L9 Flashcards
Eruption
‘The movements a tooth makes to attain and maintain its position in the jaw’
Physiologically, a tooth will move in all directions except
apically
to attain and maintain its position in the jaw: also, normally, tooth movement
occurs throughout life.
The movements of eruption are a cellular activity involving
different cell types of
the periodontal ligament (PDL): cementoblasts, osteoblast, osteoclasts and fibroblasts
The fibroblast, most numerous
cell type of the PDL, is thought to play a key role
in tooth movement. It serves several functions within the PDL:
* secretes extracellular substance (fibroblast),
* lysis the extracellular substance (‘fibroclast’),
* may beome a contracting cell called a ‘fibromyocyte’ myofibroblast:
this cell is connected (by integrins: receptors) to principal collagen fiber bundles
that connect the tooth to alveolar bone, when it contracts the collagen fibers
contract moving the tooth:
Note: Fibronectin is a glycoprotein that exhibits elastic properties and
serves as an integrin (similar to cell integrins) that binds to
extracellular components such as collagen
Stages of Eruption:
1 Preemergence 2 Prefunctional 3 Functional
Prefunctional stage:
main direction
of movement is occlusially
Functional stage: main direction
of movement is
mesially
Histologic Views of Preemergence Stages
Starts with appearance of dental lamina and continues to the appearance of reduced enamel epithelium and Hertwig’s root sheath, just prior to start of root dentin formation. During this stage the teeth move in different directions, but their main direction of movement is facially
Preemergence
The primary tooth germ and secondary dental lamina stages
The main direction of movement is facially
Fate of enamel organ cell layers:
Reduced enamel epithelium: attached to completed enamel surface by a
basal lamina, last secretion of ameloblasts
2. Hertwig’s epithelial root sheath: inner and outer layers of enamel organ
1 Preemergence
Outer and inner layers coming together forming Hertwig’s epithelial root sheath
The sheath serves 2 main functions:
1. Determines number of root canals of tooth
2. Determines root dentin outline
Preemergence stage: before start of
root formation
Diaphragm of Hertwig’s epithelial root sheath (arrows)
Number of root canals are determined at this stage
Review of physiologic events occurring during the Preemergence Stage
Start of odontogenesis: epithelial stages and tooth germ appearances
- Differentiation of ameloblasts, odontoblasts and fibroblasts
- Appearance of crown dentin
- Enamel formation begins and is maturing
- Appearance of pulp chamber
- Establishment of dentinoenamel junction
- Enamel organ over the enamel becomes the
reduced enamel epithelium that is attached to the enamel by a
basal lamina - The outer and inner layers of the enamel organ
come together at the cevical loop to form: Hertwig’s epithelial root sheath (diaphragm) - Number of root canals a tooth will have is determined at this stage
- Bone crypt: undergoing remodeling
2 Prefunctional stage of eruption:
Starts with root dentin formation and ends when tooth reaches occlusial plane:
Main direction of movement is occlusially
End of prefunctional stage:
Beginning of functional stage
Prefuctional begins with start of
root dentin formation
Ends at the occlusial plane
Main movement is occlusially or vertically
Events occurring during prefunctional stage:
*Start of root dentin formation
* Break=up Hertwig’s sheath
*Cementum formation (cementogenesis)
*Formation of periodontal ligament
*Rreduced enamel epithelium becomes the junctional epithelium:
Note: as was true for the reduced enamel epithelium , the junctional epithelium is
attached to the tooth surface by a basal lamina
*Enterance into oral cavity
Functional stage
Functional stage starts at
occlusial plane and continues throughout life
Main movement is mesially: Referred to as ‘physialogic mesial drift’
Functional stage: Events at this stage:
- Continued dentin and cementum formation
* Age changes: decreased pulp volume, attrition, and etc.
Hertwig’s epithelial sheath (HERS):
covers root dentin – determines root dentin outline
apical end continues as diaphragm:determines number of root canals
Reduced enamel epithelium (REE)
is attached to surface
by a basal lamina
Prefunctional stage
Hertwig’s root sheath along root dentin
Left: Hertwig’s sheath in tact along forming root dentin:
Here the sheath is preforming one of its functions: determining root dentin outline
Right: cementogenesis at cervical most part of root dentin
Fate: Hertwig’s Epithelial Root Sheath Cells
- Sheath: diagragm stage – number of root canals determined: tooth erupts
occlusially from this level - Sheath elongates: determines root dentin outline.
diaphragm remains at relatively same level - Root dentin forms: Sheath breaks - up: Cementum forms on exposed dentin:
this sequence continues apically until root dentin is completed
Epithelial rest of Malassez
Remanents of epithelial cells from Hertwig’s root sheath in the periodontal ligament
Appear during prefuctional stage of eruption
A = During the prefunctional stage of eruption, the merged oral epithelium and
reduced enamel epithelium undergo
cell death
B = The coronal most part emerges into oral cavity
