The blood vessel system in the periodontal ligament of the equine cheek teeth--part II: The micro-architecture and its functional implications in a constantly remodelling system.

The micro-vasculature of the equine periodontal ligament (PDL) was investigated using corrosion casts for scanning electron microscopy. Specimens from eight healthy warm-blooded horses were examined. Specific vascular features such as large ampullae with a diameter of up to 300 microm and blind vascular casts were found in the specimens of the equine PDL. The ampullae-shaped venules occurred only in the peripheral layer, where they were formed at the confluence of several vessels. Two types of blind stumps were identified. The first was developed in all three layers, while the second was found only in the peripheral layer. Blood-flow-regulating structures were not present in the equine PDL. As vascular valves are absent, the blood can flow in all directions. This polydirectional shift exists in all vascular layers. The vascular ampullae, present exclusively in the peripheral layer, are discussed here as specific features of a mechanism that can receive masticatory forces. Apart from its mechanical role, the vasculature must constantly adapt to the spatial changes resulting from the life-long eruption of the equine cheek tooth. This is facilitated by vascular sprouting and intussusceptive vascular remodelling. The blind ending stumps of the corrosion casts are typical equivalents of vascular sprouting. The large ampullae are interpreted as the first step in an intussusceptive process.