B.K.B. Berkovitz

Unimpeded eruption in the root-resected lower incisor of the rat with a preliminary note on root transection

Abstract Removal of the proliferative odontogenic base (i.e. root resection) of the lower right incisor was carried out in sixteen out of twenty-nine adult male Wistar rats from five litters of approximately the same age and with an average weight of 325 g. The remaining thirteen rats acted as controls. Except for five specimens where the unimpeded eruption rate (u.e.r.) was severely reduced, the u.e.r. of the remaining eleven root-resected teeth attained control levels 4–7 days after operation, and this level was retained for approximately 12 days when, as the base of the tooth approached the level of the alveolar crest, the u.e.r. decreased. In some cases the root-resected tooth was shed soon after attaining the level of the alveolar crest, whereas in other cases it was retained in this situation without further eruption for periods of up to 6 weeks. Assuming that the mechanism of tooth eruption in the root-resected tooth is similar to that in the control tooth, the following mechanisms do not appear essential to the eruptive process: (1) root elongation, (2) pulp cell proliferation and dentine formation, (3) fundic bone deposition, (4) tissue fluid pressure; and the cushioned hammock ligament is not a necessary structure. It is suggested that the force of eruption resides in the periodontal membrane which retains its characteristic histological structure following root resection. In two cases of root transection carried out on the lower right incisor of adult male Lister hooded rats weighing 400 g each, with plaster of Paris gauze placed between the two segments to prevent ankylosis, an u.e.r. similar to control levels was attained.

The effect of the lathyritic agent aminoacetonitrile on the unimpeded eruption rate in normal and root-resected rat lower incisors

Abstract A study was carried out in three different groups of male Wistar rats of average weight 90, 180 and 300 g respectively, concerning the effects of the lathyritic agent aminoacetonitrile (AAN), administered in the drinking water as a 0.1 per cent solution, on the eruption rate of the continuously growing lower incisor. Apart from three readings in the youngest group, unimpeded eruption rate in the lathyrogen-treated animals was not significantly lower than the respective control values. Gross and radiological findings following AAN administration were in keeping with previous studies, and the disruption of the collagen of the periodontal ligament was evidenced by the ability to extract the incisor teeth following drug administration, such a procedure not being possible for control animals. These findings do not support the view that traction within maturing collagen fibres of the periodontium is the prime mover in tooth eruption. In order to assess the possible contribution of the proliferative basal tissues to the eruptive process in lathyrogen-treated animals, such tissues were surgically removed by the procedure of root resection 10 days after the commencement of AAN administration in male Wistar rats of average weight 195 g; the unimpeded eruption rate of such root-resected teeth did not differ significantly from control root-resected values, and in the absence of any apparent contribution from the collagen of the periodontal ligament or from the proliferative basal tissues, the eruptive mechanism in this situation remains obscure.

THE EFFECTS OF ROOT TRANSECTION ON THE UNIMPEDED ERUPTION RATE OF THE RABBIT MANDIBULAR INCISOR

Abstract A study of the effects of root transection on the unimpeded eruption rate of the right mandibular incisor of male Californian rabbits of average weight 3 kg was undertaken. Following a period of one week during which control unimpeded eruption rates were determined, the root was divided transversely into proximal and distal segments in 18 rabbits with the proximal segment pinned in five animals. Unimpeded eruption rates for the operated tooth were then determined thrice weekly for 5 weeks. The distal segment in six rabbits erupted at a mean daily rate of approximately 230 μm, the proximal segment showing little or no movement. In other rabbits, however, though similar rates were observed for the distal segment, the proximal segment also erupted forwards and may have contributed to this movement. Four rabbits were maintained for 6 months following transection and the consequent effects of preventing eruption on the proliferative basal tissues were similar to those previously reported for the rat. These results would indicate no significant differences in the eruptive mechanisms of the rabbit and rat mandibular incisor.

A longitudinal study of replacement patterns of teeth on the lower jaw and tongue in the rainbow trout Salmo gairdneri.

Abstract A longitudinal study of the tooth replacement patterns was undertaken to compare the situation in fish with that established for reptiles. The dentitions of two size ranges of fish, 12–15 cm and 20–23 cm length, was recorded twice weekly over periods of up to 9 months by anaesthetizing the animals in ethyl m-aminobenzoate (MS-222, Sandoz, Germany) solution and obtaining wax impressions of teeth present on the left dentary and tongue. These data were analysed to give the number of tooth positions per tooth-bearing bone, the functional life span of each tooth and the period when the position was unoccupied. Charts were constructed and a number of distinct wave patterns formed. Two types of wave were constructed linking eruption times of successively younger adjacent teeth, one sloping anteriorly the other posteriorly. In all jaws the posterior waves were shallower. Replacement waves, formed by linking alternate teeth, sloped steeply in a cephalad direction and did not appreciably change form during the experimental period. Generation time, defined as the period between the appearance of a tooth and the appearance of its successor, averaged 13 weeks for the dentary of the 20–23 cm group and, for the 12–15 cm group, 8 weeks. The teeth on the tongue lasted 16 and 12 weeks, respectively. Further analysis of generation time into the period when a tooth was recorded as present and the interval before the eruption of its successor (functional and non-functional periods) showed a good correlation between the ratio of these periods and the mean number of teeth to be expected on any impression. This percentage varied, between animals, from 55 to 75 per cent. Differences between replacement patterns in the trout and in reptiles are discussed.

