Histology of the periodontal ligament of rat mandibular incisor following root resection, with special reference to the zone of shear

Mandibular incisor apicoectomy in a Canadian Beaver

A 52-month-old Canadian beaver was presented for treatment of lip trauma resulting from overgrowth of the right mandibular incisor tooth following earlier loss of the right maxillary incisor tooth. Extraction of the affected tooth was considered, but rejected due to the length of the embedded portion of rodent mandibular incisor teeth. The lip injury was managed by crown reduction (odontoplasty) of the overgrowing incisor tooth pending a more permanent treatment plan. A 2-cm apicoectomy of the right mandibular incisor tooth was performed to arrest growth of the tooth when the beaver was 82-months-old. The remainder of the tooth continued to erupt and was completely expelled during a 9-month period with one additional odontoplasty being required. The beaver continued to feed normally with just the left maxillary and mandibular incisor teeth until its death at 118-months, with odontoplasty performed twice on the remaining incisor teeth during the 30-months following exfoliation.

A histological study of root-resected and root-transected rat incisors when eruption ceases, shortly before they are exfoliated from the socket

Resection of the odontogenic region or root transection of normal (impeded) rat lower incisors showed that eruption ceased from 1 to 13 weeks when the base of the resected teeth (87.5%) or of the distal segment of the transected ones (86%) reached the alveolar-crest region. When the operated teeth reached the crestal region, the enamel-related periodontal tissues were absent and the periodontal ligament (PDL) was the only periodontal tissue that remained. The PDL of the crestal region may be considered as mature PDL, showing a length of approx. 5-6 6 mm at the mesial face of the tooth, 4-5 mm at lingual face and 1 mm at distal face; from these limits towards the apical end of the socket the PDL becomes gradually immature. The mature PDL seems not to have a role in the process of tooth eruption. Several factors can be suggested to explain the more frequent retention, at the crestal region of the socket, of impeded rather than unimpeded incisors submitted to the same procedures. The connective tissue that develops between the base of the tooth and the bone that fills the alveolus may have more time to organize itself in impeded than in unimpeded teeth, which erupt at a faster rate; this tissue could support and retain the impeded operated teeth longer than the unimpeded ones. The decrease in the mechanical properties of the PDL in the unimpeded condition may ease the traumatic effects and lead to exfoliation. Eruption might be stopped by the increase in occlusal forces, per unit area of root surface, as the root becomes shorter; this effect is likely to be greater in impeded than unimpeded teeth.

Changes in the fibre arrangement of the rat incisor periodontal ligament in relation to various loading levels in vitro

The relationship between the fibre arrangement of the periodontal ligament and the load-deformation behaviour was investigated at various loading levels. Transverse sections of the rat incisor were loaded in the eruptive direction in vitro and the deformation fixed at predetermined loads. Sections were prepared at these deformation levels. The periodontal ligaments were examined by polarized-light and scanning-electron microscopy. At the initial ("toe') part of the load-deformation curve, the periodontal fibres were gradually pulled and bent towards the direction of loading; their wavy pattern and periodic dark and bright bands became indistinct. At the next, linear part of the curve, the running direction of the fibres changed gradually until they were straightened and stretched. At the yielding part of the curve, the periodontal fibres began to rupture. Ruptured fibres adhering to the bone surface returned to their original obliquity and showed periodic dark and bright bands. The fibrous components of the rat incisor periodontal ligament thus transmit forces to bone at the linear part of the curve when the tooth is axially loaded.

The fibrous architecture of the rat periodontal ligament in cryosections examined by scanning electron microscopy

Cryosections through the incisor and molar teeth of the rat mandible were examined, with and without hyaluronidase pretreatment, in the scanning electron microscope. In the fully erupted molar teeth the fibres of the periodontal ligament were organized into bundles which crossed the space from the alveolus to the cementum and inserted into the associated mineralized tissues. In the erupting incisor teeth, three distinct zones were evident. The outer alveolar and cemental zones were composed of coarse fibre bundles which inserted into the adjacent mineralized tissues, while the middle zone was composed of collagenous laminates running along the axis of the tooth. These observations confirm a proposed model for the structure of the erupting periodontal ligament and suggest that the method used will provide further information about the role of the ligament in tooth support and eruption.

