Role of attrition and occlusal contact in the physiology of rat incisors. V. Life cycle of inner enamel epithelial cells at various rates of eruption

Dental disease in chinchillas in the UK

Dental abnormalities are common in chinchillas, although knowledge of the lesions responsible for the clinical signs is incomplete. Animals bred in the UK were examined to gain further knowledge of dental disease in this chinchilla population. Dental abnormalities, particularly those related to tooth elongation, were detected on careful external examination of 35 per cent of apparently healthy chinchillas. Incisor abnormalities were seen on clinical examination in 55 per cent of chinchillas presented because of clinical illness. In all but one case, this occurred secondarily to crown elongation of the cheek teeth or to the absence of opposing teeth, rather than being a primary skeletal problem. Clinical signs commonly attributed to malocclusion, such as ventral mandibular swelling, weight loss, dysphagia, altered chewing pattern and changed food preferences, were not specific to malocclusion, being seen associated with coronal spike formation, root elongation and advanced periodontal lesions. Caries and resorptive lesions rarely caused clinical signs in this population, but were identified during 37 per cent of postmortem examinations. Congenital absence of teeth, skeletal malocclusion and pathological loss of teeth all resulted in significant clinical signs, but were rare. It is concluded that provision of a diet with physical properties more closely matching that of wild chinchillas should improve the dental health of captive animals.

Accelerated eruption of rat lower incisor. Relationship between impeded and unimpeded eruption rates, rate of attrition, tooth length, and production of dentin and enamel

The present investigation studies the effects of persistently cutting one lower rat incisor out of occlusion. Within four days, the rate of eruption of the cut (unimpeded) incisor increased to 216% and that of the uncut (impeded) contralateral to 136% of the baseline rate. While the former remained high, the latter decreased gradually to about 90% within three weeks. The rate of attrition of the impeded incisor increased to 233% of the baseline rate within two days, then fell abruptly, and remained at a slightly lower level than the rate of eruption. Accordingly, the length of the erupted part of the impeded incisor decreased initially, but increased gradually after about four days. Measurements made on SEM micrographs of the series of transverse tooth segments obtained when cutting the incisor out of occlusion, showed that growth-related increase in mesiodistal tooth width was arrested from the 10th segment, dentin thickness decreased gradually to about 50% in the 12th segment, and enamel thickness, after an initial increase, decreased to about 80% in the 11th segment. The present study provides experimental evidence that accelerated eruption affects morphogenesis and histogenesis of the rat lower incisor. An impeded incisor, especially the contralateral, may not serve as an ideal control.

The effect of epidermal growth factor on neonatal incisor differentiation in the mouse

The effect of epidermal growth factor (EGF) on cellular differentiation of the neonatal mouse mandibular incisor was examined autoradiographically using tritiated thymidine ([3H]TDR) and tritiated proline ([3H]PRO). On days 0 (day of birth), 1, and 2, EGF was administered (3 micrograms/g body wt) sc to neonates. Mice were killed on Days 1, 4, 7, 10, and 13 after birth and were injected with either [3H]TDR or [3H]PRO 1 hr before death. [3H]TDR was used to analyze cell proliferation in eight cell types in the developing mouse incisor including upper (lingual) and lower (buccal) pulpal fibroblasts, preodontoblasts, inner and outer enamel epithelial cells (IEE and OEE), stratum intermedium (SI), stellate reticulum (SR), and periodontal ligament (PDL) fibroblasts. [3H]PRO was used to analyze protein synthesis in ameloblasts, and their secretion products (enamel and dentin), as well as PDL fibroblasts. The selected EGF injection scheme elicited acceleration of incisor eruption with minimal growth retardation. At Day 1, the upper and lower pulp, preodontoblasts, SI, and SR showed a significant decrease in labeling index (LI) 24 hr after a single EGF injection. After multiple injections (Days 0, 1, 2), two LI patterns were observed. In lower pulp, preodontoblasts, IEE, SI, SR, and OEE, a posteruptive change in LI was observed. In contrast, the upper pulp and PDL regions demonstrated a direct temporal relationship with eruption. Autoradiographic analysis with [3H]PRO indicated that EGF treatment caused significant increases in grain counts per unit area in ameloblast, odontoblast, and PDL regions studied. Significant differences were found in all four regions studied (ameloblasts, enamel, odontoblasts, dentin) at the 45-microns-tall ameloblast level as well as ameloblasts and odontoblasts at the 30-microns level at 13 days of age. The PDL demonstrated significant differences at all locations studied (base, 30 microns, 45 microns,) in 4-, 7-, and 13-day-old mice. Morphologically, EGF-treated groups demonstrated premature differentiation of ameloblasts and odontoblasts at the light microscopic level. The data indicate that EGF alters DNA and protein synthesis as well as differentiation patterns during the eruption process. While EGF affects both DNA and protein synthesis, the alteration of differentiation may be secondary to mitogenic effects on proliferative compartments. In order to determine the cellular target for EGF within the newborn mouse incisor, in vivo 125I-EGF binding was analyzed autoradiographically.(ABSTRACT TRUNCATED AT 400 WORDS)

Cell turnover in the odontogenic organ of the rat incisor as visualized by graphic reconstructions following a single injection of 3H-thymidine

