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PubMed1985

The effects of X-ray irradiation of the head region of eight-day-old rats on the development of molar and incisor teeth.

The effects of irradiation on the development of molars and incisors up to the age of 57 days was studied applying locally a single dose of 9.6 Gy of X-rays to the head region of eight-day-old female rats. Radiography of jaws revealed the absence of the third molar in all irradiated rats, as a result of germ stunting which was, at the moment of irradiation, in the phase of histodifferentiation. In the case of the first and second molar, being at the time of irradiation in the phase of root formation, changes were noted in the development and morphological features of the roots. In animals sacrificed 18 days after irradiation, the radiographs of jaws showed a break in the continuity of the incisors located under the mesial root of the first molar. In all animals the break in continuity of the incisors became macroscopically visible between the 28th and 42nd day after irradiation. 15 days after the loss of incisors, the incisival functional occlusion was restored in all animals.

Age Factors↗
PubMed1985

Effect of organic and inorganic mercury compounds on the growth of incisor and tibia in rats.

The pharmacological effects of methyl mercury chloride (MMC) and mercuric chloride (HgCl2) (2-16 mg/Kg per day subcutaneously, for 6 d) upon the growth were studied in the incisors and proximal tibiae of immature rats histologically. Lead acetate was used as a time marker. 1. Mercury compounds slightly affected the body weight gains of the rats but apparently inhibited the longitudinal growth of proximal tibia and the effect increased with higher dosages. 2. Mercury compounds definitely inhibited not only the longitudinal growth (incisor growth) but also the appositional growth (dentin formation) of incisal dentin. 3. The inhibitory effect on the growth was ranked as follows: bone growth greater than dentin formation greater than incisor growth. 4. The actions of MMC on the growth of incisors and proximal tibiae appeared gradually and the response was biphasic; stimulatory and then inhibitory. The inhibitory effect appeared even after the injection was discontinued and appeared more extremely than during the injections. 5. In HgCl2 groups the inhibitory effect on the growth appeared rapidly. The effect increased with higher dosages and became stronger as the injections were repeated. However, this effect was weakened promptly after the injection was discontinued. 6. The repeated injections of mercury compounds hardly affected the level of serum calcium but disturbed the calcification of incisal dentin. From the above-mentioned results a possible mechanism was discussed. It is suggestive that MMC acts directly upon a cell and is transported into it. Once MMC was introduced into a cell it is slowly demethylated to inorganic Hg and acts as the demethylated mercury. When accumulated mercury is slight in volume, it stimulates and then inhibits the cell function with increasing mercury. However HgCl2 binds directly with an effector cell membrane in loose fashion. This may cause the ready reversibility of the effect.

Animals↗
PubMed1984

Dental caries of primary incisors in children with cleft lip and palate.

Visual dental examinations were performed on 64 children ages 18 months through 4 years presenting at a Craniofacial Defects Team charged with diagnostic and referral services. Of the 41 children with cleft lip and/or cleft palate, 13 (32%) had dental caries of one or more maxillary primary incisors. One of the remaining 23 children examined experienced caries of the maxillary primary incisors. In incisors of children having clefts of the alveolus, caries tended to correspond to sites of enamel deficiency in caries-free children with alveolar clefts. Caries patterns in children with clefts involving only the palate resembled "nursing carries". Children with clefts are at significant risk for caries of the primary incisors; a dental prevention program is to be emphasized in referring these children for care.

Child, Preschool↗
PubMed1981

[Diurnal layer formation in the incisor dentin of rodents].

The growth of dentin of the constantly growing lower incisors in rodents was studied using tetracycline as a vital marker of the growing parts of bones and teeth. In the common vole in nature the incisors grow in length at a rate of 490 micrometers per day; a basal-apical gradient of growth of the tooth walls in thickness was found: the minimal daily increment at the tooth base and the maximal in the apical third, it decreases again at the very apex. In the peripulp part of dentin of the apical incisor half there are diurnal layers seen both on polished sections and stained slides the formation of which correlates with the diurnal rhythm of skeleton growth. The diurnal dentin layers form in the apical incisor half only. The odontoblasts appear to begin to function rhythmically from the middle of their life on only. A similar phenomenon was found in a natural population of large-toothed redbacked voles. The topography of dentin layers in the laboratory rats is similar to that in the voles but the layers are not distinct and the basal-apical gradient of dentin growth is observed in young animals, rather than in the old ones.

