Distinct fibro-vascular arrangements in the periodontal ligament of the horse

Abstract

The periodontal ligament (PDL) of the hypsodont cheek teeth of the horse meets specific requirements of tooth support: (1) to fix the tooth; (2) to withstand pressure and tension during the laterolateral mastication movement; (3) to restore the original tooth position. These requirements are assumed to be met by the periodontal collagen fiber apparatus and by the vascular system. Distinct fibro-vascular arrangements are described here in order to reveal the structural prerequisites for these distinct functions. Specimens from nine horses were processed for (1) collagen labeling with fluorescein, (2) oxytalan fiber staining, (3) immunostaining of smooth muscle actin, and (4) TEM examinations. Blood vessels were found to be integrated in the PDL in three distinct ways: The sheaths formed by veil cells (type-I arrangement) isolate distinct groups of blood vessels from deformations of the surrounding tissue. In contrast, the distinct fibro-vascular arrangements of anchored blood vessels (type-II arrangement) and ballooned venules (type-III arrangement) are assumed to accept the forces generated by masticatory tooth movements. The anchored blood vessels sustain tractive forces by means of what is called a lateral compression mechanism, whereas the ballooned venules act as a hemodynamic cushion.