Light and electron microscopic study of calcified dental tissues in normal horses

The structures of normal equine enamel, dentine and cement were examined in 50 teeth from 16 horses. Prior to examination these teeth were fractured, or sectioned by a lathe or diamond saw. Specimens were examined without further treatment or after acid etching or decalcification. Measurements of enamel thickness showed that in the transverse plane enamel thickness varied greatly throughout its folds, but that its thickness remained constant throughout the lengths of the tooth. Enamel was thickest in areas parallel to the long axis of the maxilla.

Three types of enamel were identified on scanning electron microscopic (SEM) examination and were termed equine types 1, 2 and 3 enamel according to the shapes of their prisms on transverse section and the amount and appearance of interprismatic enamel they contained. Equine type 1 enamel contained rows of prisms that were oval on cross section and were separated by thick interprismatic enamel plates. Equine type 1 enamel prisms were oriented at an angle of approximately 45° with both the amelodentinal junction and the occlusal surface. This enamel was found adjacent to the amelodentinal junction. Equine type 2 enamel contained prisms which ranged from circular to "horseshoe" shape on cross section and which were separated from each another by thin organic prism sheaths, with no interprismatic enamel present. Equine type 2 enamel prisms were oriented at a wide variety of oblique angles to the amelodentinal junction and the occlusal surface. This enamel was found adjacent to the amelocemental junction. Equine type 3 enamel was composed of round shaped prisms completely surrounded by large quantities of interprismatic material and this enamel was inconsistently present in a thin layer at the amelodentinal and amelocemental junctions. The largest component of peripheral enamel of upper teeth was type 1, compared to type 2 in the lower teeth. Incisor enamel contained increased proportion of type 2 enamel and unlike in cheek teeth, this consisted of bands of prisms that were aligned vertically, obliquely or horizontally to the occlusal surface. The diameters of types 1 and 2 enamel prisms were significantly greater in incisors than in cheek teeth.

Dentinal tubules were branched at the amelodentinal junction and extended towards the pulp cavity following straight vertical, oblique or "S" shaped courses depending on their origin in the teeth. The tubules of primary dentine were surrounded by a variable thickness of peritubular dentine and their position within the peritubular dentine varied from central to asymmetrical depending on their orientation in the teeth and the relationship between the origin of the dentinal tubules and the main pulp cavities. In primary dentine, the amount of intertubular dentine decreased and conversely the diameter of dentinal tubules and the amount of peritubular dentine increased, from the amelodentinal junction towards the junction of primary and secondary dentine. In regular secondary dentine peritubular dentine was absent, but the amounts of intertubular dentine and the density of dentinal tubules were increased compared to primary dentine. In irregular secondary dentine, the lumina of ail dentinal tubules were obliterated. The number of dentinal tubules containing odontoblast processes increased from the amelodentinal junction to the junction of primary and secondary dentine. Dentinal tubules with double odontoblast processes were occasionally found. Regular secondary dentine contained many dentinal tubules without odontoblast processes. Peritubular dentine had a compact appearance and a smooth surface in untreated sections, but when etched it became rough and obliquely oriented canaliculae became apparent.

Equine cement contained lacunae which usually contained one, but occasionally two cementocytes. Significant differences were present between the diameters of cellular lacunae of infundibular and peripheral cement of upper and lower cheek teeth, but the ratio of lacunae: total cement volume and the number of lacunae/ unit area were non-significantly different between the same regions of equine cheek teeth cement. Two types of cementa! defects were found, the first appearing as small round spaces along the amelocemental junction of both upper and lower cheek teeth. The second type was found only in infundibular cement and represented the site of the former vasculature. Peripheral cement was deposited both directly, i.e. on unresorbed or resorbed enamel surface or indirectly, where the cement was separated from enamel by a thin calcified organic layer.