Animals↗
PubMed1977

The effect of adrenal glucocorticoid administration on eruption rates and tissue dimensions in rat mandibular incisors.

A quantitative investigation was undertaken to evaluate the changes which occur in the dental tissues of the rat incisor following the systemic administration of adrenal glucocorticoid hormone. Of the 53 rats used in the 5 week experiment, 21 acted as untreated controls while the remaining 32 each received weekly subcutaneous injections of 0-1 ml Depo-Medrone. Throughout the experimental period the mandibular incisor eruption rates of a selection of both groups of rats were recorded. Transverse ground sections were prepared from the left mandibular incisor of each animal. Measurements taken on sections prepared at a standard reference level enabled comparisons to be made between control and hormone-treated animals in the dimensions of their incisor dentine and pulp. The results indicated that maintaining mature rats in a hyperglucocorticoid state caused a significant increase in incisal eruption rate, together with a thinning of the dentine walls and an increase in size of the pulp chambers of these teeth.

Adrenal Glands↗
PubMed1996

Clinical and histological observations of continuously formed bilateral mandibular deciduous central incisor roots after traumatic coronal destruction.

We encountered a rare case of continuously forming bilateral mandibular deciduous central incisor roots after traumatic coronal destruction. The patient, 3 year 3 month old boy, had sustained an external injury which had caused the crowns of hist bilateral deciduous central incisors to fall out when he was 12 months old. Oral examination revealed two small, hard tissues resembling tooth structures on the alveolar ridge in the area of the lost mandibular central incisors. X-ray examination revealed them to be deciduous central incisor roots. We observed the extracted roots histologically and found that the lesion of the crown fracture and pulpal exposure on these roots was completely covered by newly formed irregular dentin and cementoid tissues. In this case, the causes of the continuous root formation were considered to be that the pulp tissues of traumatized teeth were able survive, so the injured lesion was quickly healed and covered with gingival tissue. The vital pulp produced the reparative hard tissue, sealing the site of pulpal exposure and continuing root formation after the trauma.

Child, Preschool↗
PubMed1997

A pilot study to determine the order of emergence of permanent central incisors and permanent first molars of children in the Colchester area of the U.K.

Over the period of one calendar month, 505 children in the Colchester area of England were examined. The data gathered from the 12 non-Caucasian children was not used, leaving 493 Caucasian children (255 boys and 238 girls). Their mean age was 6.48 years, with a range of 4.49-8.75 years and 164 children had no permanent teeth leaving 329 children with a total of 2,238 permanent teeth present. A permanent tooth was deemed to have emerged if any part of it was visible in the mouth. The forensic significance of the results of this pilot study are that there is no significant difference (t = 0.407, P > 0.2) between the age of emergence of permanent central incisors (mean age 6.23 years) and permanent first molars (mean age 6.29 years). On the whole, lower central incisors emerge before upper central incisors and there is no significant difference (t = 0.899, P > 0.2) in the emergence times of upper and lower permanent first molars. There is no significant difference (t = 0.876, P > 0.2) in the emergence times of permanent central incisors for boys and girls (6.22 and 6.25 years respectively) and there is no significant difference (t = 1.413, P > 0.1) in the emergence times of permanent first molars for boys and girls (6.40 and 6.08 years respectively).

Age Factors↗
PubMed2006

Effect of Er,Cr:YSGG laser irradiation on eruption of rat mandibular incisor after disturbance of the enamel organ in the pulp.

