An ultrastructural study of periodontal ligament fibroblasts in relation to their possible role in tooth eruption and intracellular collagen degradation in the rat.

Though periodontal fibroblasts were polarized when viewed in logitudinal section, their outline in other planes suggested that they are flat, irregular discs, lacking any polarity in either morphology or distribution of intracytoplasmic contents. In longitudinal sections containing blood vessels, the ligament could be divided into 2 zones according to the morphology of the fibroblasts. In transverse section, however, no such division was apparent between blood vessels, the elongated fibroblasts of the inner dental zone reaching the alveolar wall. The overall morphology of molar fibroblasts resembled that of the incisor fibroblasts, but without specific orientation. The findings provide no evidence that (1) fibroblasts of the periodontal ligament generate the eruptive force by their migratory activity, or (2) there is a coherent morphological zone of shear within the periodontal ligament. Stereology showed no significant differences in the distribution of intracellular collagen profiles within fibroblasts of the inner dental zone of the incisor ligament. Fibroblasts within the molar ligament contained significantly more profiles. If the time course for intracellular collagen digestion was of the order of 30–40 min, and the intracellular collagen was all ingested from the extracellular compartment and did not represent newly-synthesized collagen, the number of intracellular collagen profiles could account for the complete turnover of periodontal collagen. Around the blood vessels, mitochondria-rich, branching, unmyelinated nerve fibres were at least 35 μm long and were in continuity with myelinated nerves. Encapsulated, unmyelinated nerves were also seen near blood vessels.