R.C. Shore

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.

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.

Model to explain differential movement of periodontal fibroblasts

Abstract Opinion has turned towards active migration of periodontal fibroblasts as being responsible for generating the eruptive force. The main experimental evidence supporting this hypothesis is the presence of differential rates of fibroblast migration within the periodontal ligament, interpreted as resulting from active movement. A model is described which can explain differential rates of fibroblast migration within the periodontal ligament without recourse to active fibroblast movement.

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

Abstract An ultrastructural study of the periodontal ligament following root resection of the rat incisor, when the basal proliferative tissues are removed, was undertaken to determine the location of its zone of shear. Fourteen days after root resection, the vacated socket was lined with connective tissue which, for at least 1 mm behind the base of the erupting tooth, was similar in structure and dimensions to the normal functional ligament above the base of the tooth. More distant from the tooth base, some loss of structure and organisation was evident, probably related to loss of function. By the 23rd day after resection, the socket began to fill with loose connective tissue and by the 29th there was bone deposition within the socket. The results may indicate that the zone of shear is located immediately adjacent to the tooth and not towards the mid-region of the ligament.

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.

The ultrastructure of the enamel aspect of the rat incisor periodontium in normal and root-resected teeth

Abstract The ultrastructure of the connective tissue adjacent to the enamel organ and of the periodontal ligament proper of the rat lower incisor tooth was studied. Two zones on the enamel side could be distinguished; an inner enamel zone with many layers of flattened fibroblasts between which were loosely-packed collagen fibrils orientated parallel to the longitudinal axis of the tooth, and an outer alveolar zone consisting of cells, ground substance, blood vessels and few, if any, collagen fibrils. Comparisons were made between this tissue and the periodontal ligament proper. Following root resection, changes in the enamel aspect of the periodontium and enamel organ were observed. For approximately 200 μm behind the resected tooth, the socket was lined with enamel-organ cells possessing processes which projected into the tooth space, and beyond this by fibroblasts. The line of demarcation between the last enamel organ cell and first fibroblast was indicated by a basal lamina. More distant from the base of the tooth, the thickness of the connective tissue lining became reduced until the inner enamel zone was represented by only two or three fibroblast layers.

High mitochondrial density within peripheral nerve fibres of the periodontal ligament of the rat incisor

Abstract An electron microscopic study of the periodontal ligament of the rat lower incisor revealed the presence of unmyelinated nerve fibres containing high concentrations of mitochondria. The diameter of such fibres varied from 4 to 0.5 μm. The fibres were probably sensory and occurred singly. The cytoplasm of the cells ensheathing the fibres contained rough endoplasmic reticulum which appeared to be dilated whilst the processes showed large numbers of vesicles and occasional desmosomes. Leptomeric type organelles were also present in the ensheathing cells. The significance of the high mitochondrial density is discussed.

The effects of preventing movement of the rat incisor on the structure of its periodontal ligament.

A quantitative, ultrastructural study was made on the periodontal ligaments of rat mandibular incisors immobilized for 18 days. Fibroblasts and their organelles (i.e. endoplasmic reticulum, mitochondria, microtubules, microfilament bundles, lysosomes, intracellular collagen profiles and intercellular contacts), oxytalan fibres, collagen fibrils and ground substance were quantified. There was re-orientation of tissue at the lateral borders of the ligament and an apparent increase in size of epithelial cell rests. Within the matrix, the mean collagen-fibril diameter decreased and the amount of ground substance increased. No change occurred in oxytalan fibres. Within the cells, effects were seen only in cell contacts and in the relative volume of intracellular collagen, lending no support to the view that fibroblast activity generates the force responsible for tooth eruption.

Fenestrated capillaries in the periodontal ligaments of the erupting and erupted rat molar

Quantitative ultrastructural studies were conducted on eight rats (four aged 21 days and four aged 8 weeks) using the periodontal tissues around the mesial root of the mandibular first molar. The periodontal ligament of the erupting tooth contained significantly more capillary fenestrations than the erupted tooth, both in terms of the number per unit area of endothelium (2.2/microns2) and in terms of total number per cubic millimetre of tissue (30.5 X 10(6)/mm3). Differences were also discerned with respect to the percentage of capillaries in the tissue. Thus, the periodontal vasculature demonstrates marked morphological changes which may be related to the eruptive phase of the tooth.

Model to explain apparent occlusal movement of extracellular protein of periodontal ligament of the rat incisor

Abstract Previous studies of the fate of tritiated proline within the periodontal ligament of rat incisors reported a shift of the peak of labelling occlusally at a rate comparable with eruption. This was interpreted as evidence for occlusal movement of extracellular protein within the ligament. A model is presented which explains these observations without requiring any movement of extracellular protein.