HORMONAL CONTROL OF TOOTH ERUPTION

I. THE EFFECT OF THYROIDECTOMY ON THE UPPER RAT INCISOR AND THE RESPONSE TO GROWTH HORMONE, THYROXIN, OR THE COMBINATION OF BOTH

¹ Division of Dental Medicine, College of Dentistry, the Institute of Experimental Biology and the George Williams Hooper Foundation for Medical Research, University of California, San Francisco and Berkeley, Calif.

For the study of the hormonal control of tooth eruption in this first of three endocrine experiments, sixty male and female rats of the Long-Evans strain were subjected to thyroidectomy at birth. One group of them served as operated controls and three groups received from the thirty-second to the sixty-third day of life, replacement therapy with growth hormone, thyroxin, or the combination of both. A group of intact 63-day-old rats was included as normal controls.

1. Thyroidectomy reduced the eruption rate 45 per cent, during a tenweek period, while the size of the tooth structures remained 20 to 25 per cent under normal average. Histologically, reduction of vascularity-stunted growth, as well as differentiation, and early atrophy of the odontogenic epithelium were conspicuous thyroprivic symptoms.

2. Administration of growth hormone elicited a slight increase in eruption rate (10 per cent) and in tooth size, while a considerable growth spurt of the maxillary bone over normal dimensions was observed. Histologically, connective tissues were rejuvenated, but young epithelial cells failed to undergo immediate histodifferentiation.

3. Injection of thyroxin increased the eruption rate (27 per cent) more than dental dimensions. There was a marked increase in vascularization of the dental structures and accelerated proliferation and histodifferentiation of epithelial tissues. Calcification, especially of the enamel matrix, however, was deficient.

4. When both growth hormone and thyroxin were injected simultaneously, the eruption rate and dental dimensions exceeded only slightly the thyroxin-treated group, but the histologic aspect revealed improved structures hardly to be distinguished from normal.