Mineralization of the enamel of ovine permanent central incisor teeth using microhardness and histological techniques

Comparison of Different Decalcification Methods Using Rat Mandibles as a Model

Selection of decalcification agents is an essential consideration when processing mineralized tissues because the integrity and immunohistochemical characteristics of the tissues may be affected. Here, we report results obtained from the decalcification of rat mandibles using 10% ethylenediaminetetraacetic acid (EDTA) at room temperature (RT), 10% EDTA at 37C, 5% nitric acid, and 10% formic acid at RT. Decalcification endpoints were determined by microcomputed tomography. Morphological preservation and antigenicity were evaluated by hematoxylin and eosin staining and immunohistochemistry. Decalcification of the anterior and posterior portions of the mandible took 220 and 191 hr in 10% EDTA RT, 102 and 73 hr in 10% EDTA 37C, 13.5 and 4.3 hr in 5% nitric acid, and 140 and 36 hr in 10% formic acid, respectively. Decalcification in 10% EDTA at 37C was accelerated, but 10% EDTA at RT provided optimal results for immunohistochemistry and cellular and structural details. Decalcification using 5% nitric acid was accomplished in the shortest time and exhibited good cellular and architectural morphology, whereas 10% formic acid was suboptimal with respect to tissue and cellular morphology. Despite being the slowest method, EDTA at RT is still the recommended method for decalcifying mineralized tissues; however, if rapid decalcification is needed, 5% nitric acid is the best option, yielding acceptable tissue integrity and speed.

Comparison of the microhardness of enamel, primary and regular secondary dentine of the incisors of donkeys and horses

The microhardness of the enamel, primary dentine and regular secondary dentine of seven donkey and six horse incisors was determined with a Knoop indenter at the subocclusal and mid-tooth level. The mean microhardnesses of the donkey incisor enamel, primary dentine and secondary dentine were 264.6 63.00 and 53.6 Knoop Hardness Number, respectively. There was no significant difference between the microhardness of the enamel and primary dentine on the incisors of the donkeys and horses, but the microhardness of the regular secondary dentine of the donkeys' incisors at the mid-tooth level was slightly but significantly less than that of the horses. There was also a difference in the microhardness of the secondary dentine between the subocclusal and mid-tooth levels in both donkey and horse incisors.

Dental wear in horses in relation to the microhardness of enamel and dentine

The microhardness of enamel, primary dentine and secondary dentine was determined in the incisor teeth of 39 horses of three different breeds, trotter horses, Belgian draft horses and Arab horses. Using a microhardness tester fitted with a Knoop diamond indenter, the overall Knoop Hardness Number was determined for each tissue, and the influence of breed and age on the hardness were evaluated. Enamel and secondary dentine were significantly harder in Arab horses than in trotters and Belgian draft horses, but there were no significant differences between draft horses and trotter horses in the hardness of their enamel and dentine.

Proton microprobe assessment of the distribution of fluoride in the enamel and dentine of developing central incisors of sheep and changes induced by daily fluoride supplements

Ten sheep were given 0.5 mg fluoride (F) and 10 sheep 0.2 mg F/kg body wt orally for periods of 1-6 months while 8 sheep received no additional F. One incisor from each sheep was sectioned longitudinally in the midline and, using the proton microprobe, multiple scans for calcium and F were made across the enamel and dentine. F was determined by proton-induced gamma-ray emission and calcium by X-ray emission. Tooth length and hence the stage of ameloblast activity for each of the 28 teeth at the start of the experiment was determined using a tetracycline marker. In addition, the stage of enamel development of the eight control teeth (no dietary F) at the time of their extraction was assessed from their macroscopic appearance. Continuous changes in F levels occurred in both enamel and dentine throughout tooth development and also in the mature enamel and associated dentine after ameloblast regression. All scans for all stages of tooth development and all F treatments showed a high F concentration at the enamel surface. Early in the secretory phase, a wide-based F peak occupied the entire width of the enamel with a similar F peak in the dentine. In the control teeth, no consistent increase in F concentration occurred at the enamel surface during later development. When F supplements were started early in the maturation phase an increase in F concentration only at the enamel surface was recorded. When F supplements were also given during the secretory phase, higher F concentrations were recorded not only at the enamel surface but also for the inner enamel and dentine plateau. These findings, based on a small number of sheep, indicate that further research is needed to clarify the method and control of F uptake and to determine the changes in these processes during the different stages of tooth development.