To investigate the efficacy of Er,Cr:YSGG (erbium,chromium:yttrium scandium gallium garnet) laser irradiation in root canal preparation and to evaluate its effect on eruption of rat incisors after disturbance of the enamel organ in the pulp, 20 canals of lower left incisor teeth were prepared by K-files followed by Er,Cr:YSGG laser irradiation, and 20 canals of right incisors were subjected to K-files only (control). At 1 week after irradiation, both sides of incisors erupted at the same level from the gingival margin. Histological findings showed that laser irradiation produced a slightly larger damage in the pulp than that of control. Scanning electron microscope observation revealed that laser-treated surface revealed a rough, irregular, and very clean surface; there was almost no evidence of debris or smear layer, and dentinal tubules were opened. Adequate power of Er,Cr:YSGG laser irradiation is effective in root canal preparation without disturbance of the eruption.

Animals↗
PubMed1989

Effects of a low calcium maternal and weaning diet on the thickness and microhardness of rat incisor enamel and dentine.

The deposition and mineralization of incisor hard tissues have been studied in rat pups nursed by mothers on a low calcium diet or weaned with the maternal diet. Animals were killed at 30 days (control and low calcium diets; maternally fed) or 60 days (after 30 days weaning on maternal diet). The degree of mineralization of enamel and dentine was evaluated by a microhardness method on thick transverse sections. The enamel and dentine thickness, and the diameters of the incisor sections and pulp cavity were measured on microradiographs from the sections. Microhardness values of enamel were similar in groups killed after 30 days maternal feeding, but the microhardness of root enamel was 73-74% less in the low calcium-diet weaned group. Peripulpar dentine had mean microhardness values lower than controls in the group fed maternally for 30 days, whereas the whole root dentine appeared significantly less hard in the low calcium-diet weaned group than in the controls. A significant reduction of the incisor bucco-lingual diameter was observed only in this last experimental group. Enamel thickness was significantly lower in the roots of both experimental groups and in the necks of the low calcium weaned group. The reduction in dentine thickness was greater (from -30 to -56%); in the root it was more evident on the lingual aspect. Thus calcium deficiency in the mother's diet did not influence either the deposition or the mineralization of the pup's incisor enamel and dentine. However, when the offspring were weaned with the maternal calcium-deficient diet, mineralization of the tooth hard tissue was retarded.

Animals↗
PubMed1992

Quantitative electron-microscopic analyses of pulpal nerve fibres in the mouse lower incisor after neonatal capsaicin treatment.

A single dose of capsaicin (50 mg/kg) was injected subcutaneously into four mice on day 2 of life; four untreated mice were used as controls. Six months later, a drop of 30 microM capsaicin was instilled on to the cornea of all the mice and the number of times the eyes were wiped was counted to assess the effect of capsaicin on trigeminal sensory neurones. Ultrathin cross-sections were made of the apical pulp of the incisors on both sides of control (n = 8) and capsaicin-treated animals (n = 8). Electron micrographs of pulp nerves were taken and enlarged to a final magnification of x34,000. The numbers of unmyelinated axons in the pulps of all 16 incisors and of unmyelinated axons per Schwann cell in the pulps of four incisors each from the control and capsaicin-treated groups were counted. The short diameters of unmyelinated axons were measured with a computer-operated image analyser. The number of eye wipings was eight-fold less in the capsaicin-treated than in the normal group. This finding clearly indicated that capsaicin irreversibly affected the chemogenic nociceptive trigeminal neurones. The mean number of unmyelinated axons was 345 in controls and 217 (37.1% reduction) in capsaicin-treated animals. The number of unmyelinated axons of less than 0.6 microns dia was 41.5% less in capsaicin-treated mice than in controls. Thus, fine unmyelinated axons in the mouse incisor pulp are capsaicin sensitive, and they are assumed to be nociceptive fibres conveying pain stimuli from the tooth. Capsaicin affected Schwann cells, even those with few unmyelinated axons.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

Inhibitory influence of sympathetic nerves on afferent nerve-induced extravasation in the rat incisor pulp upon direct electrical stimulation of the tooth.

