Oral Histology &Embryology Dr.

Ban

Lec 24

Tooth eruption

Tooth eruption is a complex event that involves many types of tissues, and
developing teeth become surrounded by alveolar bone but must maintain a cavity
or crypt that is free from bone to allow for their morphogenesis.

The location and timing of tooth development and eruption must be highly
coordinated in order to achieve a functional and esthetic adult dentition. Eruption
begins only after crown formation is complete, progresses at various speeds during
different stages, and continues after occlusal contact to increase and maintain facial
height throughout life and to compensate for masticatory wear.

Tooth eruption is the process by which developing teeth move from their site of
development in the maxilla or mandible, emerge through the soft tissue of the jaws
and the over lying mucosa to enter the oral cavity.The eruption of a tooth requires
a complex spatial and temporal coordination of molecular and cellular events
among several tissues.

Physiologic tooth movement is described as consisting of the following:

1. Preeruptive tooth movement
2. Eruptive tooth movement
3. Posteruptive tooth movement

1
Oral Histology &Embryology Dr.Ban

Three stages of tooth eruption. (A) Schematic diagrams of tooth eruption in mice during
tooth root formation. Pre-eruptive tooth movement: tooth germs within alveolar bone
before beginning to erupt. Eruptive tooth movement: movement from its position within
the alveolar bone to its functional position in occlusion. Posteruptive tooth movement:
tooth maintains its fully erupted functional position in occlusion, while the jaws continue to
grow. (B) Histologic sections of molars at different time points during tooth root formation
in mice.

Pattern of tooth movement

The teeth develop within bony crypts of the mandible and maxilla, surrounded by
the dental follicles, and initially connected to the oral epithelium via the dental
lamina.
The permanent successors to the primary teeth initially are located in the same
bony crypt as the primary tooth, lingual (palatal) to the primary tooth

A-Pre-eruptive tooth movement: Made by the deciduous and permanent tooth

germs within tissues of the jaw before they begin to erupt.

When the deciduous tooth germs first differentiate, they are extremely small, and a
good deal of space is available for them in the developing jaw. Because they grow
rapidly, they become crowded. A lengthening of the jaws, which permits the
deciduous second molar tooth germs to move backward and the anterior germs to
2
Oral Histology &Embryology Dr.Ban

move forward gradually, alleviates this crowding. At the same time the tooth
germs are moving bodily outward and upward (or downward, as the case may be)
with the increasing length and width and height of the jaws. Preeruptive
movements occur in an intraosseous location and are reflected in the patterns of
bony remodeling within the crypt wall which is achieved by the selective
deposition and removal of bone by osteoblastic and osteoclastic activity.

Preeruptive movements of teeth are a combination of two factors:

1-total bodily movement of the tooth germ : During bodily movement in a mesial
direction, bone resorption occurs on the mesial surface of the crypt wall, and bone
deposition occurs on the distal wall as a filling-in process.

2- eccentric growth in which one part of the tooth germ remains fixed while the
rest continues to grow, leading to a change in the center of the tooth germ. This
growth explains, for example, how the deciduous incisors maintain their position
relative to the oral mucosa as the jaws increase in height.During eccentric growth,
only bony resorption occurs, thus altering the shape of the crypt to accommodate
the altering shape of the tooth germ.

Histologic section showing teeth from the permanent

dentition developing between the roots of the
corresponding deciduous teeth. The roots of the
molar on the left hand side are being resorbed.
3
Oral Histology &Embryology Dr.Ban

The permanent molars, which have no deciduous predecessors, also exhibit

movement. For example, the upper permanent molars, which develop in the
tuberosity of the maxilla, at first have their occlusal surfaces facing distally and
swing around only when the maxilla has grown sufficiently to provide the
necessary space. Similarly, mandibular molars develop with their occlusal surfaces
inclined mesially and only become upright as room becomes available.

4
Oral Histology &Embryology Dr.Ban

B-Eruptive tooth movement

During the phase of eruptive tooth movement the tooth moves from its position
within the bone of the jaw to its functional position in occlusion, and the principal
direction of movement is occlusal or axial .

The mechanisms of eruption for deciduous and permanent teeth are similar,
resulting in the axial or occlusal movement of the tooth from its developmental
position within the jaw to its final functional position in the occlusal plane.

The prefunctional eruptive phase starts with the initiation of root formation and
ends when the teeth reach occlusal contact. Major events occuring during this
phase are:

1.Root formation requires space for the elongation of the roots. The first step in
root formation is proliferation of the epithelial root sheath, which in time causes
initiation of root dentin and formation of the pulp tissues of the forming root.

Root formation also causes an increase in the fibrous tissue of the surrounding
dental follicle .

2. Movement occurs incisally or occlusally through the bony crypt of the jaws to
reach the oral mucosa. The movement is the result of a need for space in which the
enlarging roots can form. The reduced enamel epithelium next contacts and fuses
with the oral epithelium.

3. Penetration of the tooth's crown tip through the fused epithelial layers allows
entrance of the crown enamel into the oral cavity. Only the organic developmental
cuticle (primary), secreted earlier by the ameloblasts, covers the enamel

4. Intraoral occlusal or incisal movement of the erupting tooth continues until

clinical contact with the opposing crown occurs. The crown continues to move

5
Oral Histology &Embryology Dr.Ban

through the mucosa, causing gradual exposure of the crown surface, with an
increasingly apical shift of the gingival attachment.

Histologic features

The prefunctional eruptive phase is characterized by significant changes in the

tissues overlying, surrounding, and underlying the erupting teeth.

