Effects of various agents on staining of the maturation pattern at the surface of rat incisor enamel

Claudin-16 Deficiency Impairs Tight Junction Function in Ameloblasts, Leading to Abnormal Enamel Formation

Claudin-16 protein (CLDN16) is a component of tight junctions (TJ) with a restrictive distribution so far demonstrated mainly in the kidney. Here, we demonstrate the expression of CLDN16 also in the tooth germ and show that claudin-16 gene (CLDN16) mutations result in amelogenesis imperfecta (AI) in the 5 studied patients with familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC). To investigate the role of CLDN16 in tooth formation, we studied a murine model of FHHNC and showed that CLDN16 deficiency led to altered secretory ameloblast TJ structure, lowering of extracellular pH in the forming enamel matrix, and abnormal enamel matrix protein processing, resulting in an enamel phenotype closely resembling human AI. This study unravels an association of FHHNC owing to CLDN16 mutations with AI, which is directly related to the loss of function of CLDN16 during amelogenesis. Overall, this study indicates for the first time the importance of a TJ protein in tooth formation and underlines the need to establish a specific dental follow-up for these patients.

X-ray microtomographic study of mineral distribution in enamel of mandibular rat incisors

X-ray microtomography was used to study the mineral concentrations in sequential slices of enamel of 5 mandibular incisors which showed an increase from approximately 1.0 to approximately 2.7 g cm(-3) from the apex towards the incisal end. For points at the same distance from the apex, there were differences up to 0.6 g cm(-3) between the teeth. The change of mean concentrations in the slices with distance could be modelled as (different) saturating exponentials. Under the assumption of a uniform growth rate of a mandibular incisor of 0.6 mm per day and a common time origin for the start of maturation (taken as a mineral concentration of 1 g cm(-3)), the distances were transformed to a common time frame to give a pooled data set. A single saturating exponential could be fitted to this pooled transformed data; this was: Cm = 2.84-1.94exp (-0.18d) where Cm is the mean mineral concentration (g cm(-3)) and d the time (days) from the start of maturation. This gives an asymptotic concentration of 2.84 g cm(-3) towards the incisal end, with a time constant of 7.7 days. The mineral concentration distribution functions were found to be more positively skewed closer to the apex, but more negatively skewed towards the incisal end. The difference between the higher mineral concentration in the outer enamel and the enamel near the amelodentinal junction (ADJ) was approximately 3%. The direction of maximum increase in concentration from the outer enamel surface to the ADJ meets the boundary of the ADJ at approximately 80 degrees. Three dimensional surface rendering of isodensity contours showed that the previously described C-shaped pattern of mineralisation is not solely a surface phenomenon, but extends through the depth of the enamel.

Cellular and chemical events during enamel maturation

This review focuses on the process of enamel maturation, a series of events associated with slow, progressive growth in the width and thickness of apatitic crystals. This developmental step causes gradual physical hardening and transformation of soft, newly formed enamel into one of the most durable mineralized tissues produced biologically. Enamel is the secretory product of specialized epithelial cells, the ameloblasts, which make this covering on the crowns of teeth in two steps. First, they roughly "map out" the location and limits (overall thickness) of the entire extracellular layer as a protein-rich, acellular, and avascular matrix filled with thin, ribbon-like crystals of carbonated hydroxyapatite. These initial crystals are organized spatially into rod and interrod territories as they form, and rod crystals are lengthened by Tomes' processes in tandem with appositional movement of ameloblasts away from the dentin surface. Once the full thickness of enamel has been formed, ameloblasts initiate a series of repetitive morphological changes at the enamel surface in which tight junctions and deep membrane infoldings periodically appear (ruffle-ended), then disappear for short intervals (smooth-ended), from the apical ends of the cells. As this happens, the enamel covered by these cells changes rhythmically in net pH from mildly acidic (ruffle-ended) to near-physiologic (smooth-ended) as mineral crystals slowly expand into the "spaces" (volume) formerly occupied by matrix proteins and water. Matrix proteins are processed and degraded by proteinases throughout amelogenesis, but they undergo more rapid destruction once ameloblast modulation begins. Ruffle-ended ameloblasts appear to function primarily as a regulatory and transport epithelium for controlling the movement of calcium and other ions such as bicarbonate into enamel to maintain buffering capacity and driving forces optimized for surface crystal growth. The reason ruffle-ended ameloblasts become smooth-ended periodically is unknown, although this event seems to be crucial for sustaining long-term crystal growth.

Enamel mineralization and the role of ameloblasts in calcium transport

Amelogenesis is a dynamic and unique process of cell-matrix interactions in that matrix synthesis, degradation and resorption all proceed simultaneously, coupled with mineral depositions in a compartment between ameloblasts and dentin or dental papilla. Accumulation of data suggest the role of ameloblasts in tooth morphogenesis and matrix formation, but no fully acceptable explanation has been given concerning the role of ameloblasts in calcium transport. In this article, old and new points of issue raised regarding the role of ameloblasts in calcium acquisition are reviewed and possible mechanisms whereby the ameloblasts prevent the rise of cytosolic calcium while actively or less actively transporting calcium are elaborated upon based on recent findings.

