Location and ultrastructure of the first cementum formed in rabbit incisors.

The morphology of the continuously erupting rabbit incisor differs characteristically from that of the rodent incisor and is incompletely known. In this study, permanent incisors from fetal New Zealand white rabbits aged 26-30 days in utero were processed for transmission electron microscopy. Examination of longitudinal and cross-sections revealed that disintegration of the epithelial sheath on root analog surfaces was followed by cementogenesis. The first layer of cementum matrix was seen on the mesial aspect and consisted of a finely granular ground substance and a few embedded collagen fibrils which extended to the cementodentinal junction. Within the period 27-30 days in utero, acellular cementum was formed on all root-analog surfaces and, in addition, formed a fine coating over the enamel. Although the initially formed cementum layer contained fewer collagen fibrils than subsequent layers, a layer of so-called intermediate cementum was not observed.