Phenotyping Transgenic Mice: Mineral Density Determination of Incisor Enamel

The use of the mouse dentition as a model for the study of odontogenesis using transgenic animals is widespread. Tissue phenotyping is a central requirement for this research but there are currently no readily available methodologies for high resolution, quantitative measurement of mineral density in murine teeth. Transverse microradiography (TMR) has been used widely to determine mineral density in human teeth, particularly in re-mineralisation / de-mineralisation studies. Objectives: To optimise TMR for use on wild-type mouse incisors to provide baseline data on mineral density throughout enamel development and to correlate this with enamel architecture and mineral composition. Methods: Lower incisors from wild-type mice were dissected free, embedded in methylmethacrylate and sectioned transverse to the tooth long axis with a peripheral diamond disc. Sections were taken at points throughout enamel development, polished plano-parallel to a thickness of 100 µm, acid etched for 15 sec with 35 % phosphoric acid to remove the smear layer and thoroughly washed in distilled water. Mineral density across the full enamel thickness was determined by TMR. An aluminium step wedge was included to aid quantitation. The enhanced images of the microradiographs were analyzed under standard conditions of light intensity and magnification. The same sections were then viewed by SEM and enamel elemental composition determined by EDS. Results: The mineral density values for enamel at the end of secretion, during maturation and in fully mature tissue were 50 %, 72 % and 82 % respectively. The average Ca:P ratio was 1.65. Conclusions: It is possible to modify standard TMR techniques to accurately determine mineral density in all stages of mouse incisor enamel development. These values can then be directly correlated with structure and composition, providing an objective and quantitative measure of mineral levels in the murine model.

Supported by the Wellcome Trust (programme grant 075945).