Effect of Experimental Hyperparathyroidism on the Incisor of the Rat. ∗

The microscopic anatomy of the incisors was studied in 2 groups of rats:

A. 29 rats that were given 1 to 15 injections of parathyroid hormone. Doses varied from 10 to 150 units. Weight: 85-145 gm. B. 7 controls from same colony.

Significant alterations were observed only in Group A: 1. Enamel hypoplasia was found to be formed at the time of the first injection in 5 animals. 2. The alveolar bone showed an abnormal increase in osteoclasts in 14 and a fibrous change of the bone marrow in 3 of the animals that were given 3 or more injections of parathyroid hormone. 3. The principal changes were found in the dentin. Each experimental animal showed a primary hypocalcified stripe in the dentin that was being calcified during the immediate effect of the first injection and a secondary hypercaleified stripe in the dentin that was being calcified subsequently. The extent of the secondary stripe varied with the number and unitage of the doses injected and the duration of the experiment.

Two animals that were given 12 daily injections of 10 units each of parathyroid hormone showed a normal histologic picture.

The absence of reaction in the 3 animals that were given inactivated hormone preparation gives conclusive proof that the effects observed were due to the parathyroid hormone and not to any foreign protein impurities.

It is believed that the experimental pattern in the dentin represents its response to the changes in the calcium and phosphorous metabolism induced by the injections of the parathyroid hormone. The dentin reaction can be explained best by the theory that the parathyroid hormone controls a fraction of the serum calcium. The findings suggest the feasibility of a method of standardization of parathyroid hormone based on the changes shown to occur in the dentin.