Ontogeny of circadian dentinogenesis in the rat incisor.

Periodic growth increments are found universally in the dentine of animals. The goal here was to determine when and how circadian dentine growth increments develop ontogenetically. A total of 97 rat pups, obtained from 13 mothers of the Wistar strain, were injected with nitrilotriacetato lead at appropriate intervals to chronologically label the dentine. Labelled rat pups were killed 5-50 days after birth. Histological transverse sections of the demineralized dentine were obtained from the maxillary incisors of the pups and stained with haematoxylin. Two types of increments were observed in the circumpulpal dentine. The first type was a deeply stained circadian band which appeared at intervals of about 16-24 microns. The other was a faintly stained, narrow, ultradian increment, at intervals of approx. 6-8 microns. The circadian increments were first detected during the second to third week after birth and were observed consistently afterwards. Before the circadian increments developed, the ultradian increments were predominant. These two rhythms seemed to be independent of each other and sometimes coexisted in one section. Thus, the two types of dentine increments may be due to two independent mechanisms.