Eruptive Mechanism and Movement in the First Molar of the Rat

Abstract

This histologic study on the eruptive movement of the rat first molar is based on serial frontal sections through the lower first molar region of 20 ia and 34 normal rats ranging in age from 13 days' insemination age to 30 days after birth. Since the ia rats are characterized by absence of bone resorption, a number of fixed points are retained in the ia mandible. These fixed points permit accurate comparisons of succeeding developmental stages in the ic animals. A comparison of ia and normal rats was made to study the areas and role of bone resorption during tooth movements. The findings follow. 1. The developing tooth of the rat first molar is initiated at 13 days' insemination age. Between 20 and 21 days' insemination age, the inner enamel epithelium differentiates into ameloblasts, and adjacent connective tissue cells form odontoblasts, with subsequent dentin formation at 21 days' insemination age. At 3 days after birth, enamel formation begins. At 10 days after birth, the formation of the root begins, and Hertwig's epithelial sheath appears. The tooth “breaks through” the oral epithelium around 17 days after birth and comes into functional occlusion at 23 days. Roots are completed at 30 days after birth. 2. The movement of the tooth germ (eruption) begins soon after its initiation and continues until 30 days of age. Prior to the beginning of root formation, the tooth germ moves upward and outward and thus keeps a constant relation with the growing alveolar process. Associated with the beginning of root formation, the tooth moves more rapidly toward the oral cavity and tilts outward. This movement is accompanied by selective, rapid bone formation on crypt wall. After the tooth comes into functional occlusion the apposition of bone in the crypt ceases. 3. The cervical loop region of the tooth germ grows deeper into the jaws for a short distance. 4. During development the crown of the first molar is inclined in the jaws in such a way that its ocelusal surface faces upward and inward. This tilted position of the tooth is corrected during eruption. The tilted position of the developing crown is due to the fact that the molar teeth of “adult size” develop in a relatively smaller jaw. 5. In early stages of development, the dental follicle consists of loosely arranged mesenchymal cells with abundant intercellular fluid. With age the follicle changes to a highly cellular structure with fusiform cells arranged parallel to the tooth surface. During these stages, the follicle is wider on the lingual and inferolingual sides of the tooth germ. These are the sites from which the tooth germ migrates. Still later, collagen fibers appear among the fusiform cells. When the tooth erupts into the oral cavity, further organization of periodontal fibers occur, and they extend from the bone to the cementum. When the tooth comes into functional occlusion, the periodontal membrane becomes densely collagenized and the periodontal space widens. The widening is a result of resorption on the socket wall. 6. The eruption is not a result of root elongation, bone apposition, dentin or enamel formation, but probably is a result of growth of the follicle. This growth is analogous to the growth in a suture. The connective tissue growth in the follicle leads to migration of tooth from the crypt. It is suggested that as in the bones at a suture, the growth of the root and apposition at the bottom of the crypt are the results of tooth migration and not its cause. © 1958, SAGE Publications. All rights reserved.