A quantitative comparison of the ultrastructure of the periodontal ligaments of impeded and unimpeded rat incisors

The periodontal ligaments of impeded and unimpeded rat mandibular incisors were examined to find structural correlates for the known functional differences between the tissues. The structures quantified were fibroblasts (area and membrane length, endoplasmic reticulum, mitochondria, microtubules, lysosomes, intracellular collagen profiles, intercellular contacts), oxytalan fibres, collagen fibrils and ground substance. The only changes seen on rendering a tooth unimpeded were an increase in the number of microtubules within the fibroblasts, an increase in the number of simplified desmosomes between the fibroblasts and a decreased amount of ground substance within the extracellular matrix. The results show that it is possible for a connective tissue to undergo marked changes in function, turnover and biomechanical properties without major structural changes.

FIBRIL DIAMETERS IN THE EXTRACELLULAR MATRIX OF THE PERIODONTAL CONNECTIVE TISSUES OF THE RAT

The diameters of collagen fibrils were measured in three varieties of periodontal connective tissue in the rat mandible (incisor periodontal ligament, incisor enamel-related connective tissue and molar periodontal ligament). Despite structural and functional differences between the tissues, a similar range of collagen fibril diameters was seen in each (average 45 nm). Variation between animals and between adjacent fiber bundles in any given tissue was observed. Even within any single fiber bundle, the fibrils had a variety of diameters. Although it has been claimed that collagen fibrils have diameters which are multiples of 8 nm, such a relationship could not be discerned for periodontal collagen fibrils. It was estimated that about 65 per cent of a periodontal collagen fiber bundle or sheet is composed of ground substance. Despite reports of abundant oxytalan fibers in periodontal tissues at the light microscope level, only one fiber having characteristics of oxytalan was observed in this electron microscopic study. Some aspects of the functional significance of the findings are discussed.

A quantitative assessment of the effects of axially directed extrusive loads on displacement of the impeded and unimpeded rabbit mandibular incisor.

Abstract Extrusive loads of 0.01–1.0 N were applied to the impeded mandibular incisors of 12 rabbits. Tooth position was monitored continuously with a variable capacitance displacement transducer. On suddenly applying an extrusive load, a biphasic displacement of the tooth was recorded. A rapid extrusive phase was followed by a more gradual extrusion. A similar, but intrusive, biphasic recovery response was seen on suddenly removing the load. The group-mean displacements showed that for all phases the responses were force-dependent, though not linearly graded. For a given load, the group-mean displacements during the first phases of the extrusive and recovery cycles did not show significant differences. However, the displacement during the second phase of the extrusive cycle was significantly greater than that of the recovery cycle for a load between 0.2 and 1.0 N. Extrusive loads were also applied to unimpeded teeth. Though no differences between impeded and unimpeded teeth were observed in the biphasic displacement patterns, significantly greater mobility was seen for the unimpeded incisors in all phases. The findings show that biomechanical changes of the periodontal tissues, of importance to studies on tooth eruption, occur in the chronically unimpeded tooth.

Interactions between thyroxine, hydrocortisone and cyclophosphamide in their effects on the eruption of the rat mandibular incisor.

The effects on the unimpeded eruption rates of the rat mandibular incisor of daily doses of thyroxine (0.5 mg/kg) and hydrocortisone (12.5 mg/kg) and of a single injection of cyclophosphamide (40 mg/kg), either alone or in combination, were studied over approximately 15 days. Both the thyroxine and hydrocortisone alone produced increases in eruption, whereas a decrease was recorded with cyclophosphamide alone. When thyroxine was combined with hydrocortisone, the increase in eruption rate was greater than that with thyroxine or hydrocortisone alone. When cyclophosphamide was combined with hydrocortisone, there was no difference compared with cyclophosphamide alone. These results obtained with drug combinations are consistent with eruption having a multifactorial basis.

The effects of axially-directed extrusive loads on movements of the mandibular incisor of the rabbit

Abstract Using a variable capacitance displacement tranducer to record tooth movements continuously, the effects of applying axially-directed extrusive loads of up to 200 g (2 N) on movements of the mandibular incisors of 6 male Californian rabbits (average weight 3.4 kg) were studied. On applying a load of 1 g (0.01 N) or more, a biphasic movement of the tooth was recorded which could be clearly distinguished from eruptive movements. Initially, a rapid, almost instantaneous, extrusive phase was observed, followed by a second phase during which there was a more gradual extrusion. A similar, but intrusive, biphasic recovery response was observed on removing the load. The responses to the extrusive loads were force-dependent. Immediately after the animals were killed, responses were similar to those in live animals.

Fenestrated capillaries in the connective tissues of the periodontal ligament.

A quantitative ultrastructural study was undertaken to assess the number and distribution of fenestrated capillaries in the periodontal ligaments of the rat dentition. Experiments were conducted on four animals using the right mandibular incisors and first molars. For the incisor, analysis was performed in three sites along the length of the periodontal ligament. Regardless of site, there was 0.1 fenestration per micron2 of endothelium. In the basal region, the incisor periodontal ligament contained 3.5 X 10(6) fenestrations per mm3 of tissue whereas more occlusally it contained 1 X 10(6) fenestrations per mm3 of tissue. For the molar, the capillaries showed more fenestrations, 0.4 fenestration per micron2 of endothelium. There were also more per unit volume of tissue, 12 X 10(6) fenestrations per mm3. Thus, the vasculatures of the rat incisor and molar periodontal ligaments exhibit significant numbers of fenestrations which, in the case of the incisor, are not distributed uniformly along its length.