The microvascular venous pool and its ultrastructural associations in mouse molar periodontal ligament — Periodontal Microvasculature & Nerves

This transmission electron microscopic investigation revealed that the periodontal ligament of mouse mandibular molars contained a microvascular bed typified by a predominance of vessels corresponding in luminal diameter with postcapillary-venules. Vessels in the cervical region showed large areas of the endothelium and basement membrane directly exposed to the investing collagen fibre bundles. Cervical venous capillaries had a ratio of luminal diameter to wall thickness averaging 20:1, whereas postcapillary-sized venules approximated 30:1. In the apical third of the ligament the venous vessels became significantly larger and morphologically conformed to ‘apericytic' postcapillary venules, since they appeared to be devoid of pericytes, and comprised endothelial-like tubes with only the basement membrane and occasional veil cells intervening between the endothelium and the enveloping collagen fibre bundles. The ratio of luminal diameter to wall thickness exceeded 60:1 in these apical vessels. This venous capillary pool of the ligament consisted of vessels which differed ultrastructurally from the postcapillary venules described as being typical of other microcirculatory systems.

Nerve endings, indicated by aggregations of mitochondria, were present adjacent to encapsulated myelinated and unmyelinated nerves located alongside cervical venous capillaries and postcapillary-sized vessels. Encapsulated nerves and nerve groups in the apical region corresponded with descriptions of putative periodontal mechanoreceptors in man, but differed in being related to postcapillary-sized vessels and not collecting veins as reported in human ligament. Unmyelinated axons were frequently located within 0.25 microns of the endothelium of venous capillaries and venules. The oxytalan meshwork fibres were associated with all categories of vessels observed including capillaries having luminal diameters less than six microns.

These findings demonstrate that the mouse periodontal ligament has unique, and previously unreported, vascular and neural anatomical features related to its operant role and dynamic behaviour in supporting masticatory loads. Some functional implications of the present findings are discussed.