Preliminary characterization of the craniofacial morphology of the B6C3Fea/a-din/+ mouse strain
Dense incisors (din) is an autosomal recessive mutation mapped to Chr 16 near the pituitary transcription factor gene Pit1, but not a mutation in Pit1. Complete incisor eruption past the gingiva does not occur in homozygous (din/din) mice. Continued dentin formation gradually occludes the pulp chambers creating a dense incisor (H.O. Sweet et al. 1996. J Hered. 87, 162-7).
Homozygous (din/din) and control (+/?) B6C3Fea/a-din /+ mice were collected in groups (n=2-8) of age 4, 8, 12, and 16 weeks. Bone mineral density (BMD) scans (PIXImus, LUNAR, Madison, WI) were completed of skulls and bodies separately. The skulls were prepared by incomplete maceration in KOH, alizarin red stained, and stored in glycerin. Seven measurements of the skull along with ear pinnae length were then taken with digital calipers (Stoelting, Wood Dale, Ill). The skull measurements are as follows: total skull length, (tip of the nasal bone to back of the occipital bone) nose length, (nasal bone to coronal suture) skull height, (lowest point of the angular process of lower jaw to frontal bone) skull width, (measured at the widest points of the parietal bone) inner canthal distance, and length of both jaws.
Most skull measurements and overall body mass proved to be significantly dependent on genotype. However, the data showed no genotypic effect on any of the following allometric comparisons at any age; skull length/nose length, skull height/skull length, skull length/skull width, skull height/skull width. This indicates the din mutation effects growth in all areas of the skull equally.
Hand Caliper Measurements
The following graphs show the effect of genotype at different ages on the percent fat, skull BMD, whole body BMD (g/cm2) and the ratio of skull BMD to whole body BMD. The percent fat measurements indicate a reduction in body fat in the din/din homozygotes. This could be explained as the effect of incomplete incisor eruption on consumption. Stomach contents inspected during necropsy indicate that the homozygotes do continue to eat. However their total caloric intake may be reduced. The skull BMD and whole body BMD of the din/din homozygotes are significantly less at 8 and 12 weeks of age. The ratios of skull BMD to whole body BMD indicate that the mineralization defect caused by the din mutation is not specific to the skull.
