Morphometry of neural structures in the mouse periodontal ligament mesial to the mandibular first molar

The periodontal ligament mesial to the mandibular first molars of three mice was analysed stereologically between the alveolar crest and the tooth apex. Ultrathin tissue sections were collected at statistically predetermined intervals, 50 and 200 microns apart, examined in the TEM and quantified using standard point counting procedures (Gundersen et al. 1988). Findings include evidence that, in the mouse, some unmyelinated axons arise from myelinated axons and that anatomically discrete arrangements of unmyelinated axons occur in the apericytic wall sections of postcapillary-sized venules. Morphometric data indicate that unmyelinated axons constitute approximately 95 percent of all periodontal axons. The greatest relative proportion of myelinated axons is the bone third of the ligament, at depths between 600 and 800 microns, where the ratio of unmyelinated to myelinated axons is 5:1. Ultrastructurally, the ligament contains a variety of anatomically discrete neural structures in juxtaposition to periodontal blood vessels. These structures include nerve endings contiguous with K-cells, partially exposed terminal axons, preterminal and terminal axons protruding into the vessel lumina, and lamellated receptors. Mitochondria-rich terminals and fine nerve endings approximated pericytes in the walls of postcapillary-sized venules and arteriovenous anastomoses. Typically, these neural structures were characterised by the presence of an associated oxytalan fibre meshwork. This study also provides quantitative parameters for axon distribution within the ligament.