Previous studies have shown that sympathetic nerve stimulation reduces afferent nerve-induced vasodilation by mechanisms unrelated to vasoconstriction in the rat incisor pulp. The present investigation concerned whether similar modulatory mechanisms might also influence neurogenic plasma extravasation in dental pulp. Rat mandibular incisors were electrically stimulated and blood flow reactions in the pulp were recorded by laser Doppler flowmetry. Plasma extravasation in the incisor pulp, gingiva and lip were indirectly assessed by the Evans-blue method. Stimulation of teeth with 50 microA (5 min) did not cause increased dye accumulation in the stimulated pulps whereas stimulation with 100 microA significantly increased the dye content in ipsilateral pulps by 32% as compared to controls; 100 microA stimulation was without effect in unilaterally denervated animals. Tooth stimulation with 50 microA (5 min), in the presence of either the alpha-adrenergic blocker phenoxybenzamine (3 mg/kg), or the alpha 1-adrenergic antagonist prazosin (50 micrograms/kg), as well as in chronically sympathectomized animals, significantly increased the Evans-blue content in the stimulated pulps by 47, 83 and 86%, respectively. Application of short trains of impulses (same number of impulses as for the continuous stimulation but producing minimal vasoconstriction) resulted in some dye accumulation, which was enhanced in the ipsilateral pulps in the presence of prazosin (100 micrograms/kg) or after acute resection of the superior cervical sympathetic ganglion by 70 and 64%, respectively. The Evans-blue content in the lip and gingiva was uninfluenced by the tooth stimulation. The results indicate that activation of sympathetic nerves inhibits the afferent nerve-induced plasma extravasation in rat incisor pulp and this effect is mediated by alpha-adrenoceptors not associated with vasoconstriction.

Animals↗
PubMed1999

Ameloblast apoptosis and IGF-1 receptor expression in the continuously erupting rat incisor model.

Enamel-producing cells (ameloblasts) pass through several phenotypic and functional stages during enamel formation. In the transition between secretory and maturation stages, about one quarter of the ameloblasts suddenly undergo apoptosis. We have studied this phenomenon using the continuously erupting rat incisor model. A special feature of this model is that all stages of ameloblast differentiation are presented within a single longitudinal section of the developing tooth. This permits investigation of the temporal sequence of gene and growth factor receptor expression during ameloblast differentiation and apoptosis. We describe the light and electron microscopic morphology of ameloblast apoptosis and the pattern of insulin-like growth factor-1 receptor expression by ameloblasts in the continuously erupting rat incisor model. In the developing rat incisor, ameloblast apoptosis is associated with downregulated expression of the insulin-like growth factor-1 receptor. These data are consistent with the hypothesis that ameloblasts are "hard wired" for apoptosis and that insulin-like growth factor-1 receptor expression is required to block the default apoptotic pathway. Possible mechanisms of insulin-like growth factor-1 inhibition of ameloblast apoptosis are presented. The rat incisor model may be useful in studies of physiological apoptosis as it presents apoptosis in a predictable pattern in adult tissues.

Journal Article↗
PubMed1991

Reinforcement of bonded crown fractured incisors with porcelain veneers.

A method is described by which porcelain laminate veneers are used to reinforce crown-fractured incisors which have been restored by reattachment of enamel-dentin fragments using enamel etching and a dentin bonding system. In an experimental model using sheep incisors, it was found that fracture strength equal to that of intact incisors could be achieved by employing this method. This is in contrast to fracture strengths of reattached enamel-dentin tooth fragments without porcelain laminates which were only 50% of intact incisors. It is suggested that porcelain laminate veneers may be used to supplement fragment bonding, thereby enhancing dental esthetics and function.

Acid Etching, Dental↗
PubMed1991

Healing of the guinea pig incisor after partial pulp removal.