Overlying the teeth

 The dental follicle changes and forms a pathway for the erupting teeth. A
zone of degenerating connective tissue fibers and cells immediately overlying
the teeth appears first .
 During the process, the blood vessels decrease in number, and nerve fibers
break up into pieces and degenerate.
 The altered tissue area overlying the teeth becomes visible as an inverted
triangular area known as the eruption pathway. In the periphery of this zone,
the follicular fibers; regarded as the gubernacular cord , are directed toward
the mucosa. As the successional tooth erupts, its gubernacular canal is
widened rapidly by local osteoclastic activity, delineating the eruptive
pathway for the tooth.In a dried skull, holes can be identified in the jaws on
the lingual aspects of the deciduous teeth. These holes, which once
contained the gubernacular cords, are termed gubernacular canals

6
Oral Histology &Embryology Dr.Ban

Macrophages appear in the eruption pathway tissue. These cells cause the
release of hydrolytic enzymes that aid in the destruction of the cells and
fibers in this area with the loss of blood vessels and nerves.Osteoclasts are
found along the borders of the resorptive bone overlying the teeth. This bone
loss adjacent to the teeth keeps pace with the eruptive movements of the
teeth.

Surrounding the teeth

The tissues around the teeth change from delicately fine fibers lying parallel to the
surface of the tooth to bundles of fibers attached to the tooth surface and extending
toward the periodontium. The first fibers to appear are those in the cervical area as

7
Oral Histology &Embryology Dr.Ban

root formation begins. As the root elongates, bundles of fibers appear on the root
surface. Fibroblasts are the active cells in both the formation and the degradation
of the collagen fibers. With tooth eruption, the alveolar bone crypt increases in
height to accommodate the forming root. After the teeth attain functional
occlusion, the fibers gain their natural orientation. During eruption collagen fiber
formation and fiber turnover are rapid,occurring within 24hours. This mechanism
allow fibers to attach and release and attach in rapid succession.

Gradually the fibers organize and increase in number and density as the tooth
erupts into the oral cavity. Blood vessels then become more dominant and exert
additional pressure on the erupting tooth.

Underlying the teeth

As the crown of a tooth begins to erupt, it gradually moves occlusally, providing

space underlying the tooth for the root to lengthen . In the fundic region these
changes in the soft tissue and the bone surrounding the root apex are believed to be
largely compensatory for the lengthening of the root. Fibroblasts form collagen
around the root apex, and these fiber bundles become attached to the cementum as
it begins to form on the apical dentin. Fibroblasts appear in great numbers in the
fundic area, and some of these fibers form strands that mature into calcified
trabeculae. These trabeculae form a network, or bony ladder, at the tooth apex
filling the space left behind as the tooth begins eruptive movement .

Gradually this delicate bone ladder becomes denser as additional bony plates
appear . The bony plates remain until the teeth are in functional occlusion at the
end of this phase. Dense bone then forms around the tooth's apex, and bundles of
fibers attach to the apical cementum and extend to the adjacent alveolar bone to
provide more support.

8
Oral Histology &Embryology Dr.Ban

c-Functional eruptive phase

This movement maintains the tooth position in occlusion by compensation for

occlusal and proximal tooth wear. The post-eruptive phase starts when the teeth
attain occlusion and continues for as long as each tooth remains in the oral cavity
The final eruptive phase takes place after the teeth are functioning and continues as
long as the teeth are present in the mouth. During this period of root completion,
the height of the alveolar process undergoes a compensating increase. The fundic
alveolar plates resorb to adjust for formation of the root tip apex.

The root canal narrows as a result of root tip maturation, during which the apical
fibers develop to help cushion the forces of occlusal impact. Root completion
continues for a considerable length of time, even after the teeth begin to function.

At that time, the mineral density of the alveolar bone increases, and the principal
fibers of the periodontal ligament increase in dimension and change orientation in
their mature state. These fibers separate into groups oriented about the gingiva, the
alveolar crest, and the alveolar surface around the root. Such fibers stabilize the
tooth to a greater degree, and the blood vessels become more highly organized in
the spaces between the bundles of fibers.
9
Oral Histology &Embryology Dr.Ban

Later in life, attrition and abrasion may wear down the occlusal or incisal surface
of the teeth, causing the teeth to erupt slightly to compensate for this loss of tooth
structure. Any such change results in deposition of cementum on the root's apex.

MECHANISM OF TOOTH MOVEMENT

(THEORIES OF TOOTH ERUPTION)

1- Root growth:
The root growth theory supposes that the proliferating root impinges on a fixed
case, thus converting an apically directed force into occlusal movement.
2- Bone remodeling:
The bone remodeling theory supposes that selective deposition and resorption of
bone brings about eruption.
3-Vascular pressure in periapical area:
supposes that a local increase in tissue fluid pressure in the periapical region is
sufficient to move the tooth.
4-Periodontal ligament traction:
The ligament traction theory proposes that the cells and fibers of the ligament pull
the tooth into occlusion

CLINICAL CONSIDERATIONS

1-Tooth eruption is a complex and multistep process that includes different types
of tooth growth and movements within the bony crypt in order for the tooth to
erupt into the genetically designated area of the maxilla or mandible. To
accomplish eruption, bone remodeling by osteoclasts (resorption of bone) and

10
Oral Histology &Embryology Dr.Ban

osteoblasts (bone deposition) must take place in a coordinated manner.

2-Teeth are considered submerged when eruption is prevented because of
crowding or tipping of the adjacent teeth into the space created by the missing
primary tooth.
3-Lack of eruption resulting from failure of root formation may be caused by
crowding of teeth, crown-to-root fusion, or may occur due to hereditary disease
like osteopetrosis(defect in osteoclasts).

11