Enamel mineralization in the absence of maturation stage ameloblasts.

The role of maturation stage ameloblasts is not clear yet. The aim of this study was to verify to which extent enamel mineralizes in the absence of these cells. Maturation stage ameloblasts and adjacent dental follicle cells from rat lower incisors were surgically removed and the limits of this removal were marked by notches made in the enamel. Histological analysis confirmed that the ameloblasts had been removed within the limits of the notches. The teeth erupted and when the notches appeared in the mouth, the enamel in the experimental teeth was hard but whitish compared to the yellowish colour of the contralateral incisors used as control. SEM images revealed similar enamel rod arrangement in both groups. Decreased mineral content was observed in some specimens by polarized light microscopy, and microhardness values were much lower in the experimental teeth. FTIR analysis showed that higher amounts of protein were found in most experimental teeth, compared with the control teeth. Enamel proteins could not be resolved on 15% SDS-PAGE gels, suggesting that most of them were below 5kDa. These results suggest that the enamel matured in the absence of ameloblasts has increased protein content and a much lower mineral content, suggesting that maturation stage ameloblasts are essential for proper enamel mineralization.

Growth and the modeling/remodeling of the alveolar bone of the rat incisor.

The modeling and remodeling of the rat incisor alveolar bone was followed as the animals grew. The weight of the hemimandible, the length of the socket, and the width of the lower incisor were measured. Osteoclasts and resorption areas were identified by tartrate-resistant acid phosphatase staining. Fluorochrome markers were used to detect and measure osteogenic activities. In the socket related to the periodontal ligament, osteoclasts appeared in scattered sites as well as isolated sites of osteogenic activity, apparently without any variation related to the age of the animals. At the socket facing the dental follicle of young rats, the inner surface was lined with osteoclasts. The number of osteoclasts decreased steadily as the rats grew. In 1-year-old rats, in addition to a few scattered osteoclasts, the internal aspect of the labial wall showed some sites lined with osteoblasts and cement lines indicative of prior bone formation. In young rats, there was a continuous osteogenic activity at the external surface of this wall. The thickness of the labial wall of the socket remained apparently constant; therefore, bone resorption must have occurred at the internal side of the wall. Such osteogenic activity was not observed in old rats. The main forces acting on rat incisors, biting and eruption, are continuous through the life of the animals. Thus, these results indicate that the modeling of the alveolar bone related to the dental follicle, in young rats, can only be associated with another force, specifically, the growth of the incisor.

Development of functional dentin incisors after a partial resection of the odontogenic organ of rat incisors.

The resection of the labial half of the odontogenic organ of rat incisors resulted in the development of teeth without enamel. Ten out of 26 operated rats developed a functional dentin incisor, i.e. a continuously growing and erupting tooth. These teeth were a little shorter and much thinner than normal incisors. The dentin and pulp presented a normal structure. Periodontal ligament and cement started to develop at the lingual face and gradually all tooth faces were invested by these tissues. The original socket space, to accommodate a thinner tooth, was narrowed by newly formed bone around the inner face of the socket. Eleven rats developed defective dentin incisors; these teeth showed signs of growth, however, their eruption was impaired. The operation failed in five rats. The odontogenic organ of the dentin incisor presented islands of epithelial cells at the labial aspect of a dense mass of mesenchyme cells. These islands, formed by densely packed, dark-staining cells encircling a few pale-staining cells, merged gradually, forming a root sheath and a cervical loop limiting a long apical foramen. The bulk of the bulbous part (apical bud) was absent; thus, there was no differentiation of ameloblasts and of the crown-analogue part of the incisor. The growth and eruptive behaviour of the dentin incisor, similar to that of a normal incisor, indicates that it has to bear a stem cell niche to retain its regenerative capacity. As in the apical bud, this niche is apparently located at the stellate reticulum of the cervical loop. The putative molecular mechanisms related to either the maintenance of the stem cell niche or the differentiation of the enamel organ and the root sheath are discussed. These data and our results, showing the development of a functional dentin incisor, suggest that the root-analogue part of the rodent incisor is an anatomic-physiological entity.

Alveolar bone Sharpey fibers of the rat incisor in normal and altered functional conditions examined by scanning electron microscopy.

The morphology and the area density of Sharpey fibers in the socket of the rat incisor under normo-, hyper-, and hypofunctional conditions were evaluated using scanning electron microscopy. Sharpey fibers appeared either as dome-shaped projections, when highly mineralized, or as depressions when less mineralized. Near the alveolar crest, most of the fibers were fully mineralized and arranged in compact longitudinal rows. Toward the basal end of the socket, the rows became interrupted, forming islets of gradually smaller size and number. The density of the Sharpey fibers was higher (P < 0.01) in the mesial and distal faces than in the lingual face in most of the socket length. In normofunctional conditions, in all faces the density decreased 70 to 90 times from the crestal toward the basal region of the socket (P < 0.01). The degree of mineralization of the Sharpey fibers also decreased steadily in the same direction, indicating that, for support, the periodontal ligament matures from basal to incisal and is fully developed only in the crestal region. In hyper- and hypofunctional conditions, the same distribution was observed. The area density of the Sharpey fibers in the hyperfunctional condition showed a slight increase at the basal levels of the socket mesial and distal faces (P < 0.01 or P < 0.05). In hypofunctional incisors, the density decreased significantly (P < 0.01) at the mesial and distal faces in all regions of the socket, and at the lingual face, the decrease (P < 0.05) was restricted to the incisal regions. This may be one of the factors for the weakening of the periodontal ligament in hypofunctional incisors.

