Eruption of primary and permanent dentitions

Eruption of teeth is the process by which developing teeth move through the jaw bones and overlying mucosa, to emerge in their respective place within the oral cavity eventually.

Now, humans are diphyodonts, which means we have two sets of teeth during our lifetime.

The first set, the primary teeth, also called deciduous teeth, or baby teeth, start erupting at around six months of age.

And, by about six years of age, the primary teeth fall out and are replaced by a set of permanent teeth that stays with you for the rest of your life.

Tooth eruption occurs in 3 phases, the pre-eruptive, eruptive, and post eruptive phase.

The driving force behind the eruption of teeth is still unknown but is thought to be due to a combination of a few factors, which include signals originating from the dental follicle; root formation and elongation, which could drive the tooth in order to acquire space within the jaw; and formation of PDL which provides an occlusal force, aiding in eruption.

Okay, let’s look at all the stages one by one starting with the pre-eruptive phase.

In this phase, the tooth germs are growing within the jawbones.

As the bones grow in length, width, and height, more space becomes available for the tooth germs to spread out and grow without affecting each other.

Lengthening of the jawbones allows the anterior tooth germs to move mesially, or forward, and the molar tooth germs to move distally, or backward.

At the same time, as the bone widens, the tooth germs move buccally, or outward, and as the height increases, the tooth germs move closer to the surface; that is, the mandibular tooth germs move upwards, and the maxillary tooth germs move downwards.

In this phase, movement of the tooth germ is achieved by two methods - bodily movements and eccentric growth.

In bodily movement, there is a remodeling of the bony crypts through resorption of bony crypt wall in the direction that the tooth is moving.

This is followed by depositing new bone on the opposite crypt wall, hence creating a path for the tooth germ.

Eccentric growth is where a part of the tooth grows relatively faster than the rest of the tooth.

Eccentric growth helps the crown of the tooth reorient itself as the jawbone keeps growing.

Now the position of permanent tooth germs is also changing in relation to the primary tooth germs.

At the beginning of the pre-eruptive phase, the successional tooth germs lie lingual to their primary predecessors, with their occlusal surfaces on the same plane.

And, by the end of the pre-eruptive phase, the permanent anterior teeth ends up lingual and apical to the primary anterior teeth.

And the permanent premolars come to rest underneath the divergent roots of the primary molars.

However, this change in position is more due to the movement of the primary tooth and growth of the jaws, rather than the movement of the permanent tooth itself.

Now, you’d think that without a primary predecessor, the permanent molars would be developing unhindered.

But their location right near the ends of the jaw is a very cramped up space.

And so, the permanent molar tooth germs in the maxilla grow with their crowns tilted distally, and those in the mandible have their crowns tilted mesially.

Only when the jaw grows to a sufficient size can the permanent molars move into a more vertical position.

Next, the tooth germs enter the eruptive phase, or the pre-functional eruptive phase, which begins simultaneously with root formation.

The tooth ascends through the bony crypt and the connective tissue and finally comes into contact with the oral epithelium.

The reduced enamel epithelium over the tooth crown then proliferates along with the overlying oral epithelium to form a single membrane.

And this epithelium is pierced by the tooth to finally enter the oral cavity.

The fused epithelium lining the sides of the tooth is now called the functional, or attachment epithelium.

Through the rest of the eruptive phase, the tooth gradually rises into the oral cavity till it reaches the plane of functional occlusion.

Now, several changes are happening in tissues surrounding the teeth that help in the eruptive phase.

In the region above each tooth, a path, called the eruptive pathway, is cleared through the bone and connective tissue.

The reduced enamel epithelium covering the dental follicle secretes colony stimulating factor 1, or CSF-1, and transforming growth factor beta-1, or TGF-beta-1, into the surrounding tissue.

The CSF-1 and TGF-beta-1 attract tons of monocytes that then differentiate into osteoclasts.

These osteoclasts eat away, or resorb, the part of the bony crypt overlying the tooth.

Without the bony crypt, the dental follicular cells become continuous with the cells of the lamina propria of the overlying oral mucosa.

The reduced enamel epithelium then goes on to secrete some enzymes that break down overlying the connective tissue, nerves and blood vessels, forming an eruption pathway for the tooth.

The lack of blood vessels and nerves helps in a relatively bloodless and painless eruption.

Now, as the tooth erupts, the epithelial root sheath begins to proliferate, and starts to form the root.

At the same time, bundles of collagen fibers start appearing in the surrounding dental follicle.

The collagen bundles get invaded by fibroblasts that lay down fibrous tissue, and thickens the bundles to form the periodontal ligament, or PDL.