscus: an immunohistochemical approach

In athletic horses, diseases leading to lameness are of major importance due to the loss of performance and the resultant economic concerns. Although, in the hindlimb, stifle lesions are frequent due to the large and complex joint, and meniscal tears have been identified as the most common soft tissue injuries in this joint, little is known about the mechanism that causes the painful sensation and thus the lameness. The aim of our study was therefore to highlight any peripheral fibres involved in meniscal nociception in five healthy anterior horns of the equine medial meniscus, which is the most common locale reported for equine meniscal injuries. Immunohistochemical stainings were performed using antibodies against Substance P in order to identify nociceptive fibres; antibodies against tyrosine hydroxylase for detecting postganglionic sympathetic fibres; and against glial fibrillary acidic proteins in order to identify Schwann cells. Fundamentally, our work highlights for the first time the presence of nociceptive and sympathetic fibres in equine menisci. They were found in the abaxial part of the anterior horn of the equine medial meniscus. This study suggests thus that when the abaxial part is injured, the meniscus itself could be the source of pain. This finding provides a better understanding of the nociceptive and sympathetic innervation of the equine meniscus which could contribute towards improving the therapeutic strategies in order to alleviate pain in cases of equine meniscal injury

A Preliminary Study Into Meniscal Innervation in 2 Horses With Different Degrees of Chondropathy in the Femorotibial Joint

Osteoarthritis is a painful degenerative disease, sometimes responsible for the premature ending of the careers of sporting horses. In humans, meniscal neurogenesis has been suggested to contribute to pain in osteoarthritis. To the best of our knowledge, no study on this subject has been performed in equine species. The aim of this report was to compare the density of the nociceptive and sympathetic nerves in the menisci of two 11- and 13-year-old mares, one presenting a “low-chondropathy grade” in its stifle and the other a “high-chondropathy grade”. The cranial horns of the medial menisci, a commonly reported site of equine meniscal injuries, were harvested after a maximum of 48 hours postmortem. A macroscopic and microscopic grading system was established using a modified Pauli score. Nociceptive fibers were highlighted by immunohistochemical staining with an antibody against Substance P (SP) and postganglionic sympathetic fibers with an antibody against tyrosine hydroxylase (TH). Descriptive statistics were calculated, and a nonparametric statistical test was used to analyze the data. The meniscus from the “high-chondropathy horse (HCH)” showed a higher density and frequency of postganglionic sympathetic nerves compared with the meniscus of the “low-chondropathy horse (LCH)”. No difference was observed regarding nociceptive nerves. This report shows possible meniscal sympathetic efferent-mediated pain in a “highly chondropathic horse”. Although these results cannot be extended to the equine population as a whole because of the small sample size, they are nevertheless interesting as they provide new information regarding the possible etiology of the pain in equine stifle osteoarthritis

Mechanoreceptors in the anterior horn of the equine medial meniscus: an immunohistochemical approach.

Mechanoreceptors are “encapsulated sensory end-organs” involved in proprioceptive function. Given the high incidence of meniscal injuries in horses, the clinical interest in these mechanoreceptors, particularly in the meniscus, and the lack of information concerning them in equine menisci, our objective was to study these corpuscles in the anterior horn of the equine medial meniscus, which is the most common localization reported for equine meniscal injuries. An immunohistochemical approach to detect Schwann cells and nerve fibres allowed us to localize and identify these corpuscles within the meniscus. Three types of mechanoreceptors were identified and localized between the abaxial quarter and the abaxial third of the meniscus: the Ruffini, Pacini and Golgi corpuscles. In conclusion, from a purely fundamental point of view, our work highlights for the first time the presence of MCR at the level of the anterior horn of the equine medial meniscus and proposes a classification based on specific immunocytochemical techniques. This morphological approach could serve as a basis for clinical studies, in order to evaluate the impact of these corpuscles on the poor sportive prognosis in equine meniscal tears