Regulation of amelogenin gene expression during tooth development

The amelogenins are the predominant matrix proteins in developing enamel and are crucial for proper enamel mineralization. Transgenic mice were constructed in order to identify the segment of the amelogenin gene required for specific expression in enamel organ cells. A 3.5 kb fragment of the bovine X-chromosomal amelogenin gene that includes a TATA box, the transcription initiation site, and 32 bp of exon 1 was linked to the beta galactosidase gene and injected into fertilized mouse eggs. Newborn transgene positive mice expressed beta galactosidase activity in developing teeth treated with the chromogenic substrate Xgal. Foci of ameloblasts were positive in newborn mice; stain intensity and number of positive ameloblasts increased in 1-day and 2-day postnatal mice. Some of the adjacent stratum intermedium cells also were positive in the later stages. Targeting of the transgene to the enamel organ was specific; the only other cells observed to be positive were macrophages, which have endogenous beta galactosidase activity.

Banding patterns in rat incisor enamel stained by histochemical complexing methods for calcium

A characteristic banding pattern can be visualized at the surface of the rat incisor in the maturation zone of amelogenesis by staining with glyoxal bis(2-hydroxyanil) (GBHA). Other banding patterns can be obtained with certain histological and fluorochrome stains and by radioautography following 45Ca injection. In this study, several histochemical reagents known to complex with different states of calcium were used to stain the surface of enamel. Rat incisors were quickly dissected and immediately immersed in solutions containing the following calcium-binding reagents: arsenazo III, calmagite, murexide, N,N-naphthaloylhydroxylamine, and calcein. Routinely, one contralateral lower incisor from each pair was counterstained with GBHA in order to relate each of the staining patterns to the banded distribution of maturation ameloblasts that is reflected by the characteristic GBHA staining pattern in the enamel. Each of the reagents used in this study demonstrated a staining pattern consisting of a series of broad bands running transversely and obliquely across the enamel. In all cases, the dyes stained predominantly that enamel associated with ruffle-ended ameloblasts, i.e. enamel left unstained by GBHA. Some of the reagents also stained enamel in the secretion zone. The appearance and distribution of the staining patterns reflect the banded distribution of maturation ameloblasts and appear to be controlled on a time scale related to the rapid modulation of these cells.

Biophysical analyses of sequential bands of enamel related to ruffle-ended and smooth-ended maturation ameloblasts

During amelogenesis in the rat incisor, modulating ruffle-ended (RA) and smooth-ended (SA) ameloblasts are distributed as bands in the enamel organ of the maturation zone. This distribution of the two cell types has been shown to be precisely correlated with a banding of the underlying enamel, as shown by staining and other cyclical indicators (Takano et al., 1982a,b). Several biophysical approaches have been taken here to characterize the enamel bands sequentially and to determine whether the appearance of such bands is attributable to differences in inorganic composition possibly related to RA and SA. Sprague-Dawley rats were decapitated under ether anesthesia, lower incisors were dissected from surrounding alveolar bone, and enamel organs were wiped from the teeth with moistened gauze. Analyses were performed on either the surface of intact enamel or on individual strips of enamel dissected from the tooth surface, by use of the translucent bands that appear during drying as reference marks for the positions of the overlying cell type. X-ray diffraction (XRD), infrared spectroscopy (IR), x-ray photoelectron spectroscopy (XPS), and wave-length-dispersive electron probe x-ray micro-analysis (WDS) all failed to detect significant differences between analysis areas; data were characteristic of enamel apatite having typical XRD maxima (hkl = 002, 211, 112, 300), IR absorption bands (PO4(3-) and CO3(2-], XPS Ca and P binding energies (Ca2p = 350.5, 346.9 eV; P2p = 133.7, 132.7 eV), and WDS Ca/P molar ratios (1.33-1.49).(ABSTRACT TRUNCATED AT 250 WORDS)

Cyclical incorporation of 33P into rat incisor enamel in vivo as visualized by whole-mount radioautography

Phosphorus uptake during amelogenesis was investigated in the continuously erupting rat incisor. Five minutes after intravenous injection of 33P-labelled ortho phosphoric acid, whole-mount radioautography of entire incisors revealed heavy labelling in the form of bands and narrow parallel stripes at the surface of the enamel in the maturation zone. There was relatively little labelling over enamel in the secretion zone and over pigmented enamel. Thus 33P is incorporated cyclically into maturing enamel and is visualized as (1) a banded pattern that reflects the modulation of ruffle-ended and smooth-ended maturation ameloblasts and (2) a striped pattern that reflects the distribution of newly-formed protein secreted by maturation ameloblasts. Presumably these P incorporation patterns are closely related to other cyclical events known to occur during enamel maturation.