Odontogenic cysts of the anterior mandible of sheep

Cysts of this site are quite common in mature ewes in New Zealand and the British Isles, although their true prevalence is not yet known. In most examples, an unerupted incisor is embedded in their wall but some do not contain a tooth. The cysts are lined by parakeratinized stratified squamous epithelium and generally exhibit a flat interface between the epithelium and the connective tissue. They may become secondarily infected, in which case they exhibit chronic inflammation. In some cases, foci of cementum are attached to the epithelial lining. When a tooth is involved, the cysts arise from various locations along its crown, which in sheep is normally covered by a thin layer of cementum. Some of these unerupted teeth exhibit irregular hypercementosis and/or external resorption which is repaired by cementum. In some examples there is a fistula between the cyst and the oral cavity, either through a deep periodontal pocket or from the incisal edge of the unerupted tooth piercing the overlying mucosa.

Nature and mechanisms of dental fluorosis in animals

Recent studies of dental fluorosis in animals have been reviewed. The aim was to describe studies which have provided information which may be relevant to explaining the mechanisms involved in human dental fluorosis. Studies on rats, sheep, and pigs have provided details of dosage regimens which produce lesions which resemble those described in human fluorosis. In the pig and rat, the plasma fluoride concentrations associated with these dental lesions are of the same order of magnitude as those which may occur in man. Three different kinds of studies in different species have shown that fluoride affects processes occurring during enamel maturation. One study on rats has indicated that fluoride may reduce matrix removal during maturation by an effect on enamel proteases. Many studies have demonstrated accumulation of fluoride in secretory enamel and that fluoride concentrations in maturation enamel are lower than in secretory enamel. This phenomenon had previously been explained by the binding of fluoride to enamel proteins, but recent studies indicate that such binding does not occur. The hypothesis that enamel fluorosis might be caused by general effects of fluoride on calcium metabolism has not been supported by more recent studies. It was concluded that, although the mechanisms involved in dental fluorosis remain obscure, recent animal studies do seem to have provided new information which may prove to be important for our understanding of mechanisms whereby fluoride causes dental fluorosis in man.

Developmental defects of enamel--historical and present-day perspectives of their pathogenesis

Developmental defects of enamel are visible deviations from the normal translucent appearance of tooth enamel resulting from enamel organ dysfunction. In the past, information about the activities of the ameloblasts has determined the terminology used to describe the lesions. Advances in our understanding of the complicated secretory and maturation phases of amelogenesis have required a re-appraisal of the concepts of defect formation. The phase of ameloblast activity, the duration of the disturbance, and its severity leading to temporary or permanent inactivity of the cells determine the appearance of the three common types of lesions--hypoplasia, and diffuse and demarcated opacities.

Proton microprobe determination of fluorine profiles in the enamel and dentine of erupting incisors from sheep given low and high daily doses of fluoride

Developmental defects in incisors were induced by daily oral ingestion of sodium fluoride solutions. Teeth extracted at eruption from sheep that had been subjected to four different fluoride regimens--0.2 or 0.5 mg F/kg body weight daily for 6 months, 2 or 6 mg F/kg body weight daily for 21 days--were analysed for fluorine by gamma emission using a proton microprobe. Calcium and zinc profiles were also measured using proton-induced X-ray emission. Diffuse opacities, similar in appearance to mild human fluorosis, were produced by the first two regimens, whereas the last two produced hypoplastic lesions. Different distributions of fluoride were found in the unerupted enamel and dentine, and these patterns reflected variations in both the duration and concentration of the fluoride dose used to induce the fluorotic lesions.

The macroscopic and scanning electron-microscopic appearance and microhardness of the enamel, and the related histological changes in the enamel organ of erupting sheep incisors resulting from a prolonged low daily dose of fluoride

The hypothesis that diffuse opacities in enamel result from a chronic, mild disturbance to ameloblast activities was tested using fluoride. Three sheep (HF) were dosed orally with 0.5, and 3 (LF) with 0.2 mg fluoride/kg body weight daily for 6 months. A control sheep (C) received no additional fluoride. The 7 sheep were killed at or close to the time of emergence of their permanent central incisors. One tooth from each sheep was sectioned longitudinally. The enamel related to the secretory (S) and maturation (M) phases of ameloblast activity at the start of fluoride dosing was determined from a tetracycline marker. The pattern of mineralization of the outer 150 micron of the cut labial enamel was assessed using microhardness testing. The SEM appearance of the acid-etched outer enamel was compared in S and M zones in 5 teeth. The enamel of the C tooth was translucent. Diffuse opacities, similar in appearance to human fluorosis, were present in all fluoride-treated teeth. Hardness values in the outer 70 micron of the enamel decreased as the fluoride dose increased and, in the HF teeth, were lower in the S zone than in the M zone. Fluoride given during the M phase induced a surface hypomineralization which increased in degree and depth when fluoride was also given during the S phase. The SEM appearance of M and S enamel was similar in 2 LF and 1 HF teeth but, in the other HF tooth, S enamel but not M enamel had a disordered prism structure and loosely-packed crystals in an abnormal organic matrix. Histological examination revealed that ameloblasts remained in only 4 of the 7 teeth and that their regression and the formation of the cementum adjacent only to the labial enamel were progressing abnormally.

