We used a fluorescent dye, FM1-43 to investigate mechanotransduction mechanisms in the hair cells of lateral line organs of Xenopus larvae. FM1-43 specifically labeled the hair cells. The photo-oxidation technique was performed with election microscopy to examine the labeling sites and their mechanisms. The results showed that the labeling sites were mitochondria and rough endoplasmic reticulum throughout the cytoplasm. Endocytic activity of the hair cells was limited to endosomes and small granules located at the apical part of the cells. Blockers of the mechanosensitive cation channel (neomycin, gentamicin, streptomycin, and amiloride) effectively inhibited FM1-43 labeling. One of the blockers, amiloride, was found to bind to hair cells when its fluorescence was examined. Divalent cations such as Mg2+ and Ca2+, but not monovalent cations such as Na+ and K+, inhibited FM1-43 labeling when they were added in excess amounts. These results suggest that FM1-43 internalizes hair cells via the putative mechanosensitive cation channel in the plasma membrane.

Ameloblasts responsible for tooth enamel formation are classified into two different phases: secretion and maturation. At the transition between these secretion and maturation stages, a considerable number of cells die. In this study, we examined the morphology of degenerating ameloblasts by conventional electron microscopy, and DNA cleavage in degenerating ameloblast nuclei by the in situ terminal transferase assay. The results suggest that apoptosis (programmed cell death) in ameloblasts, including DNA ligation is induced at the transitional stage. The nuclear fragments, chromatin condensation and DNA relocation in apoptotic nuclei were examined quantitatively by post-embedding anti-DNA immunogold electron microscopy and the in situ terminal transferase assay combined with electron microscopy. Numerical analysis revealed that immunogold labeling density in the condensed chromatin of apoptotic nuclei was comparable on the average to that in the perinuclear heterochromatin of normal nuclei, and that individual apoptotic nuclear fragments exhibited highly variable to that of normal heterochromatin, to fragments with densities twice as high as that of normal heterochromatin. The in situ terminal transferase assay combined with electron microscopy detected DNA ends exposed by ultrathin sectioning as well as DNA cleavage by a putative endonuclease. In conclusion, the state of the DNA, including its ligation and degeneration, changes gradually during chromatin condensation and nuclear fragmentation of apoptosis.

SUMMARY Apoptosis of dental pulp cells of rat incisors was investigated by the TUNEL method and electron microscopy. The results showed that a considerable amount of apoptosis occurred in the pulp, increasing in extent with incisal direction. OX6, ED1, and ED2 antibodies were used to detect macrophages and dendritic cells in combination with immunoelectron microscopy. Apoptotic fragments were eliminated mainly by MHC Class II-expressing cells, including dendritic cells positive for the OX6 antibody, and by MHC Class II-negative macrophages. Macrophages and dendritic cells positive for OX6, ED1, or ED2 increased from the apical to incisal direction of the incisor. These results indicate that apoptosis contributes to normal pulp formation and maintenance.

No abstract

The patterns of fluorescence associated with maturation ameloblasts of mandibular incisors labeled with 7-nitrobenz-2-oxa-1,3-diazole-phallacidin (NBD-phallacidin) for the detection of F-actin were investigated in normal and fluoride-treated rats. In normal rats, bands of smooth-ended ameloblasts (SA) exhibited intense fluorescence at their proximal ends only. Bands of ruffle-ended ameloblasts (RA) exhibited strong fluorescence at their distal ends as well as at their proximal ends. Regional differences in degree of intensity within the bands and between bands were displayed. In the apical part of the RA bands the proximal fluorescence was intense; it then decreased in an incisal direction; and it finally was absent close to the adjacent SA band. The incisal extension of strong proximal fluorescence in RA bands was short in early maturation and long in late maturation. The fluorescence pattern at both ends of the ameloblasts was cyclically repeated throughout the region of ameloblast modulation corresponding to the numbers of SA bands. In rats receiving 113 ppm fluoride in their drinking water for 2 months the number of fluorescence and ameloblast modulation cycles was reduced equally indicating that the cyclic F-actin localization is a phenomenon related to ameloblast modulation. Electron microscopy revealed that areas of strong fluorescence contained filament bundles, presumably actin filaments, in relation to continuous junctions occluding the interameloblast spaces. Areas of weak or no fluorescence were related to discontinuous macular junctions. The results suggest that the changes in F-actin distribution correlate well with junctional complex development, and therefore, possible functions related to the intermeloblast spaces within the RA bands may be redistributed as the ameloblasts are carried incisally by the erupting incisor.

