Correlation of apical and lateral membrane modulations of maturation ameloblasts

The cadherin-catenin complex is expressed alternately with the adenomatous polyposis coli protein during rat incisor amelogenesis

E-cadherin, a calcium-dependent cell-cell adhesion molecule, is expressed in highly specific spatiotemporal patterns throughout metazoan development, notably at sites of embryonic induction. E-cadherin also plays a critical role in regulating cell motility/adhesion, cell proliferation, and apoptosis. We have used the continuously erupting rat incisor as a system for examining the expression of E-cadherin and the associated catenins [alpha-, beta-, gamma-catenin (plakoglobin) and p120(ctn)] during amelogenesis. Using immunhistochemical techniques, we observed expression of alpha-catenin and gamma-catenin in ameloblasts throughout amelogenesis. In contrast, expression of E-cadherin, beta-catenin, and p120(ctn) was strong in presecretory, transitional, and reduced stage ameloblasts (Stages I, III, and V) but was dramatically lower in secretory and maturation stage ameloblasts (Stages II and IV). This expression alternates with the expression pattern we previously reported for the adenomatous polyposis coli protein (APC), a tumor suppressor that competes with E-cadherin for binding to beta-catenin. We suggest that alternate expression of APC and the cadherin-catenin complex is critical for the alterations in cell-cell adhesion and other differentiated cellular characteristics, such as cytoskeletal alterations, that are required for the formation of enamel by ameloblasts.

Adenomatous polyposis coli protein is expressed in alternate stages of the ameloblast life cycle

Mutations of the adenomatous polyposis coli gene protein (APC) are associated with familial polyposis and also sporadic colon adenomas, both preconditions to cancer formation. Some familial polyposis patients also develop Gardner's syndrome, a condition characterized by supernumerary teeth, mandibular osteomas, and other maladies. We investigated participation of APC in normal tooth development. Using a monoclonal antibody to study APC expression in the forming rat incisor, we found no APC staining in differentiating ameloblasts, then strong staining in secreting ameloblasts and stratum intermedium cells, followed by cells in the transition stage which did not stain. Intense APC staining resumes in maturation-stage ameloblasts and proximal papillary cells. APC staining disappears again in reduced ameloblasts at the conclusion of amelogenesis. APC staining was not seen in any other odontogenic cells. We report a unique system in which APC expression is upregulated and downregulated twice during the normal life cycle of ameloblasts. APC, therefore, is important in the normal maturation of both colonic epithelium and odontogenic epithelium. At this point, we cannot rule out any of the known functions of APC, which include: modulation of cell adhesion by binding to catenin, regulation of beta-catenin as a differentiative signaling molecule, and promotion of microtubule assembly. In this respect, the rat incisor enamel organ provides a unique tissue for studying the regulation and functions of APC.

Morphology of the enamel organ in the miniature swine

In recent years, the dentition of the pig has been increasingly used as a model for the study of amelogenesis. Indeed, much of our current knowledge on enamel formation derives from biochemical and physicochemical analyses of the organic and inorganic components, respectively, of porcine enamel. As an extension of this previous work, and as the first step in our attempt to correlate known enamel matrix and mineral changes with adjacent enamel organ morphology, the present study was undertaken to provide a description of the morphological events occurring in the enamel organ during porcine amelogenesis. Two-week-old miniature swine (minipigs) were fixed by vascular perfusion with glutaraldehyde, the deciduous teeth present at this age were embedded in Epon resin and sectioned, and the cells of the enamel organ at each of the various developmental stages of amelogenesis were examined by light and transmission electron microscopy. In many respects, the morphology of the porcine enamel organ was similar to that previously described in other mammalian species. On the other hand, several particularities were noted and these are discussed in the context of available data correlating cell ultrastructure with putative function during enamel formation.

Ultrastructural study of tracer permeability through the cat and ferret enamel organ

The access of exogenous materials to the developing enamel surface has been intensively studied in rodents, but not in other mammalian species. This ultrastructural study investigates the permeability of injected horseradish peroxidase (HRP) and lanthanum tracers in cat and ferret tooth buds. In cat enamel organs fixed by immersion, lanthanum did not escape the capillaries overlying secretory stage tooth buds, but it did permeate up to the distal junctions of ruffle-ended (RA) and the proximal junctions of smooth-ended (SA) ameloblasts. Perfusion fixation with lanthanum compromised junctional integrity of cat ameloblasts at all stages of development. Similarly, HRP rarely escaped the capillaries associated with cat secretory stage enamel organs. However, unlike lanthanum, HRP was mostly confined to the vasculature of maturation stage enamel organs in immersion fixed cats at all time intervals examined. In ferrets, HRP penetrated up to, but not beyond, the distal junctional complexes of secretory ameloblasts. In maturation stage enamel organs, HRP coated the papillary and RA cells, but did not penetrate the RA distal cell junctions. HRP did permeate the extracellular spaces of SA to reach the underlying enamel surface. Ameloblasts in transitional phases of SA and RA endocytosed HRP at the distal cell surface. This data leads to several conclusions. First, HRP localization in the ferret paralleled that observed in rodents. Second, the results of cat enamel organs substantiate previous studies showing perfusion fixation can increase vascular and intercellular permeability to lanthanum. However, in cats fixed by immersion, both lanthanum and HRP were restricted to capillaries associated with the secretory stage enamel organ, and only lanthanum escaped maturation stage capillaries. It is suggested that variations in the fenestrations and distribution of capillaries associated with the cat enamel organ may differentially retain some materials and permit other materials to escape with relative ease.

