The term active tooth eruption implies the emergence of a crown into the oral cavity. In general, however, the term refers to the total life span of the tooth, from the beginning of crown development until the tooth is lost or the individual dies. This eruptive process is usually divided into three stages, and although there may be some difference in the terminology, they refer to the same mechanism.
The preeruptive stage begins as the crown starts to develop. Recall that dental lamina formation—bud, cap, and bell stages, as well as the calcification of the crown—takes place in the connective tissue beneath the oral epithelium. During this time the bone of the maxilla or mandible surrounds the developing primary tooth in a U-shaped crypt or beginning socket (Fig. 22-1).
The eruptive movement associated with the preeruptive stage is of two varieties—spatial and excentric. In spatial movement the crown develops while the bottom of the socket fills in with bone, pushing the crown toward the surface. A similar facial movement accompanies jaw growth. In excentric, or off-center, growth, the crown of a tooth does not grow in a perfectly symmetrical pattern. As the crown enlarges, it grows more in one area than in another, and so the tooth seems to be moving because the center of the tooth is shifting. This can be visualized by blowing up a small round balloon to a diameter of 3 to 4 inches. Put a mark on the center of the balloon and continue to blow it up to a diameter of 8 to 10 inches. Again mark the center of the balloon. Because the balloon walls may not be of equal thickness, one area may expand more than another. Thus the center of the balloon moves from the original marking. This same principle can be applied to the developing crown. It appears to have moved because the center point of the developing crown has shifted. This is the activity of the preeruptive stage. It involves crown growth and some movement toward the surface while the crypt is developing.
The eruptive stage or prefunctional eruptive stage begins with the development of the root. In Chapter 21 the development of the root and Hertwig’s epithelial root sheath were discussed. The root develops in a crypt of bone. As it begins forming, osteoclasts temporarily may deepen the crypt by resorbing bone at the bottom to accommodate for the increase in root length. While the root continues to lengthen, the tooth begins to move toward the surface of the oral cavity. As it approaches the oral cavity, the alveolar bone is growing to keep pace with it. However, in time the tooth moves faster than the growing alveolar bone and approaches the surface of the oral epithelium and breaks into the oral cavity.
The crown of the tooth is surrounded by reduced enamel epithelium. Around the reduced enamel epithelium there are cells of the dental sac, or follicle, that covers the crown. Cells of the dental follicle form a cord of connective tissue epithelium. This fibrous cord is known as the gubernacular cord and forms a gubernacular canal that leads the way and, with the help of macrophages and osteoclasts, breaks down the bone between the tooth and the surface oral epithelium for the primary tooth to erupt. As the tooth moves to the surface, the reduced enamel epithelium moves with it until it compresses the connective tissue and causes it to disintegrate. The reduced enamel epithelium then contacts the oral epithelium, and these two layers fuse into one layer—the united oral epithelium. The tooth breaks through this layer and emerges into the oral cavity. It is believed that this breakdown of epithelium is caused by an enzyme probably produced by the reduced enamel epithelium. This stage continues until the erupting teeth meet the opposing teeth.
For both primary and secondary dentition, tooth movement in the eruptive stage tends to be occlusal and facial, more facial in the anteriors than in the posteriors. When we think about the pathway for the secondary teeth, we have to consider their mechanism for development. As discussed in Chapter 19, the successional lamina buds off the dental lamina and forms a permanent tooth at its end, still partially attached by the successional lamina. As the permanent