A histological and microradiographical study was made of eruption and healing of guinea pig lower incisors after endodontic treatment compared with intact incisors. One incisor per animal was cut perpendicularly at the level of the gingiva. One third to one half of the pulp was removed. The experimental periods were 0, 4, 7, 16, 21 and 31 days. Unstained, HE or Brown and Brenn stained sections were studied microscopically. Some teeth were sectioned undecalcified and studied microradiographically. In intact incisors the dentin matrix deposition and mineralization were described. In the dentin, incisally to the pulp proper, narrow canals, the walls of which consisted of mineralized material, were found opening at the occlusal surface. Histologically, in the treated teeth, a beginning of cell-rich fibrodentin deposition at the original dentin wall was present at Day 4. At Day 7 a substantial walling off of the pulp is achieved by cell-rich and some cell-poor fibrodentin. After 16 days large amounts of cell-rich and cell-poor fibrodentin were covered by some tertiary dentin. After 31 days the amount of tertiary dentin was larger than after 16 days and also covered the incisal part of the root canal walls. Further apically some vasodentin and newly formed normal, secondary, dentin was observed. In course of this period the cells of the cell-rich and cell-poor fibrodentin first showed necrobiosis, then necrosis. The cell-rich fibrodentin appeared not fully mineralized. All blood vessels and strands of soft tissue which ran in canals in the fibrodentin came to a dead end. Thus the root canal was fully obturated. Although the teeth appeared (nearly) normal to the naked eye and radiographically after 31 days, histologically they were not. Large amounts of fibrodentin still being present incisally which showed the dual potential of the guinea pig pulp to produce both fibrodentin and orthodentin.

Animals↗
PubMed1996

Occurrence of amorphous and crystalline mineral deposits at the epithelial-mesenchymal interface of incisors in the calcium-loaded rat: implication of novel calcium binding domains.

BACKGROUND: In order to clarify the regulatory factors that promote precipitation of enamel crystals in mammalian tooth germs, possible calcium binding domains were visualized at the epithelial-mesenchymal interface of rat incisor teeth by means of electron microscopy and X-ray microanalysis. METHODS: Adult rats were loaded with calcium (30 mM Ca) by vascular perfusion and further loaded through fixation and dehydration in the presence of high doses of calcium. RESULTS: Electron microscopy of anhydrously prepared Epon sections of the calcium-loaded rat incisors revealed numerous electron-dense granular deposits, enriched with calcium and phosphorus, scattering in the fibrous mantle dentin matrix and the intercellular spaces of the inner enamel epithelium, but not in the pulp tissues including the odontoblastic cells layer. The electron-dense deposits were specific for the enamel-related portion and were never shown to occur in the cementum-related portion. Proceeding incisally, dense deposits in both the mantle dentin matrix and presecretory ameloblast layer gradually converted to fine needlelike figures resembling the early enamel crystallites. Mineral deposits in experimental rats disappeared concomitant with the onset of normal mineralization of mantle dentin. There was no spatial correlation between the dense deposits and either stippled material or matrix vesicles. CONCLUSIONS: These results indicate the presence of novel calcium-binding domains in the enamel-related portion of the epithelial-mesenchymal interface of rat incisors that form enamellike crystallites under calcium-loaded conditions. A contribution of these putative calcium-binding domains in the induction and spontaneous formation of enamel crystals is suggested.

Animals↗
PubMed1998

Comparison of biomechanical properties of the incisor periodontal ligament among different species.

BACKGROUND: The aim of this study was to obtain a more precise understanding of the mechanical properties of the periodontal ligament in continuously erupting incisors by comparing the shear stress-strain relations among teeth from four closely related species. METHODS: Four species of experimental animals (mice, hamsters, rats, and rabbits) were used. Transverse sections of the left mandibular incisors were cut from the incisal, middle, and basal regions of each incisor. The tooth was pushed out of the alveolar bone in an extrusive direction at 5 mm/min using a materials testing machine. The maximum shear stress, maximum shear strain, tangent modulus, and failure strain energy density were estimated from the resulting stress-strain curve. Polarized light microscopic observations of collagen fibers were also made. RESULTS: All the biomechanical measures tended to decrease from the incisal toward the basal regions in all species. There were large species differences, especially in the incisal region, with the greatest maximum shear stress and failure strain energy density in hamsters. The greatest tangent modulus and the smallest maximum shear strain were observed in mice. The birefringent fiber architectures of the periodontal ligaments in the four species appeared to be similarly organized; the incisal periodontal ligament appeared to have more organized and thicker collagen fibres than did the middle and basal ligaments in the four species. CONCLUSIONS: These results suggest that the regional differences in the biomechanical properties of the periodontal ligament depend upon the developmental stages of the periodontal collagen fibers that may be related to the general arrangement, diameters, and densities of the collagen fiber bundles and the fiber insertions into the alveolar bone and cementum. The species differences in the biomechanical properties may be due to differences in the width of the periodontal ligament and the waviness as well as the strength and stiffness of the periodontal collagen fibers.

