[The morphological study of experimental caries produced in rat molars]

Since there is some disagreement over the validity of using the rat for various experiments of relevance to dental caries in the human. The process of carious formation in the lower first and second molars of the rat was observed histologically. 120 new born Wistar rats were divided into two groups, one on a carious diet and the other on a standard diet. Both control and experimental animals were killed at preselected intervals namely 1 week, 4 weeks and 7 weeks after weaning. The maxillary and mandibular tooth bearing segments were removed and fixed in glutaraldehyde and osmium. After clinical examination and assessment of the caries status made with the aid of a binocular microscope, the material was sectioned without decalcification and prepared for light, electron, or microradiogram. From the results it was concluded that, 1. The caries process in the rat is extremely rapid much faster than in the human for example complete breakdown of the crown occurs within 28 days of weaning. 2. Frequently destruction of the dentin occurs before mineral loss is detectable in the enamel. 3. Rarely are any of the characteristic features of the early human caries lesion, such as the surface zone, body of the lesion, the dark zone and the translucent zone, to be found. 4. When a surface zone is visible, the dentine is usually in an advanced state of destruction. 5. The histopathology of rat caries does not therefore follow the same pattern as in the human. It is deduced that the reasons for these differences with the human lesion are due to. a) Rat molar enamel is extremely thin being only 100 microns in the thickest part of the fissure enamel whereas it is 2.5mm in the human. b) Rat molar enamel has a lower mineral density than the outer two thirds of human enamel, and is even lower (2.74) than the average value for the inner layer deciduous human enamel. (2.85-2.92) (Weidmann, et al., 1967) c) Rat enamel is extremely porous allowing easy diffusion of fluid and dyes. d) This porosity may be related to incomplete maturation of the enamel when the teeth erupt.