[Diurnal layer formation in the incisor dentin of rodents]

The growth of dentin of the constantly growing lower incisors in rodents was studied using tetracycline as a vital marker of the growing parts of bones and teeth. In the common vole in nature the incisors grow in length at a rate of 490 micrometers per day; a basal-apical gradient of growth of the tooth walls in thickness was found: the minimal daily increment at the tooth base and the maximal in the apical third, it decreases again at the very apex. In the peripulp part of dentin of the apical incisor half there are diurnal layers seen both on polished sections and stained slides the formation of which correlates with the diurnal rhythm of skeleton growth. The diurnal dentin layers form in the apical incisor half only. The odontoblasts appear to begin to function rhythmically from the middle of their life on only. A similar phenomenon was found in a natural population of large-toothed redbacked voles. The topography of dentin layers in the laboratory rats is similar to that in the voles but the layers are not distinct and the basal-apical gradient of dentin growth is observed in young animals, rather than in the old ones.