Cell sizes and apposition of dental hard tissues in rats

The daily apposition of the enamel and dentin and the sizes of ameloblasts and odontoblasts in the first upper molar of 2- to 9-day-old and of 12-day-old rats were examined by light microscopy of freeze-dried sections. The formation of enamel in the first molar started at the age of 2 days and was completed in most of the analyzed areas at the age of 9 days. On each day the enamel on the mesial sides of the cusps was about twice as thick as on the distal sides. The differences in thickness of dentin between the mesial and distal sides of the cusps were not as marked as was seen for the enamel. However, the rapid formation of dentin seemed to continue longer on the mesial sides than on the distal sides. The height of the ameloblasts and odontoblasts showed similar differences between the mesial and distal sides as was seen in the formation of enamel and dentin. In addition, these cells showed a gradual increase and decrease in height during the observation period, with a maximal height in the analyzed area at the age of 4-6 days.

Histological, macroscopic and microhardness observations of fluoride-induced changes in the enamel organ and enamel of sheep incisor teeth

When aged 8.5 months, 10 sheep born in the same week were given 4 mg fluoride (F)/kg body weight orally for 26 days. Three sheep received no F. Sheep were killed at the end of the treatment period and later at selected stages of tooth development. The macroscopic changes in the enamel of one incisor were related to the cellular changes in the enamel organ of the contralateral tooth. A break in enamel continuity, hypoplasia, was seen on the labial enamel of 9 of the 10 F-treated sheep. Pitting of the enamel was associated with shortening of some ameloblasts and aggregations of cysts affecting cells late in their secretory phase in the first-killed sheep. In sheep killed later, these changes were associated with cells which had progressed into their maturation phase. A more extensive absence of enamel with ledge formation cervically, seen in one sheep, was associated with displacement or death of almost all the cells in their secretory phase during F treatment and consequent retention of the organic matrix. The hypoplastic lesions resulted from secretory-cell reaction during the period of F dosing. Diffuse patchy opacities, characterized by an irregular hypomineralized surface zone, were only apparent in the enamel of the later-killed sheep and were associated in one sheep with abnormal ameloblast regression in the contralateral tooth. These defects possibly resulted from the long-continued release of F stored in the bones during the period of F dosing.

The production of developmental defects of enamel in the incisor teeth of penned sheep resulting from induced parasitism

Thirty-four sheep were infected with nematode parasites (Trichostrongylus spp. and Ostertagia circumcincta) during central incisor formation. The tooth enamel was examined for defects of mineralization at eruption and compared with that of 14 control sheep. The type and number of parasites given, the duration of the infection and the age of the sheep were varied. Sheep receiving high doses of parasites e.g. 200,000 T. vitrinus + 20,000 O. circumcincta showed severe systemic effects (debilitation, scouring and weight loss) which were of short duration (7-10 days) as the infections were terminated by anthelmintic treatment. Hypoplastic enamel (a break in continuity) was induced in all 5 sheep infected at 8 1/4-9 months of age and demarcated opacities (an alteration in the translucency) in 3 out of 4 sheep infected at 10 months of age. Sheep given 150,000 or 40,000 T. colubriformis showed less obvious systemic effects although the weight loss continued in some sheep for up to 54 days. Although opacities of the enamel in the form of parallel lines, sometimes with localized increase in width or with a diffuse opacity superimposed, were seen by microscopic examination (X 10) of teeth in this group, many of the teeth from the control sheep showed similar lesions. Only the hypoplastic lesions can be attributed with confidence to the experimental manipulations.

The pattern of mineralization of traumatically-induced developmental defects of sheep enamel assessed by microhardness and microradiography

An earlier study showed that some of the variation in appearance of defects of enamel in sheep incisor teeth induced by trauma during tooth development was related to the phase of activity of the ameloblasts at the time of injury. In the present study, sixteen selected defects were examined by microhardness and microradiography. It was found that in some of the fully formed teeth, the defects had retained the pattern of mineralization present at the time of trauma, i.e., mineralization had ceased. In other teeth, further mineralization of the defects occurred after trauma, resulting in well-mineralized enamel, although sometimes of incomplete thickness. It was concluded that the trauma, depending on its severity, leads to either permanent or temporary dysfunction of the ameloblasts, which also influences the appearance of the resulting lesions.

Defects of enamel in sheep resulting from trauma during tooth development

Defects of enamel were produced by inserting an instrument into the tooth crypt of developing permanent incisor teeth in sheep. The activity of the labial ameloblasts was deduced from tooth length at the time of trauma. Missing enamel (hypoplasia) resulted when secretory ameloblasts were damaged. Opacities (hypocalcification) followed trauma to the cells in their maturation phase.