Turnover of cells within the odontogenic organ was studied in three dimensions by preparing serial sections of incisors from young male rats killed at various times following a single intraperitoneal injection of 1 muCi/g body weight of 3H-thymidine. Radioautographs showed that at 1 hour after injection labeled cells were present in all cell layers throughout the entire depth of the odontogenic organ. They were encountered frequently within the inner dental epithelium and stratum intermedium but appeared less abundant within the stellate reticulum and outer dental epithelium. With time, the frequency of labeled cells in each layer declined progressively, and more rapidly at the anterior and labial side of the odontogenic organ than toward its posterior and lingual side. Hence labeled cells were observed over the longest time interval in regions where cell layers were in closest proximity to the opening of the apical foramen, that is, near the apical and cervical loops. By 32 days after injection, numerous labeled cells could still be identified within the outer dental epithelium and stellate reticulum near the apical loop (bulbous part of the odontogenic organ) and the outer dental epithelium near the cervical loops ("U"-shaped part of the odontogenic organ). These findings support the hypothesis that cells originate within the bulbous part of the odontogenic organ and migrate anteriorly through the "U"-shaped and root sheath parts of the odontogenic organ during renewal of the incisor. It appears that individual stem cell compartments may be maintained for surface (outer/inner dental epithelium) and intermediate layers (stellate reticulum/stratum intermedium) in the odontogenic epithelium.

The effect of a magnesium deficient diet and cortisone on the growth of the rat incisor

Incisors of Mg-deficient rats showed a marked inhibition of eruption and a decrease in mitoses of apical tissues. Cortisone administration resulted in some increase in eruption rate and cell division though these remained below the values of Mg-supplemented controls. An improvement in gross condition and a less severe histopathology of incisor tissues were also observed in cortisone-treated rats.

Quantitative analysis of cell turnover in the enamel organ of the rat incisor. Evidence for ameloblast death immediately after enamel matrix secretion

During renewal of the enamel organ in the rat incisor cohorts of epithelial cells are transported sequentially through presecretory, secretory and maturation zones to the gingival margin where the life cycles of these cells terminate. This process was examined kinetically by determining the absolute flux of cells within each of these zones of amelogenesis. It was found that the efflux of ameloblasts, stratum intermedium and papillary layer cells from the presecretory zone was about equal to the efflux plus expected growth within the secretory zone. However, between the secretory and maturation zones about 50% more ameloblasts entered the maturation zone than were required to account for the egress at the gingival margin and the expected growth. Since there was no similar imbalance between these zones for papillary layer cells, it was concluded that this discrepancy must represent a 50% reduction in the size of the ameloblast population during the maturation stage of amelogenesis. It was calculated that a little over 25% of the loss occurred immediately at the start of maturation within the region of postsecretory transition and the remaining 25% of the loss occurred throughout the subsequent regions of the maturation zone. In addition to the kinetic analysis graphic reconstructions, or surface maps, of ameloblast nuclei were prepared. These maps illustrated the characteristics of ameloblast nuclear packing within the three zones of amelogenesis and they provided quantitative confirmation that as ameloblasts progress through the maturation zone, there is a loss of cells in an amount predicted by the kinetic analysis.

Role of attrition and occlusal contact in the physiology of the rat incisor: X. The part played by the periodontal ligament in the eruptive process

Removal of the proliferating base of the rat incisor did not influence the rate of eruption which responded to changes in impediment to eruption in a fashion similar to that for control teeth. It is the peridontal ligament rather than the proliferating cells that is responsible for tooth eruption. The elements of the peridontal ligament apparently responsible for tooth movement are the mature fibroblasts.

Cellular renewal in the enamel organ and the odontoblast layer of the rat incisor as followed by radioautography using 3H-thymidine

Renewal of the cell populations of the incisor was studied in 100 gm male rats injected with a single dose of 3H-thymidine and sacrificed at various times from one hour to 32 days after injection. Radioautographs showed that a cohort of labeled cells within the enamel organ, odontoblast layer, and pulp was carried passively with the erupting incisor from the apical end towards the gingival margin where the life cycle of these cells was terminated. Labeled cells in the upper and lower incisor, although traversing different absolute lengths, were found in approximately the same functional stage of their life cycle at similar times after the injection. Thus, by one and on-half days labeled ameloblasts began inner enamel secretion and, by eight days (upper) or nine days (lower), complement outer enamel secretion. By 32 days labeled ameloblasts had traversed the entire enamel maturation zone and were located at the gingival margin. Labeled odontoblasts followed closely the movement of labeled ameloblasts. The mean rate of ameloblast migration was 567 mum/day on the upper incisor and 651 mim/day on the lower. For the odontoblasts this rate was 55 mum/day (upper) and 631 mum/day (lower). Finally, it was found that as the rat age, the duration of the life cycle for epithelial and pulp cell populations of the incisor increased because of growth within the lonitudinal axis of the tooth. It was concluded that the apical end of the incisor literally "grows backward" in the bony socket, and hence, the duration of the life cycle becomes greater simply because it takes cells longer to physically reach the gingival margin.

Circadian mitotic rhythms in the cervical tissues of the rat maxillary incisor

A pronounced 24-hour fluctuation occurs in the mitotically active tissues of the rat maxillary incisor. High and low periods of activity were seen during the daytime as well as at night. A consistent increase in the number of mitotic figures occurred approximately four hours subsequent to a change in the light-dark regimen.