Distinct Fibro‐vascular Arrangements in the Equine Periodontal Ligament

Abstract

The morphological pre‐requisites to sustain pressure loads and tractive forces during mastication are examined histologically in the periodontal ligament (PDL) of cheek teeth from 17 horses. The upper and lower jaws were dissected and perfused via the infraorbital and the inferior alveolar arteries with Bouin's solution or, alternatively, with Karnovsky's solution. After perfusion, the jaws were subdivided with a steel band saw and subsequently with a diamond saw to obtain PDL specimens from the first molar tooth. After decalcification, specimens were processed for (1) collagen labelling with fluorescein, (2) oxytalan‐fibre staining, (3) immunostaining of vascular smooth muscle actin, and (4) TEM examinations. The periodontal ligament (PDL) of equine cheek teeth is composed of two distinct functional units: (1) a collagen‐oxytalan‐fibre apparatus, (2) a specific vascular system. These elements are associated with each other in characteristic ways: (1) extremely wide, ‘ballooned’, venules are accompanied by collagen fibres that run parallel to the cemental surface. Plenty of oxytalan fibres are packed between the venous walls and the dental cementum. This is a typical feature for regions of the PDL that are supposed to receive compression (related data concerning entheses, osteoclasts, osteoblasts have been presented by Staszyk and Gasse, 2003, 2004*). (2) Regions that are defined to sustain tractive loads – instead of pressure – are distinguished by the so‐called ‘anchored’ venules. The vessels are ‘embraced’ bilaterally by thick bundles of dense collagen fibres, which are orientated in an alveolo dental direction. The fibre bundles are directly linked to the venous walls by means of a complex of oxytalan fibres, veil cells, and smooth muscle cells. These different fibrovascular arrangements (a,b) represent morphological adaptations that may be suitable to withstand either compression or traction. The first (a) provides the pre‐requisites for a ‘hemodynamic cushion’ to receive pressure forces; the latter (b) is suggested to receive tractive forces by means of an elaborate ‘lateral compression’ mechanism that shall be presented in detail.