The primary cilium, a sensory apparatus, functions as both a chemical and mechanical sensor to receive environmental stimuli. The present study focused on the primary cilia in the epithelialmesenchymal interaction during tooth development. We examined the localization and direction of projection of primary cilia in the tooth germ and oral cavity of mice by immunohistochemical observation. Adenylyl cyclase 3 (ACIII)-immunolabeled cilia were visible in the inner/outer enamel epithelium of molars at the fetal stage and then conspicuously developed in the odontoblast layer postnatally. The primary cilia in ameloblasts and odontoblasts-shown by the double staining of acetylated tubulin and γ-tubulin-were regularly arranged from postnatal Day12, projecting apart from each other. The periodontal ligament possessed ACIII-positive cilia, which gathered on both sides of the dentin/cement and alveolar bone in postnatal days. In the oral cavity, numerous long primary cilia immunoreactive for ACIII were condensed at subepithelial stromal cells in the oral processes in fetuses, while postnatally a small number of short cilia were dispersed throughout the stroma of the oral cavity. These findings suggest that the primary cilia showing stage-and regionspecific morphology are involved in the epithelial-mesenchymal interaction during tooth development via mechano-and/or chemoreception for growth factors.

GP2, a GPI-anchored glycoprotein, is a useful marker of M cells in Peyer's patches. Our immunostaining of the paranasal sinuses in mice detected a condensed distribution of GP2-immunoreactive cells within the epithelium, apart from lymphoid tissues. In the paranasal sinuses, the cells exhibited a unique morphology characterized by a slender neck portion and huge terminal bulb, quite different from M cells. Electron microscopically, the GP2 immunoreactivity centered on the luminal plasma membrane of the terminal bulb, being less intense in the baso-lateral plasma membrane and not visible at all in the cytoplasm. The cells frequently came in contact with nerve fibers containing small synaptic vesicles. These nerve fibers contained neither CGRP nor substance P-indicators of sensory neurons; moreover, no signal molecules used for a sensory function were expressed in the GP2-immunoreactive cells, implying that these nerves are efferent in nature. A weak but significant stainability in PAS reaction and an intense GP2 immunoreactivity for typical goblet cells in the tunica conjunctiva suggest that the GP2-expressing cells in paranasal sinuses are in the lineage of goblet cells.

Tooth development is regulated by various growth factors and their receptors. However, the overall mechanism of growth factor-mediated odontogenesis remains to be elucidated. The present study examined expression sites and intensities of major growth factors and receptors in the tooth germ of murine fetuses and neonates. Signals of TGF-β and CTGF in fetuses were released from the enamel epithelium, while their neonatal signals arose in odontoblasts. Moreover, BMP/Smad signaling may affect the differentiation of ameloblasts, in contrast to PDGFα whose signals may cause odontoblast differentiation. Growth factors associated with the formation of the periodontium were IGF1, IGF2, IGFBP3, CTGF, and PDGFα. Concerning cusp formation, the enamel knot selectively expressed FGF4, BMP2, and BMP4 with an expression of PDGFα in the enamel-free area. It is concluded that many molecules play critical roles in the epithelium-mesenchyme interaction of tooth germ differentiation, and their expressions are precisely controlled.Tooth germ differentiation commonly progresses in the following order: the initiation, bud, cap, bell, and root formation stages. According to the stages, progenitor constituents of the tooth germ proliferate and are differentiated to form specialized parts of teeth and associated tissues. Tooth development is precisely regulated by cross-talk between adjacent tissues and various growth factors secreted in autocrine and paracrine modes. Recent studies in this research field have identified many molecules which play important roles in signaling associated with tooth germ differentiation, but it is largely unknown which growth factors are the main contributors to odontogenesis, partially due to insufficient in vivo systematic analyses of the expression of such factors. The transforming growth factor-β (TGF-β) superfamily may take a leading role in the regulation of cell proliferation, differentiation, and apoptosis, also being one of the major signals for tooth and periodontium morphogenesis. Immunohistochemical studies of the mouse tooth germ detected TGF-β1 in the inner enamel epithelium, ameloblasts, and odontoblasts; the expression of TGF-β receptor-1 (TGF-βR1) was weak in the enamel epithelium but increased in intensity in differentiated ameloblasts (6). An in situ hybridization study using mouse embryos identified an intense mRNA expression of TGF-β2/3 in the odontoblast layer (20). Among bone morphogenetic proteins (BMPs) which belong to the TGF-β family, it has been reported that the expression sites of BMP2 and BMP4 mRNA in the mouse tooth germ shift from the enamel knot to dental papilla and odontoblasts, and their signalings are required for the epithelium-mesenchyme interac-