Dentine apposition in the mandibular first molars of albino rats

Abstract

Dentine formation was studied in the mandibular first molars of 264 albino rats (Holtzman Co.), equally distributed as to sex. Thirty-two animals were used to establish the onset of mineralization at the sites of measurements by means of vital staining with alizarin red S. Sixteen rats were divided in two groups, killed on the twentieth and twenty-third day, to test the reliability of the weaning line (21-day weaning) as a reference zone. The remaining 216 animals were divided into two groups. Group 1 animals were injected with a 0.2% solution of NaF at the age of 31, 41 and 61 days to produce three timed calciotraumatic dentine responses. The dose of F at each injection was 20, 20 and 10 mg F per kg of body wt., respectively. The increments of dentine in these rats were studied up to the age of 91 days. Group 2 animals, divided in three sub-groups, were injected at the same age as the experimental ones with physiologic saline solution and killed when 41, 61 and 91 days of age. The injections of saline did not result in calciotraumatic lines. The deposits of dentine were measured on comparable mid-coronal ground sections. The largest increments per day were observed prior to weaning, namely 13.8, 14.6 and 10.2 μ, on the cervical mesial, distal, and on the mid-occlusal surfaces, respectively. Subsequently, between 21 and 41 days, approximately 7.3 μ of dentine (per day) was deposited on the mid-occlusal surface and 6.7 μ (per day) on the distal cervical surface. The formation of dentine in the latter area had apparently subsided in 22–33 per cent of the rats prior to the age of 61 days because no evidence of the 61 day NaF injection could be observed. However, daily increments of 3.5 μ of dentine accrued on the distal cervical surface between 41 and 61 days in those rats that did show the 61 day NaF line. All the rats continued deposition of dentine on the mid-occlusal surface beyond the ages of 41 and 61 days, but the daily increments between 41 and 61 days were only 3.8 μ in the NaF injected rats as opposed to the 5.1 μ observed in the controls. This deceleration was only temporary, and the mid-occlusal dentine deposit (21–91 days) at age 91 days did not differ significantly from that observed in the saline injected animals. The rate of dentine formation was not associated with the increments in body weight. The magnitude of the dentine deposit and the size of the tooth appeared to be unrelated and there were no statistically significant sex differences. Persistence of the 21 day weaning line and of the induced calciotraumatic responses made it possible to obtain longitudinal data on dentine deposition. Because of the large individual variations in dentine widths at weaning, and because these variations increased in magnitude with age, it is suggested that future investigations of dentine formation should be planned as longitudinal studies.