The effect of partial damage to the enamel-related periodontium combined with root resection on eruption of the rat incisor eruption.

Previous work has indicated that the enamel-related periodontium (ERP) has a role in the eruptive process of the rat lower incisor. By combining partial damage of this tissue with resection of the odontogenic organ, we examined the effect of the damage on subsequent incisor eruption. The connective tissue of the enamel-related periodontium was regenerated in less than 2 weeks, showing morphology close to normal. The injured part of the enamel organ was neither regenerated nor repaired, and a cement-like tissue, continuous with the true acellular cement, was formed on the denuded enamel. Before tooth exfoliation, the operated teeth erupted at a slower rate compared with root-resected and sham-operated incisors, probably because of the absence of a substantial part of the enamel organ due to surgical damage. As with the coronal dental follicle and the enamel organ in rat molars, the enamel-related periodontium and the enamel organ of rat incisors may have some control on their eruptive process.

An ultrastructural study of the supporting tissues of sea urchin (Lytechinus variegatus) teeth

The teeth of sea urchins are connected to the calcareous jaw plates, known as pyramids, by a ligament consisting of collagen fibers and microfibrils synthesized by fibroblasts in the aboral growth zone of the tooth. This ligament needs to be sufficiently stiff to hold the teeth firmly when the animal scrapes hard surfaces, but also needs to be flexible enough to allow the teeth to move outwards during growth. To understand the mechanisms that regulate the growth and stiffness of sea urchin teeth, we have examined the ultrastructural organization of the supporting structures of Lytechinus variegatus teeth. Electron microscopy showed that collagen fibrils were mechanically attached to the jaws was formed by cavities that ramificated in the deep portions. The collagen fibrils were not mechanically linked to the jaws. These findings suggest that the stiffness of the ligament is mediated by chemical bonding between the collagen fibrils and the jaw surface. The cavities present in the pyramids greatly increased the surface area and strengthened the area for the bonding of collagen fibrils.

Kinetics of labelled precursors incorporated in the enamel matrix during amelogenesis as shown by radioautography

The kinetics of (3)H-tymidine and (3)H-proline incorporated by ameloblasts and enamel, were studied in undecalcified mouse incisors from birth to 6 days of age. Serial cross sections of unfixed right incisors were cut with a cryotome. The left incisors were fixed in paraformaldehyde, embedded in polybed as to get sagital 1 mum-thick sections. (3)H-thymidine was used to determine the apparent daily migration rate of ameloblasts, which was 513 mum in the unfixed sections and 610 mum/p.d. in the fixed ones. The semi-thin epon-embedded sections were also used to measure the lengths of the regions of the secretory and post-secretory zone of amelogenesis and to determine their growth during the experimental period. (3)H-proline was used to show the fate of the enamel proteins by correlating the radiactivity, determined by silver grain counts, with the migration rate of the ameloblasts. The results showed that the (3)H-proline labeled protein reached a peak of radiactivity at 4 h over ameloblasts and between 24 and 48 h after injection over enamel. In the unfixed section of the righ incisor a second peak of reaction was shown at48 h over ameloblasts and at72 h over enamel matrix. All these peaks were related to ameloblasts and enamel of the secretory zone. These results were interpreted as the evidence of reabsorption and reutilization of labeled proteins broken down in the young enamel, but may also be explained as secretion of low molecular weight proteins which are not kept by fixation. Another evidence of reutilization of labeled compounts for the biosynthesis of enamel proteins were given by the labeling of ameloblasts and enamel formed after birth at a considerable time after the pulse of (3)H-proline.

Detecçäo histoquímica de glicoproteínas e glicosaminoglicanos na mucosa respiratória de ratas albinas durante o ciclo estral, prenhez e puerpério / Histochemical detection of glycoproteins and glycosaminoglycans in the respiratory mucosa of albino rats during estrous cycle, pregnancy and puerperio

Neste trabalho procuramos determinar com métodos histoquímicos as possíveis alteraçöes do muco presente na mucosa respiratória de ratas albinas durante o ciclo estral, prenhez e puerpério. Com base nos resultados obtidos, foi possível concluir que: a - Näo ocorrem alteraçöes da natureza do muco epitelial e supra-epitelial durante as fases estudadas; b - Durante a prenhez desaparece a alcianofilia da substância fundamental da lâmina própria, que reaparece durante o puerpério