We examined the processing of apoptotic ameloblast fragments at the transitional stage by immunocgtocbemistry and electron microscopy. Both macrophages and epithelial cells in the enamel organ ingested apoptotic bodies. MHC Class II antigen expression and the phagolysosomal nature of the macrophages were further examined by immunoelecaon microscopy using OX6 and ED1 antibodies followed by HRP or gold-conjugated secondary antibodies. The results showed that macrophages in the enamel organ at the transitional

Dendritic cells in the enamel organ of rat incisors were examined with immunocytochemistry using an anti-cystatin C antibody for immature dendritic cells and macrophages, OX6 for MHC Class II, ED1 for macrophages and dendritic cells, and ED2 for macrophages. Single cells positive for anti-cystatin C appeared in the enamel organ in zones at which ameloblasts secrete enamel matrix proteins. They were also present in transition and enamel maturation zones. In addition, ameloblasts, osteocytes, and osteoclasts were labeled by anti-cystatin C. ED1 and ED2 immunocytochemistry revealed that there was no macrophage population in the enamel organ of secretion, transition, or enamel maturation zone. A double labeling study showed that most anti-cystatin C-positive cells in the enamel maturation zone were also positive for OX6, whereas anti-cystatin C-positive and OX6-negative cells were prevalent in the secretion zone. The results suggest that immature dendritic cells penetrate the enamel organ of the secretion zone and begin to mature in the zones of transition and enamel maturation. (J Histochem Cytochem 48:1243-1255, 2000)

ABSTRACT. Localization of junctions between inner enamel-secretory ameloblasts was examined by immunofluorescence microscopy using antibodies against adherens junction proteins, radixin, vinculin, and A-CAM.All antibodies used stained the boundary between the ameloblasts exclusively in the plane where F-actin was abundant. This suggests that the adherens junctions in the ameloblasts are involved in cell-to-cell movement with actin-based micro filament bundles.Ameloblasts, which are responsible for the formation of tooth enamel, have junctional complexes at both their apical and distal poles during the secretory stage (12, 17). There are two secretory stages of enamel formation (19). The first stage, inner enamel secretion, occurs whenameloblasts produce rows and the secretory processes (Tomes' processes) of the ameloblasts take on an opposite orientation between adjacent rows of ameloblasts (5). The inner enamel is composed of rows of rods which are structural units of enamel, and the long axis of th rods within a row runs almost at right angles to that of the adjacent row (18). The second stage, outer enamel secretion, occurs whenameloblasts lose their row pattern and, the rods formed become uniformly straight (5, 18, 19). Studies on the structure of the inner enamel suggest that the ameloblasts slide laterally past each other during inner enamel formation and that the micro filament bundles associated with the distal junctional complexes are responsible for cellcell sliding between the rows of ameloblasts (5, 7-10). It has also been shown that the micro filament bundles in the ameloblasts are composed of actin, myosin, alphaactinin and tropomyosin (8). These cytoskeletal proteins are abundant at the cell boundary in the plane where cell-cell sliding takes place. Therefore, it is reasonable to assume that the contraction of actinbased filament bundles causes the ameloblast rows to slide laterally. As to how contraction of the filament bundles in the inner enamel-secretory ameloblasts leads to actual cell movement, to produce actual sliding between adjacent cells sometransient fixing points must be present in the sliding plane with the contraction of the filament bundles. In this study, we concentrated on adherens junction proteins, since adherens junctions are known to be highly dynamic structures as well as anchoring loci between neighboring cells or cells and substrates.Three kinds of proteins were examined by immunofluorescence microscopy: radixin, an adherens junction protein located at the cell-cell junction but not at the cell-substrate junction (13) vinculin, another adherens junction protein located at both the cell-cell and cellsubstrate junctions (2,3); and A-CAM,a Ca2+-dependent cell adhesion molecule, located at the adherens junc- tions (15,16). In this study, all three proteins were localized in the distal junctional area, and were exclusively at the cell boundary along the possible sliding plane of the inner enamel-secretory ameloblasts. MATERIALS AND METHODSMale Wistar rats (130-310 g) were sacri...