The mechanism of tooth eruption has been the subject of several investigations, and a number of theories attempting to explain this phenomenon have been proposed.Massler and Schour' summarized the theories of tooth eruption up to the time of their report and, on the basis of their review and their own investigations, concluded that the eruption of teeth is apparently related to the degree of vascularity of the periapical tissues. In a recent publication, Bryer2 supported this view. However, Miller3 and Taylor and Butcher4 did not find the eruption rate to be altered by fairly considerable alteration of blood flow to the periapical tissues. O'Brien, Bhasker, and Brodie5 reported on serial histologic sections through the first molar region of ia and heterozygous rats and were of the opinion that eruption was probably the result of growth of the tooth follicle. Engle6 has shown that the connective tissue ground substance overlying erupting teeth is altered in its histochemical staining reaction and thought to be depolymerized. Baume, Becks, and Evans7' 8, 9 in a series of experiments demonstrated the selective action of growth hormone and thyroxin on tooth eruption and were of the opinion that it was a process of growth and differentiation of maturation. Notwithstanding these investigations, the phenomenon of tooth eruption is still incompletely understood. This report is based upon a serendipitous finding in an experiment1 designed to study the pathogenesis of radio-osteonecrosis. In that experiment it was observed that crowns without any root formation erupted into the oral cavity.
MATERIALS AND METHODSEighteen Macacus rhesus monkeys were subjected to fractionated external irradiation to the mandible and maxilla. The animals reported in this paper were irradiated with dose levels of 4,500, 5,500, and 7,500 r and lived up to 31 years post-irradiation. They were irradiated 5 days a week to alternating sides of the face. The length of irradiation treatment varied from 5 to 12 weeks, depending on the dose administered. The physical factors of irradiation were: 250 KVP, 15 ma, target-skin-distance, 50 cm., half-value layer, 1.5 mm. Cu, dose rate of 44r/min. The animals were killed with an overdose of nembutal. Tissues were fixed in 10 per cent neutral formalin and decalcified in formic acid-sodium formate solution. Tissues were imbedded in celloidin, cut at 8-10 ju, and stained with hematoxylin-eosin-azure II.
