Cell sizes and apposition of dental hard tissues in rats

The daily apposition of the enamel and dentin and the sizes of ameloblasts and odontoblasts in the first upper molar of 2- to 9-day-old and of 12-day-old rats were examined by light microscopy of freeze-dried sections. The formation of enamel in the first molar started at the age of 2 days and was completed in most of the analyzed areas at the age of 9 days. On each day the enamel on the mesial sides of the cusps was about twice as thick as on the distal sides. The differences in thickness of dentin between the mesial and distal sides of the cusps were not as marked as was seen for the enamel. However, the rapid formation of dentin seemed to continue longer on the mesial sides than on the distal sides. The height of the ameloblasts and odontoblasts showed similar differences between the mesial and distal sides as was seen in the formation of enamel and dentin. In addition, these cells showed a gradual increase and decrease in height during the observation period, with a maximal height in the analyzed area at the age of 4-6 days.