Animals↗
PubMed1998

Cytochrome oxidase activity in the enamel organ during amelogenesis in rat incisors.

Cytochrome oxidase (CO), one of the membrane-bound mitochondrial enzymes involved in oxidative phosphorylation, reflects the functional activity of mitochondria. Mitochondria in the enamel organ show drastic changes in localization during amelogenesis (Smith. INSERM, 1984;125:273-282). In understanding the functional aspects of the enamel organ, it is essential that one knows the exact CO activity in the respective mitochondria. The present study examines the CO activity of mitochondria in the enamel organ of rat incisors throughout the various stages of amelogenesis using light and transmission electron microscopy. CO activity was examined histochemically according to Seligman et al. (J. Cell. Biol., 1968;38:1-14) in decalcified sections of the upper and lower incisors of the rat. In the secretory stage, half of the mitochondria in the ameloblasts accumulated in the infranuclear region were reactive for CO. Both the population and CO activity of the infranuclear mitochondria of ameloblasts decreased significantly in the later stage where the enamel matrix secretion was almost complete. The CO-reactive mitochondria in the cells of the stratum intermedium (SI) gradually increased in number throughout the secretory stage. In the maturation stage, the ameloblasts contained intensively CO-reactive giant mitochondria in the proximal region and regular sized ones in the distal cytoplasm that were mostly devoid of detectable CO reactivity. The proportion of CO-reactive mitochondria in the supranuclear region and the population of mitochondria in the infranuclear regions of the smooth-ended ameloblasts were significantly higher as compared with the respective values in the ruffle-ended ameloblasts. In the late stages of enamel maturation, ameloblasts containing a large number of ferritin-filled pigment vesicles possessed numerous CO-reactive mitochondria between those vesicles in the supranuclear region, implicating an active role of the ameloblasts in iron transfer into the maturing enamel. The papillary layer cells possessed numerous intensively CO-reactive mitochondria throughout the maturation stage. A stage-related variation in the localization of CO-reactive mitochondria in the enamel organ of rat incisors was quantitatively demonstrated. It is conceivable that maturation stage ameloblasts form a functional unit with the papillary layer cells, and operate in energy-requiring events such as active ion transport to, and water and matrix protein removal from the maturating enamel. A sign of such functional integrity among the types of the enamel organ cells (ameloblasts, cells of SI, cells of stellate reticulum, and outer enamel epithelial cells) cannot be seen in the secretory stage. The secretory ameloblasts may function in matrix formation and calcium regulation in a less cooperative manner with the other cells of the enamel organ as compared to the maturation stage ameloblasts.

Amelogenesis↗
PubMed1975

Histological and three dimensional organization of the odontogenic organ in the lower incisor of 100 gram rats.

A three dimensional reconstruction of the epithelial tissue at the apical end of the lower rat incisor was made from serial 1 mum thick cross sections. This tissue formed an elongated structure, called the odontogenic organ, which was composed of a bulbous and a "U"-shaped part. Both parts were joined to one another at the posterior aspect of the apical foramen. The bulbous part of the odontogenic organ was situated at the lingual side of the "U"-shaped part and protruded anteriorly over the pulp. It was formed by cells of the outer dental epithelium and stellate reticulum whose organization suggested that the bulbous part was important in the production of cells for renewal of all the epithelia of the incisor. The "U"-shaped part of the odontogenic organ was apparently derived from the bulbous part and delineated the pulp by forming the lateral, mesial and labial sidewalls around the apical foramen. It was composed of all the epithelial cell types recognizable as precursors to (a) cells of the enamel organ which form the enamel, and (b) Hertwig's epithelial root sheath, a part of the odontogenic organ which induces the formation of dentin on the lingual aspect of the incisor.

Ameloblasts↗