Neurotrophins are diffusible peptides that are essential for the development and maintenance of the central and peripheral nervous systems. Interestingly, they are completely absent from immature periodontal Ruffini endings (Nakakura-Ohshima et al., 1993, 1995).īrain-derived neurotrophic factor (BDNF) belongs to a family of neurotrophins that includes nerve growth factor (NGF), neurotrophin-3 (NT3), neurotrophin-4/5 (NT4/5), and neurotrophin-6 (NT6). The expanded terminal portions of mature Ruffini endings under transmission electron microscopy (TEM) demonstrated ruffle-ended outlines that were identified as microprojections of axoplasm, called the axonal spines (Byers, 1985 Maeda et al., 1989). Morphological characterizations of these structures under the light microscope have revealed extensive and complicated arborizations of their expanded terminal portions and an association with specialized Schwann cells (Byers, 1985 Maeda et al., 1989). Electrophysiologically, they are categorized as low-threshold, slowly-adapting, type II stretch receptors (Chambers et al., 1972 Biemesderfer et al., 1978). They are concentrated in the region where periodontal fibers are most stretched, such as in the middle to apical regions of the ligament of rooted teeth. The large-diameter fibers form specialized endings, which can receive mechanosensations when a tooth moves in the alveolar socket.Īmong the various types of mechanoreceptors, the Ruffini endings are the most abundant in the mammalian periodontal ligament, although their development and morphology vary among species and tooth types (Byers, 1985 Maeda et al., 1987 Sato et al., 1988, 1989, 1992 Byers and Dong, 1989 Kannari, 1990 Kannari et al., 1991). The thin fibers terminate as free endings and are implicated in the perception of pain. The former are myelinated and the latter are either myelinated or nonmyelinated. Large- and small-diameter nerve fibers are found in the periodontal ligament. Because of its rich sensory innervation, this tissue acts as a sensory apparatus for nociception and mechanoreception (Matthews, 1975 Hannam, 1982 Schroeder, 1986 Taylor, 1990). The periodontal ligament is a dense collagenous tissue that is particularly adapted for supporting the tooth and buffering occlusal forces. Furthermore, other neurotrophins may play a role in the development and/or early maturation of the periodontal nerve fibers, as indicated by the presence of nerve fibers in the BDNF-homozygous mice. These findings suggest a possible involvement of BDNF during the postnatal development and, in particular, the maturation of periodontal Ruffini endings. In the quantitative analysis, homozygous mice had the smallest percentages of PGP 9.5-immunoreactive areas at the same postnatal periods, but there were no significant differences between wild-type and heterozygous mice during PW1–3. The homozygous mice lacked the typical Ruffini endings at PW3. While the Ruffini endings of the wild-type mice appeared either ruffled or smooth, as reported previously, most of these structures showed a smooth outline in the heterozygous mice. At PW3, the Ruffini endings occurred in the periodontal ligament of the wild-type and heterozygous mice. At PW2, the heterozygous and wild-type mice started to show ramified nerve fibers resembling the mature shape of periodontal Ruffini endings. At this stage, the nerve fibers had a beaded appearance, but did not form the periodontal Ruffini endings. A similar distribution and terminal formation of PGP 9.5-immunoreactive nerve fibers was recognized in the periodontal ligament of all phenotypes at postnatal week (PW) 1. To investigate further roles of BDNF in these structures, the development, distribution, and terminal morphology of Ruffini endings were examined in the incisor periodontal ligament of heterozygous and homozygous BDNF mutant mice, as well as in the wild-type littermate by immunohistochemistry for protein gene product (PGP) 9.5, a general neuronal marker. Furthermore, adult heterozygous BDNF-mutant mice showed malformation and reduction of the periodontal Ruffini endings. The periodontal Ruffini ending has been reported to show immunoreactivity for tyrosine kinase B (trkB), the high-affinity receptor for brain-derived neurotrophic factor (BDNF), in the periodontal ligament of the rat incisor.
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