The document summarizes the process of primary tooth shedding and replacement by permanent teeth. It describes how odontoclasts, cells similar to osteoclasts, initiate root resorption through secretion of acids and enzymes. This causes dissolution of the dental hard tissues and degradation of the organic matrix. Shedding occurs through intermittent periods of root resorption by odontoclasts and recovery periods where tissues are repaired, until the tooth is loosened and lost.

1. By Hesham Dameer

2. Definition : shedding or exfoliation of deciduous teeth
is a term given to describe the physiologic process that
ultimately leads to replacement of the deciduous teeth
by their corresponding permanent successors .
Significance :
1) the small deciduous teeth cannot grow in size to
accommodate the growing jaw, thus , another
generation of teeth is needed to fulfill this requisity .
1) Growth of muscle of mastication from infant to
adult leading to increase the masticatory force so
the periodontal ligament of deciduous teeth can not
withstand the masticatory force .
.

3. Pattern of shedding :
a. Shedding of deciduous anterior teeth :
the permanent incisors and canines tooth germs
initially develop in apicolingual position to their
deciduous predecessors , so, the permanent anterior
tooth germs move into an incisolabial direction and, in
latter stages, they are frequently located apical to their
deciduous predecessors . Thus, the resorptive process is
initiated on apico-lingual root surface, and then
proceeds in a transverse plane apically . This secures
the replacement of the primary teeth by their permanent
successors in the exact position

5. b. Shedding of deciduous molars :
the premolars begin their development lingual to
their corresponding primary molars .
In later stage, however , they are frequently found
between the divergent roots of the primary molars
.Therefore resorption of the roots of deciduous
molars first begins on their inner surfaces because
The early developing bicuspids are found between
them, then Come to lie apical to deciduous molars .
At this time , the developing premolars become away
and pressure is relieved from deciduous root so the
areas of early resorption are repaired by
deposition of new-cementum like tissue.

6. Later when the bicuspids begin to erupt
resorption of the deciduous molars is again
initiated and continuous until the roots are
completely lost and the tooth is shed .

7. Histology of shedding
Odontoclast
exfoliation of the primary teeth takes place by a
continuous resorption of their roots by cells having
identical histology to osteoclasts (osteoclasts are
bone resorbing cells ) .
Since these cells are concerned in the resorption of
the dental tissues ,so they are referred to as
odontoclasts . These cells are capable to resorb all
dental hard tissues even the enamel .
Odontoclasts resorb hard tissue by separating
mineral from the collagen matrix through the action
of hydrolytic enzymes

8. Origin of Odontoclast :
It is presume that, they have the same origin of
osteoclasts , that is monocytes . An alternative origin
of odontoclasts is the undifferentiated mesenchymal
cells .
Morphology :
The odontoclasts are easily recognized , with light
microscope , in clusters rather than singly and appear
occupying hollowed-out shallow depressions known
Howship's lacunae .
By scanning electron microscope: these lacunae seem
not as small focal bays but rather long shallow troughs
. This indicates that during the resorptive process , the
odontoclasts continuously move inside the resorbing
dentin .

9. An odontoclast is a large cell that is characterized by
multiple nuclei and a cytoplasm with a homogeneous,
"foamy" appearance. This appearance is due to a high
concentration of vesicles and vacuoles. At a site of
active dentin resorption, the odontoclast forms a
specialized cell membrane, the "ruffled border“ ( brush
border ) , which faces the surface of the dentin tissue.
The ruffled border facilitates removal of the dentin
matrix. The ruffled border increases surface area
interface for dentin resorption. The mineral portion of
the matrix (called hydroxyapatite) includes calcium and
phosphate ions. These ions are absorbed into small
vesicles which move across the cell and eventually are
released into the extracellular fluid, thus increasing
levels of the ions in the blood.

10. By transmission electron microscope :
 The portion of cell membrane facing the resorbing
bone is thrown into numerous folds that may invaginate
the cytoplasm up to 2-3 micrometer deep. The regional
cytoplasm adjacent to the brush border appear devoid of
cell organelles but rich in actin and myosin (the
attachment zone ) which are presumed to provide an
attaching system for odontoclast to the dentin surface .
 The remainder of the odontoclasts is heavily laden
with mitochondria and vesicles especially concentrated
beside the ruffled border . Also the cytoplasm contains a
large number of nuclei, well developed , tightly packed
Golgi saccules while several small vesicles (presumed
to be primary lysosomes) are located peripheral to these
saccules .

11. Distribution : the odontoblasts occupy variable positions
and this depends on the different pattern of resorption
occurring in the different teeth . They are located on
the root surface , to resorb both cementum and dentin
in relation to the site of pressure exerted by the
erupting permanent successor . The odontoclasts have
occasionally found in the root canals or pulp chamber
lying against the predentin surface .
The pattern of resorption of single rooted teeth
ultimately leads to shedding of the primary teeth before
their roots are completely resorbed and the erupting
permanent tooth compresses on the outer root surface
therefore, the odontoclasts are not found in the pulp
chamber but on the root surface . However, in
multirooted teeth, odontoclasts are seen in the pulp
chamber .

12. Odontoclast

15. Histological features of teeth undergoing
shedding:
Root surfaces exhibit resorption lacunae and
odontoclast cells are often associated with these
concavities. It is significant that periodontal fibroblasts
in the area show signs of impaired function. 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.

20. Mechanism of action during resorption of
mineralized tissues:
odontoclasts 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 odontoclasts cells are then able to
break down this collagenous organic matrix.

23. The histochemical study have evidenced that the
odontoclasts characteristically contain a high level
of acid phosphatase activity .
Mechanism of shedding :
I) Initiation of shedding
Two factors are presumed to initiate shedding of
the primary teeth :
a. pressure factor which leads into the
differentiation of the odontoclasts
* pressure of the erupting permanent tooth .
* augmentation of the masticatory forces
b. genetic factor is probably responsible for the
initiation of root resorption and an ultimate
shedding of primary teeth .

24. II) Process of shedding :
The scanning and transmission electronmicrographs
demonstrate the presence of mineral crystallites in the
depth of the brush border enfolding .This denotes that,
during resorption, the minerals of the dental hard
tissues are primarily removed .
It is presumed that the intracellular vesicles are thought
to be primary lysosomes, which discharge their content
extracellularly among the brush border, thus creating an
acidic medium . Such medium causes minerals
dissolution. However the removal of the dissolved
minerals could be mechanically facilitated by the
folding of the brush border .
The disintegration of the organic matrix occurs by
secretion of the proteolytic enzymes by odontoclast

25. into smaller molecular components and then degraded
intracellularly by the vesicle containing-acid
phosphatase enzyme found beside the brush border .

26. Tissue and cellular changes:
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.

27. Intermittent resorption
During root resorption,
periods of resorption
are alternated by
periods of cementum
repair.
Cementoblasts
deposit cementum in
areas of resorption
forming a reversal
line. Some
cementoblasts
become embedded in
the cementum and
are then called
Cementocytes.
A, Reversal line; B, Cementoblasts; C,
Cementocyte.

28. Dr.Hesham Dameer