A Determination of the Time of Eruption

Rat Teeth in study of Caries A Determination of the Time of Eruption, and Rate of Calcification of the Teeth of the Normal Albino and Piebald Rate as a Guide for Experimental Work in Dental Decay.

By Don C. Lyons, D.D.S, M.S., Ph.D.

Because of inability to find any source of information regarding successive stages of the development of rat teeth to use as a guide in experimental work, the following examination was made.

Albino and Piebald rats were taken at random from the stock colony of the Biological Chemistry Department of the Michigan State College at intervals of approximately twenty-four hours. These rats were etherized, their heads removed, and then placed in a 10% formalin solution to harden.

After fixing the heads were split laterally in the mid-line, taking care to bisect the tissue which contained the developing incisor teeth.

The heads were then x-rayed, 1.50 to 2.0 seconds exposure being used, 10 mil-amp, and 8.75 inches target distance. The x-rays were used as a convenient method of demonstrating the calcification progress of the teeth. The machine used was a well known dental x-ray machine. The x-rays revealed the following data:

It was first noted that at the time of birth two small thin lines showing slight depositions of calcium salts are present which represent the embryonic or developing jaws. In the posterior portion there is a slight thickening of this line representing the developing molar teeth.

Forty-eight hours later a fine triangular shaped wedge of calcifying bone is demonstrable in the anterior portion of the developing jaws and represents the beginning of the upper incisors. Twenty-four hours later a similar calcification is found in the lower jaw.

The molar anlage continues to increase in size during this time so that by the fifth day after birth definite triangular wedges of calcification are to be seen in the x-rays, these being the cusps of the developing molar teeth.

The tips of the incisor teeth break through the gums and into the mouth at the sixth day, while those of the molars appear at the ninth or tenth day. It is interesting to note that this corresponds very closely to the time at which the young rat opens its eyes, although in some colonies of rats, the eyes do not open until later.

Calcification proceeds from here at a very rapid rate in the normal rat. The incisor teeth are almost completely formed by the fifteenth day. It is impossible to tell from the x-ray alone just when the calcification of these teeth may be said to be complete because of their continuous growth throughout life.

The crowns or enamel portion of the molars are complete by the twentieth day when the roots also begin to show some calcification. Calcification of the molar teeth is complete by the thirty-third day after birth.

Calcification of all teeth proceed at an equal rate. All molar and incisor teeth being in the process of development at the same time.

This determination of the development time of the teeth of the normal Albino and Piebald rats furnishes a valuable guide in nutritional experiments on the developing teeth and in the experimental production of dental caries.