Cyclic changes in the properties of maturing enamel in the bovine permanent incisor as revealed by glyoxal bis(2-hydroxyanil) staining

Staining patterns of the surface and interior of maturing enamel of these tooth germs were examined using the glyoxal bis(2-hydroxyanil) (GBHA) solution which chelates with calcium loosely bound to hydroxyapatite to form insoluble red precipitates. An intense red, band-like pattern of GBHA staining, 1-2 mm wide, first appeared on the incisal portion of lingual enamel surface as complete loops, concentrically arranged; these gradually increased in number. Most of the later-formed bands encircled the entire periphery of the maturing enamel surface. GBHA also revealed reactive areas on the labio-lingual, cut and ground surface of maturing enamel, corresponding exactly to the surface GBHA bands. GBHA did not stain EDTA-etched surface enamel, but did reveal regular staining patterns on the ground surface, disclosed after EDTA treatment. These observations suggest an intimate correlation between the properties of interior and surface enamel, at least with regard to the state of local calcium. The maturation of bovine incisor enamel may start at the lingual aspect of the tooth crown.

Demonstration by staining and radioautography of cyclical distributions of protein at the enamel surface in rat incisors

Staining patterns in the enamel during the maturation stage of amelogenesis reflect the banded distribution of ruffle-ended and smooth-ended ameloblasts. This study investigated the possibility that proteins at the enamel surface may be distributed cyclically according to cyclical changes in ameloblast morphology. Dissected lower rat incisors were wiped free of their enamel organs and immediately immersed in fixative containing one of the following heavy metal and histological stains: uranyl acetate, lead citrate, Coomassie blue, alcian blue and ruthenium red. Other animals were injected with [35S]methionine to label newly-formed enamel proteins. Their incisors were dissected, the enamel organs were wiped from the enamel surface, and the teeth were processed as whole mounts for radioautography. Teeth stained by heavy metals were also viewed by back-scattered electron imaging. The in-situ staining revealed that proteins were distributed in bands and stripes across maturing enamel. Radioautography revealed that the proteins in the stripes were newly-synthesized and secreted into the enamel by certain maturation ameloblasts. We conclude that the enamel organ expresses cyclical activity in part through secretion of proteins.

Cyclic induction and rapid movement of sequential waves of new smooth-ended ameloblast modulation bands in rat incisors as visualized by polychrome fluorescent labeling and GBHA-staining of maturing enamel

The movement of smooth-ended ameloblast modulation bands was studied in continuously erupting Tincisors of male Wistar rats, with fluorochromes such as calcein (green), xylenol orange (red), tetracycline (yellow), and calcein blue (turquoise) used to label maturing enamel intensely at sites delimiting the location of smooth-ended ameloblasts at the time of injection. Hence, a fluorescent label of one color was injected to establish a reference position at time "0" followed by one or more fluorescent labels of different colors, or by in vitro enamel staining with glyoxal bis(2-hydroxyanil)(GBHA), at various times after the initial injection. For example, rats injected with calcein followed by xylenol orange at 10 min or two, four, six, or 12 hr later showed zero, 367, 888, 1259, and 2833 μm incisal movement, respectively, of the red bands relative to companion green fluorescent bands in the mandibular incisors. If one takes into account the eruption rate for these teeth (27.1 μm per hr), these data were indicative of a coordinated, wave-like movement of smooth-ended ameloblast modulation bands incisally along the length of the tooth at a mean rate of 243 μm per hr. Measurements and graphic plots of the distribution of smooth-ended ameloblast bands in histological sections and in GBHA-stained teeth revealed not only that such bands were positioned at all possible locations along the length of the maturation zone within a group of different teeth, but also that the average interband distance equaled about 2100 μm in the apical part of the maturation zone. Hence, new modulation waves appear to arise near the region of post-secretory transition and travel along the ameloblast layer toward the gingival margin about once every 8.5 hours. This suggests that a given cohort of ameloblasts may modulate as frequently as three times a day and complete a minimum of 45 modulation cycles by the end of enamel maturation.

Cyclic localization of actin and its relationship to junctional complexes in maturation ameloblasts of the rat incisor

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.

Modification of the enamel maturation pattern by vinblastine as revealed by glyoxal bis(2-hydroxyanil) staining and 45calcium radioautography

Patterns characteristic of enamel maturation can be visualized at the surface of the rat incisor by staining with glyoxal bis(2-hydroxyanil) (GBHA) and radioautography following 45calcium injection. In this study, the effects of vinblastine on enamel maturation were monitored by these two methods. At 4 h after injection of vinblastine, the darkly-stained GBHA bands had widened incisally into the interband regions when compared to normal, control teeth. Radioautography at 5 min after calcium injection in vinblastine-treated animals (4 h) showed a modified maturation pattern of weaker labeling and less distinct banding. At 8 h after vinblastine injection, most of the enamel stained uniformly with GBHA, and bands and interband regions could not be resolved. Radioautography at 5 min after calcium injection showed that the 8 h vinblastine treatment removed the banding pattern, leaving only a weakly-labeled area. Vinblastine is known to destroy and prevent the formation and turnover of microtubules, and hence the formation of ruffled borders of ruffle-ended ameloblasts (Akita et al. 1983). The concomitant decrease in calcium incorporation implies that events taking place in relation to the ruffled border may affect calcium exchange or accretion within the enamel.