An immunohistochemical study examining the role of collagen type VI in the rodent periodontal ligament

The distribution of collagen types I and VI has been examined in the periodontal ligaments of rat incisor and molar teeth using cryosections and immunohistochemical staining procedures. The stain for collagen type I was uniform in all the ligaments examined. Uniform staining for collagen type VI was evident only in the ligament of the fully erupted molars, and the stain was absent from the mid-zone of the ligaments of the erupting molars and incisors. The staining pattern of the collagen type VI antibodies is consistent with previous reports which have suggested that the removal of collagen type VI precedes the degradation of major banded collagen fibrils in remodelling connective tissues. The removal of collagen type VI from the mid-zone of the incisor ligament may precede the remodelling events which facilitate tooth eruption.

The effect of cyclophosphamide on the eruption of impeded and resected incisors in rats

It is known that a single dose of cyclophosphamide (40 mg.kg-1 intraperitoneal) slows the eruption of unimpeded incisors but has no effect on impeded incisors. In this study incisors that were impeded when cyclophosphamide was given were subsequently made unimpeded. When they were unimpeded they erupted more slowly than the controls. Therefore the changes cyclophosphamide produces which slow eruption are formed in impeded incisors but have an effect only in unimpeded incisors. It has been reported that cyclophosphamide slows the eruption of resected incisors when it is given before the resection. In a second experiment cyclophosphamide was given after the resection and had no effect on eruption rates. It is suggested from these and earlier findings that cyclophosphamide slows eruption by imposing a maximum on the rate at which it can occur and that it does so through the changes it produces in the periodontal ligament. The periodontal changes make it harder for the tooth to move rather than altering the forces moving the tooth. This explanation of how cyclophosphamide slow eruption produces an alternative interpretation of some, but not all, of the findings that have been cited as evidence that there are two mechanisms of tooth eruption.

Lathyrogens and the role of collagen in the eruption of rat incisors

Collagen fibres in the periodontal ligament may have two functions: to resist displacing forces and to cause the tooth to erupt. Their function was examined in the continuously-erupting incisor of the rat using various concentrations and types of lathyrogens. Lathyrogens retarded tooth eruption and increased the quantity of salt-soluble collagen in the ligament, indicating inhibition of the maturation of salt-soluble (young) collagen into salt-insoluble (old) collagen, which would lead to reduction in the tensile strength of the fibres and decrease resistance to occlusal forces. The easy extractability of the teeth is explained by the greater susceptibility to lathyrogens of the fibres in the alveolar-related part of the periodontal ligament, compared with those in the other parts.

The effect of procedures intended to alter the interstitial fluid pressure in the sockets of resected rat incisors on their eruption rate

Eruption rates of resected incisors were measured while their sockets were kept open to the mouth. In other resected incisors, hypertonic solutions were placed in the pulp cavity, with the intention of drawing fluid by osmosis across the membrane separating the pulp cavity from the socket, to lower the pressures within the socket. These procedures slowed eruption when control resected incisors were erupting rapidly but not during their initial and final slow phases of eruption. The findings are consistent with there being two factors making resected incisors erupt, tentatively identified as the periodontal ligament and, when eruption is rapid, the interstitial pressures of the tissues in the socket.

Histological study, including ultrastructural quantification, of the periodontal ligament in the lathyritic rat mandibular dentition

A 0.15 per cent solution of aminoacetonitrile was added to the drinking water of young adult male Wistar rats for 18 days. Their right mandibular incisors were maintained unimpeded by frequent trimming. After 18 days, the periodontal ligaments of the right mandibular incisors and first molars were prepared for light and electron microscopy. For control purposes, similar material was obtained from animals fed on a normal diet ad libitum or pair-fed. With light microscopy, lathyritic incisor periodontal ligaments showed areas of normal connective tissue appearance interspersed with areas of degeneration. In the lathyritic molars, cell-free areas were observed. With electron microscopy, quantification of features in the connective tissue regions of the lathyritic ligament showed little change. In cell-free areas, many collagen fibrils had smaller fibril diameters than normal.

An in-vitro model for tooth eruption utilizing periodontal ligament fibroblasts and collagen lattices

An in-vitro model that simulates tooth eruption was devised to determine the possible role of periodontal ligament fibroblasts (PLF) in providing a major force for eruption. The model consisted of agarose-coated wells in multiwell tissue-culture plates to which wire mesh was attached at the rim and a slice of demineralized root was attached to the floor. When the wells were filled with a collagen lattice containing PLF, the lattice contracted and elevated the root slice. The root slice in models that were unattached at the rim was not elevated. Cultures without cells did not contract and root slices were not elevated. Elevation was inhibited by cytochalasin D and Colcemid. Histologically, cultures that had elevated showed orientated cells and collagen fibres. The results suggest that PLF may be capable of generating forces that are sufficient to produce tooth eruption.