Postnatal development of protein gene product 9.5- and calcitonin gene-related peptide-like immunoreactive nerve fibers in the rat temporomandibular joint.

Abstract

The aim of this study was to investigate the little-known postnatal development of innervation in the temporomandibular joint (TMJ). We therefore investigated the protein gene product 9.5-like immunoreactive (PGP-LI) and calcitonin gene-related peptide-like immunoreactive (CGRP-LI) nerves in the rat TMJ from days 0-24 in wholemount preparations and frozen sections by immunohistochemistry using the avidin-biotin-peroxidase complex method. At birth, PGP-LI and CGRP-LI nerves were observed only in the disk attachment but not in the peripheral portion of the disk. From days 3-6, the peptidergic nerves associated with the development of synovial cells were first found in the anterior (3rd day) or posterior (6th day) peripheral portion of the disk in which synovial cells later appear. From days 7-10, incisal occlusion started and simultaneously the distinct networks of the nerves were formed in four peripheral portions of the disk. On the 14th day, the PGP-LI nerves were first more numerous than the CGRP-LI nerves only in the anterior disk attachment. From days 18-24, the occlusion between the first molars started. Simultaneously, the distribution and number of the nerves were comparable to those of adult rats. However, from days 0-24, the central portion of the disk (actual disk) did not contain any nerves. These findings suggest that functional stimuli such as sucking and mastication might be associated with the development of nerves in the TMJ during the early and late postnatal days, whereas the actual disk except for both disk attachment and peripheral portion of the disk is not innervated and does not contribute to the afferent sensory information from the TMJ. The presence of peptidergic nerves prior to the appearance of synovial lining cells furthermore suggests that the neuropeptide released from these nerves may affect on the differenciation and maturation of synovial cells.