Distribution and coexistence of neuropeptides in nerve fibres in the temporomandibular joint of late gestation fetal sheep

The density and distribution of nerve fibres immunoreactive to antisera for PGP 9.5 (general neuronal marker), calcitonin gene related peptide (CGRP) and substance P (SP) (markers for sensory neurons), as well as neuropeptide Y (NPY), vasoactive intestinal peptide (VIP) and tyrosine hydroxylase (TH) (markers for autonomic fibres), were examined in the temporomandibular: joint (TMJ) of late gestation fetal sheep. This work formed part of a project investigating the influence of age and osteoarthritis on the innervation of the TMJ, and was undertaken to determine whether the innervation of the joint at 140 d gestation (17 d before birth) differed from that in the mature adult. Immunofluorescence microscopy was applied to serial sections of the capsule, disc and synovial membrane of 10 joints from 5 fetuses and image analysis was used for the quantitative assessment. The capsule, synovial membrane and the disc contained fibres immunoreactive (IR) to antisera for PGP 9.5, P and CGRP. NPY-IR fibres were only visible in the loose connective tissue of the capsule. No VIP-or TH-IR nerve fibres were detected in the fetal TMJ, There was no statistically detectable difference between the density of nerve fibres immunoreactive to CGRP or PGP 9.5 antisera in the capsule or disc. Substance P-immunoreactivity (IR) was relatively weak in all samples examined. Scattered branches of CGRP-IR fibres were found deep in the disc proper. The lack of receptor endings, other than free nerve endings in the TMJ of the late fetal sheep, might be a reflection of the functional and anatomical immaturity of the TMJ, as reflected in the immature, gross and microscopic appearance of the disc, the inferior joint compartment and articular surface of the condyle at this stage. These results demonstrate that the capsule, synovial membrane and disc in the TMJ of fetal sheep at 140 d gestation age are innervated with sensory fibres, while autonomic fibres are located in the capsule only. The findings also support the view that the disc is innervated at an early stage of life but at a later stage the density of innervation in the central part of the disc regresses and the innervation remains only peripherally in the adult TMJ disc. Further work is required to determine (1) at what stage sympathetic fibres innervate the disc and the synovium, and (2) when the mechanoreceptive nerve endings develop.