The innervation of the synovium of the knee joint in the guinea pig: an immunohistochemical and ultrastructural study

The innervation of the knee joint synovial membrane of the guinea pig, i.e., the synoviocyte layer, the subjacent connective tissue and the connective tissue region beneath, was analyzed with immunohistofluorescence and electron microscopy. A screening of the innervation with antibodies against the general axon marker protein gene product (PGP) 9,5 - revealed the presence of nerve fibers distributed in various regions of the knee joint synovial membrane. Confirmating previous studies, some of these nerve fibers stained with antibodies to tyrosine hydroxylase (TH), neuropeptide Y (NPY), substance P (SP), calcitonin gene-related peptide (CGRP), and vasoactive intestinal polypeptide (VIP). In addition, dynorphin (DYN)-containing fibers were detected, which have not been reported previously in normal joints. In general, the immunoreactive fibers were observed close to the synoviocytes and at blood vessels. Fibers with colocalization of NPY-and TH-like immunoreactivities (LIs), as well as of DYN-and TH-LIs were demonstrated, In the electron microscope, bundles of unmyelinated fibers as well as single fibers were found in the connective tissue region below the synoviocytes. Varicose parts of the nerve fibers contained mainly small, clear vesicles. Small and large dense-cored vesicles were also seen, but less frequently. Denser portions of the plasma membranes of some axons were observed in these regions, facing the extracellular space. Myelinated fibers were also observed in some nerve bundles. These findings emphasize the complex innervation of the synovial membrane, with nerve fibers containing a host of neuroactive substances. Altogether, these fibers are probably involved in many functions such as vasoregulation and control of synovial secretion in addition to being a source of mediators in joint inflammation.