Variations in the Morphology of Mandibular First Molars of Albino Rats

¹ Forsyth Dental Infirmary and Harvard School of Dental Medicine, Boston, Massachusetts

A study of mandibular first molars of 216 albino rats showed that the maximal mesiodistal crown size was larger in male than in female rats. The mesiodistal dimension at the cervical level was similar for the molars of both sexes; therefore, it is suggested that the molars of male rats had a greater bulging of the proximal surfaces of their crowns. No sex differences in the mesial, central, and distal internal crown heights were observed.

The crowns of mandibular first molars were fully formed when the rats were about 10 days of age. The sex difference expressed in the maximal mesiodistal tooth size was thus established much earlier than sex differences in the craniofacial skeleton, which did not become apparent until the animals were about 2 months old.⁶

The maximal mesiodistal and buccolingual crown size was highly correlated. Moderate, but statistically significant, correlation was observed between the mesial and distal, central and distal, and mesial and central internal crown heights.

As a result of tooth wear, a statistically significant widening of the occlusal opening of the mesial groove and a narrowing of the distal groove took place between 41 and 61 days of age. The latter dimension continued to decrease at a rapid rate from 61 to 91 days, and the occlusal aspect of the central groove also became considerably narrower during this period. There was a decrease of 0.13 mm. in external mesial crown height in both males and females between 41 and 91 days.

The strain of rats used in this investigation has been inbred for many generations, and the animals had an opportunity for comparable nutrition during and after tooth development. Although food debris was found in all occlusal grooves of the molars, no dental caries lesions were observed clinically or microscopically.

The wide range of normal variation in the size and shape of rat molars, the early age when the crown morphology was established, as well as the marked occlusal changes due to wear, must be considered when studying the effects of nutritional, endocrine, and other factors on tooth morphology and the possible relation between the size and health of teeth.