The Effect of Acetyl Thyroxin on the Teeth of Newborn Rats.

Abstract

In the acetyl derivatives of thyroxin certain parts of the thyroxin molecule have been replaced by the acetyl group giving a compound which does not affect the metabolic rate of mammals. 1 It retains, however, its effect on the development of young organisms. Tadpoles, to which acetyl thyroxin is given by injections or feeding, develop rapidly into frogs. Newborn rats are similarly affected, going through changes of form normal for the first 25 days of life in a period of 15 days. These general changes are shown in the proportions of the skull, the development of the hair, eyes, auricles and feet. These effects, and the method of the experiments have been described elsewhere. 2 During the course of the work it was noted that the injections hastened the eruption of the incisors, but this point was not studied in detail. A brief sketch of the changes occurring in incisors and molars is presented below.

Animals were injected as in the above experiments, with the same general results. On the third, fifth, and eighth days of life, specimens were killed. In each case a normal litter mate was killed also and studied for comparison. The heads of the animals were fixed in Bourn's fixing fluid. The upper and posterior part of each head was then cut away, leaving the jaws, tongue and surrounding tissues to be sectioned. This piece was decalcified in weak nitric acid, embedded and sectioned.

In material prepared in this way, the following conditions were observed. In a normal rat at 3 days of age, the incisors began to penetrate the oral epithelium, the tips being about a third of the way from the base to the surface. In the injected animal, after only 0.1 mg. of acetyl thyroxin had been given, the tooth had grown considerably farther into the epithelium, its tip being half to three-quarters of the way through.