|Oral Histology, HSTL 451
Shedding of Primary
The student should be
1. Define shedding (exfoliation) of
2. Describe the histological features of the
tooth and its supporting tissues during shedding.
the characteristics of the cells involved in resorption of
mineralized tissues at the light and the electron microscopic
4. Describe the mechanisms involved in resorption of
5. List the factors contributing to
shedding of primary teeth and explain how these factors act. 6.
Define the terms: ankylosed, retained, shortened and submerged
Definition: Shedding is the physiological
elimination of primary teeth at specific ages.
Resorption of the roots of primary teeth starts
at the lingual apical areas in the anterior teeth and in the
interradicular areas of molars. It is related to where the erupting
permanent successors are located.
-Tissue and cellular
Shedding is an intermittent process with
periods of resorption involving alveolar bone, cementum and root
dentin resorption by clast cells, osteoclasts and odontoclasts,
respectively and recovery periods when osteoblasts and cementoblasts
replace part of the resorbed tissues. Eventually more resorption
takes place and when the tooth loses its supporting periodontal
tissues, it is shed. During this process the primary teeth become
loose during the periods of resorption and tighten during the brief
periods of apposition.
1- Increased masticatory forces.
Weakened supporting structures e. g. loss of cementum, alveolar bone
and periodontal attachment.
3- Pressure due to erupting
Clast cells: Cytological
Clast cells are large multinucleated cells
with a ruffied border and numerous lysosomes and mitochondria.
Osteoclasts and odontoclasts are morphologically similar and seem to
have the same origin and mechanism of action. The rationale for
using different names for these cells is to reflect the specific
tissue that is being resorbed.
Mechanism of action
during resorption of mineralized tissues:
Clast cells act by isolating an area of hard
tissue (bone, cementum, dentin or even enamel) using clear
cytoplasmic areas (no organelles) and through plasma membrane
associated enzymes that act as proton pumps, the isolated area's pH
is lowered making it acidic. This acidity breaks down the
hydroxyapatite crystals of the inorganic content and also denature
the collagenous organic matrix. Essentially denaturing makes the
tightly assembled collagen fibrils looser. The proteolytic enzymes
both secreted and within lysosomes in the clast cells are then able
to break down this collagenous organic matrix. It should be
emphasized that osteoclasts do not produce mammalian collagenase. In
case of resorption of mineralized tissues collagenase is not needed
since the acidic environment induced by osteoclasts denature
collagen and provide for its degradation by proteolytic enzymes. The
attachment of clast cells to the surface of a mineralized tissue
undergoing resorption is mediated by specific molecules such as avß3
Histological features of
teeth undergoing shedding:
Root surfaces exhibit
reorption lacunae and clast cells are often associated with these
concavities. It is significant that periodontal fibroblasts in the
area show signs of impaired function, most notably disrupted
secretion as well as cells with nuclear alterations indicating
apoptotic incidence. The fact that programmed cell death is seen
during shedding that occurs at specific ages is consistent with the
concept that shedding is a genetically determined process.
It should be emphasized that the pulp tissue in teeth
undergoing shedding appears histologically normal except that neural
elements seem to be missing. Thus the pulp does not contribute to
the process of shedding and plays a passive role in this process.
Retained, ankylosed, shortened and
-A retained tooth is one that
remains in the dental arch beyond the age at which it is supposed to
be shed. Many conditions cause primary teeth to be retained for
example root ankylosis or the absence of a permanent successor.
-An ankylosed tooth is one that have its root( s) fused to the
-A shortened tooth is a retained primary tooth
which is smaller than the adjacent larger permanent teeth.
submerged tooth is a retained tooth that becomes surrounded by
alveolar bone. This condition is created by the loss of adjacent
primary teeth and the accompanying resorption of their alveolar
bone. When the permanent successors erupt they have their own
alveolar bone which covers the retained tooth.
Histology of Orthodontic Tooth Movement
The student should be able to:
Describe the immediate changes in the periodontal ligament and
alveolar bone induced by orthodontic tooth movement.
the difference between effects of orthodontic tooth movement on
alveolar bone and cementum.
3. List the effects of using
excessive force during orthodontic tooth movement.
hyalinization of the periodontal ligament.
remodeling following hyalinization of the periodontal ligament.
Periodontal changes on
the tension side:
1. Alveolar bone apposition.
Straightening of periodontal ligament fiber bundles.
should be stressed that periodontal fibers are collagenous fibers
which do not stretch.
3. Cellular proliferation.
Widening of the periodontal space.
Periodontal changes on
the pressure side:
1. Alveolar bone resorption.
Compression and disorganization of periodontal ligament fibers.
3. Increased vascularity at sites of bone resorption.
Narrowing of periodontal space.
Hyalinized periodontal ligament:
Pressure induced by forces greater in
magnitude than those tolerated biologically lead to partial loss of
vitality of the periodontal ligament i.e. it becomes hyalinized. It
shows no cells or individual fibers or vascular elements. The
hyalinized periodontal ligament is not necrotic and it remodels
after the pressure is relieved.
Pattern of alveolar bone
1. Periodontal surface resorption: ocurrs when
the force applied is at the biological level or blow that level.
2. Undermining resorption: occurs adjacent to areas where
the periodontal ligament is hyalinized and clast cell resorption
starts at the medullary surface.
(Ed.): Ten Cate's Oral Histology-Development, Structure and
Function, Sixth edition, Mosby, 2003, Chapter 10, pp 282-295.