A light microscopic and ultrastructural examination of calcified dental tissues on horses: 4. Cement and the amelocemental junction.

Ultrastructural examinations showed the diameter of cement lacunae to be greater in infundibular cement than in peripheral cement of upper cheek teeth, which in turn was greater than in the peripheral cement of the lower cheek teeth. However, numbers of lacunae/unit area remained similar in these 3 dentinal region. Two types of cemental hypoplasia were found in equine cheek teeth. The first type was termed central infundibular cemental hypoplasia and was confined to the central region of infundibular cement. The cement adjacent to these frequently large defects was very porous and contained large vascular channels. In recently erupted cheek teeth, these central infundibular cemental defects were filled with connective tissue. The size of these cemental defects, the relationships of such defects with the occlusal surface and the degree of porosity of cement surrounding these defects may be important in the development of cemental caries. The second type of cemental defect was found at the amelodentinal junction of both peripheral and infundibular cement and was termed junctional cemental hypoplasia and appeared as spaces varying from focal, to long narrow defects along the amelocemental junction with the adjacent cement of normal appearance. Peripheral cement was deposited both directly, i.e. on unresorbed or resorbed enamel surfaces or indirectly, where the cement was separated from enamel by a thin calcified layer. The surface of unresorbed enamel had a pitted appearance, with the bases of the pits formed by enamel prisms and the pit walls by interprismatic enamel. The cemental surface of resorbed enamel contained depressions of variable shapes and sizes. These depressions which are believed to be caused by the resorption of enamel by odontoclasts were both focal and diffuse and were more marked on the cemental surface of infundibular as compared to peripheral enamel.