Structural and Molecular Characteristics of Axons in the Long Head of the Biceps Tendon

Abstract

ObjectivesAlthough the long head of the biceps tendon (LHBT) is a major source of shoulder pain, the innervation is not sufficiently documented. Thus, the present study aimed to characterize structural and molecular features of nervous elements in the LHBT.MethodsThe proximal part of eleven LHBTs was harvested intraoperatively. There were 8 female and 3 male specimens. Age ranged from 66 to 86 years. For structural analyses, nervous elements were viewed in the transmission electron microscope. For molecular characterization, we used a series of neuronal markers: This included general neuronal markers [antibodies against neurofilament and protein gene product 9.5 (PGP9.5)] as well as specific neuronal markers[antibodies against myelin basic protein (MBP), calcitonin gene related product (CGRP), substance P (SP), tyrosine hydroxylase (TH), and growth associated protein 43 (GAP43)]. Anti‐neurofilament and anti PGP9.5 visualized the overall innervation. Anti‐MBP visualized myelination, anti‐CGRP and anti‐SP nociceptive fibers, anti‐TH sympathetic nerve fibers, and anti‐GAP43 nerve fibers during development and regeneration. Immunolabelled sections were analyzed in the confocal laser scanning microscope.All procedures performed in this study were approved by the local medical ethics committee (Austrian workers compensation boards’ research ethics committee and institutional review board of the Medical University of Vienna, No. 06/2017) and in accordance with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. Informed consent was obtained from all persons included in this study.ResultsWe show that the LHBT contains unmyelinated as well as myelinated nerve fibers which group in nerve fascicles and follow blood vessels. Many myelinated and unmyelinated axons exhibit molecular features of nociceptive nerve fibers. Another subpopulation of unmyelinated axons exhibits molecular characteristics of sympathetic nerve fibers. Unmyelinated sympathetic fibers and unmyelinated nociceptive fibers express proteins that are found during development and regeneration.ConclusionPresent findings show that the LHBT of aged persons is richly innervated. The occurrence of growth protein GAP43 in nociceptive nerve fibers supports the hypothesis that in‐growth of nociceptive fibers is the source of chronic tendon pain.