Scanning electron microscopy of monkey secretory- and transitional-stage enamel organ cells

This scanning electron microscope (SEM) study of secretory- and transitional-stage enamel organ cells of the permanent dentition of Macaca mulatta and Macaca arctoides was undertaken because the topography of these cells in primates has not been described in the literature. Comparison of our results with murine enamel organ morphology reported previously revealed not only many similarities, but also some significant differences. Tooth buds of the permanent dentition were routinely prepared for SEM. Murine secretory-stage ameloblasts have been described to be 65-70 microns long, with smooth lateral membranes, but those of monkeys were only 30-35 microns tall, with four different lateral plasma membrane configurations: smooth, filamentous, longitudinally ridged, and transversely ridged. The filamentous form was most common. Cells were seen with either transverse or longitudinal ridges in the basal half, and with filamentous ridges in the apical portion; this indicates modulation between these forms. Because of the extraordinary similarity between these lateral membrane modulations and those of rat incisor maturation ameloblasts, a comparable function is proposed--namely, that monkey secretory ameloblasts function, in part, in the resorption and mineralization of enamel matrix. There were several layers of rounded stratum intermedium cells basal to monkey secretory-stage ameloblasts, but only one layer of cuboidal stratum intermedium in rodents. The stellate reticulum cells of rats and monkeys appeared attenuated, with large extracellular spaces. There was little or no reduction in cell length of monkey transitional-stage ameloblasts. The position of the nuclear bulge differentiated transitional- from secretory-stage ameloblasts.(ABSTRACT TRUNCATED AT 250 WORDS)

A scanning electron microscope study of monkey maturation-stage ameloblasts

The maturation-stage enamel organs of Macaca arctoides and Macaca mulatta were examined in order to determine whether the cells were similar to those of the continuously erupting rat incisor. Tooth buds of the permanent dentition were fixed in formaldehyde-glutaraldehyde and post-fixed in OsO4. The enamel organs were separated from the enamel during dehydration, critical-point-dried, metal-coated, and examined in a scanning electron microscope. The results showed that there were few differences in the morphology of maturation-stage ameloblasts of these primates compared with those of other species reported in the literature. The apical plasma membranes were either smooth- or ruffle-ended, while the later membranes had maze, microvillous, or ridge configurations, also seen in rats, and an additional configuration of interdigitating bulbous extensions. The blood vessels of the papillary layer in monkeys were about 7 micron in diameter, considerably larger than those of the rat.

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.

Correlated observations and analysis of maturation-ameloblast morphology and enamel mineralization

A combined HCl-collagenase digestion technique and scanning electron microscopy were used to isolate the enamel organ and to confirm the presence of maturation ameloblasts of both ruffle-ended (RA) and smooth-ended (SA) types on maturing enamel in kitten permanent tooth germs. EDTA perfusion of animals fixed with aldehyde produced two or three belt-like shallow grooves (from 30 to 100 micron wide) running horizontally through the maturing enamel surface, coinciding closely with the SA distribution pattern. In animals that had been perfusion-fixed with unbuffered osmium tetroxide containing 2.5% potassium pyroantimonate, SEM-EDX analysis detected K in a superficial enamel layer overlaid by the SA layer. Potassium concentration decreased gradually toward the deeper layers. Very little K penetrated the enamel under the RA layer. Energy-dispersive x-ray analysis of Ca and P concentrations in the enamel revealed an even distribution of these elements throughout the superficial layer of maturing enamel. These results suggest that the SA layer forms an access route for K and EDTA and that, in spite of the obvious morphological and functional differences between RA and SA, the maturing enamel surfaces overlaid by these two cell types show similar degrees of mineralization.

A study of post-secretory maturation ameloblasts in the cat by transmission and freeze-fracture electron-microscopy

Permanent canine and molar tooth germs of kittens were processed for thin-sectioning and freeze-fracture replication. Maturation ameloblasts were divided into ruffle-ended (RA), smooth-ended (SA) and intermediate (IA). RA had an extensive distal ruffled border consisting of deep membrane invaginations, forming complicated extracellular channels. Adjacent RA were connected by extensive distal junctional complexes (zonulae occludentes). The SA had flattened distal cell surfaces and few coated and smooth vesicles in the distal cytoplasm. Adjacent SA were connected by proximal zonulae occludentes, but had only maculae occludentes at their distal ends so permitting broad lateral extracellular spaces to communicate directly with the enamel surface. IA near the RA layer had a ruffled border consisting of deep and narrow membrane invaginations and pinosomes filled with granular material. IA next to the SA layer had no ruffled border but had pinosomes that seemed to originate directly from the distal cell surfaces. IA were polarized and connected by proximal and distal junctional complexes consisting of either zonulae or fasciae occludentes and associated gap junctions.