Oral Histology & Embryology Dr.
Ban
Lec25
Tooth shedding
The necessity for two dentitions exists because infant jaws are small and the size
and number of teeth they can support is limited. Since teeth, once formed, cannot
increase in size, a second dentition, consisting of larger and more teeth, is
required for the larger jaws of the adult. The physiologic process resulting in the
elimination of the deciduous dentition is called shedding or exfoliation
Pattern of shedding The shedding of deciduous teeth is the result of progressive
resorption of the roots of teeth and their supporting tissues ,the removal of the
dental hard tissues is accomplished by multinuclear cells similar to osteoclasts .
Pressure generated by the growing and erupting permanent tooth dictates the
pattern of deciduous tooth resorption.
At first this pressure is directed against the root surface of the deciduous tooth
itself. Because of the developmental position of the permanent incisor and canine
tooth germs and their physiologic movement in an occlusal and vestibular
direction, resorption of the roots begins on their lingual surfaces .
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Oral Histology & Embryology Dr.Ban
Later, these developing tooth germs occupy a position directly apical to the
deciduous tooth, which permits them to erupt in the position formerly occupied by
the deciduous tooth. As a result of the continued growth of the jaws and occlusal
movement of the deciduous molars, the successional tooth germs come to lie apical
to the deciduous molars.
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Oral Histology & Embryology Dr.Ban
The areas of early resorption are repaired by the deposition of a cementum-like
tissue. When the bicuspids begin to erupt, resorption of the deciduous molars is
again initiated, and this time continues until the roots are completely lost and the
tooth is shed. The bicuspids thus erupt in the position of deciduous molars.
Histology of shedding
The cells responsible for the removal of dental hard tissue are identical to
osteoclasts, and are called odontoclasts, are readily identifiable in the light
microscope as large, multinucleated cells occupying resorption bays on the
surface of a dental hard tissue.
They are smaller than osteooclast, contain fewer nuclei, and produce smaller
resorption lacunae. The cytoplasm of the odontoclast is characterized by a high
content of mitochondria and many vacuoles, Acid phosphatase activity occurs
within these vacuoles , and the surface of the cell adjacent to the resorbing hard
tissue forms a “ruffled” border . Peripheral to the ruffled border is a clear zone
represents the attachment apparatus of the odontoclast..
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Oral Histology & Embryology Dr.Ban
Odontoclasts are able to resorb all the dental hard tissues, including enamel.
They resorb all dental tissues in the same way as oseoclasts resorb bone.
Odontoclasts probably have the same origin as osteoclasts. The monocyte,
circulating in the blood, originally gives rise to all the different tissue
macrophages, including the osteoclast.
Different patterns of resorption exist for different teeth. For example, single-
rooted teeth are usually shed before root resorption is complete, therefore
odontoclasts are not found within pulp chamber of these teeth and the
odontoblasts layer remains intact. In molars, however, the roots are usually
completely resorbed and the crown is also partially resorbed before shedding.
Histology of maxilla in the mixed dentition period. Roots of erupted primary teeth are undergoing resorption.
Crowns of developing permanent teeth appear below primary teeth.
Mechanism of resorption and shedding
Odontoclasts attach to the hard-tissue surface peripherally through the clear zone,
thereby creating a sealed space lined by the ruffled border of the cell. As resorption
of the roots initiated by pressure of the underlying tooth occurs, there is a
progressive loss of surface area for attachment of the periodontal ligament fiber
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Oral Histology & Embryology Dr.Ban
bundles. The most likely sequence of events in resorption of dental hard tissue by
the odontoclast are:
1)An initial removal of minerals.
2)Extracellular dissolution of organic matrix (mainly collagen) to smaller
molecules.
Although pressure exerted by permanent successor has a key role in initiating tooth
resorption, other factors must be involved. The forces of mastication applied to the
deciduous tooth are also capable of initiating the resorption. As an individual
grows, the muscles of mastication increase in size and exert forces on the
deciduous tooth greater than its PDL can withstand. This leads to trauma to the
ligament and the initiation of resorption.
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Oral Histology & Embryology Dr.Ban
Histology of active resorption sites on primary exfoliated tooth
tooth roots.
Clinical considerations
1-Remnants of deciduous teeth :
Sometimes parts of the roots of deciduous teeth are not in the path of erupting
permanent teeth and may escape resorption. They are most frequently found in
association with the permanent premolar. Root remnants may later be found deep
in the bone, completely surrounded by and ankylosed to the bone . Progressive
resorption of the root remnants and replacement by bone may cause the
disappearance of these remnants.
2-Retained deciduous teeth :
Deciduous teeth may be retained for a long time beyond their usual shedding
schedule. Such teeth are usually without permanent successors, or their successors
are impacted. They are invariably out of function. Retained deciduous teeth are
most often the upper lateral incisor, less frequently the second permanent
premolar, especially in the mandible , and rarely the lower central incisor . A
supernumerary tooth or an odontogenic tumor may occasionally prevent the
eruption of one or more of the permanent teeth. In such cases ankylosis of the
deciduous tooth may occur.
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Oral Histology & Embryology Dr.Ban
3- Submerged deciduous teeth:
Trauma may result in damage to either the dental follicle or the developing
periodontal ligament. If this happens, the eruption of the tooth ceases, and it
becomes ankylosed to the bone of the jaw. Because of continued eruption of
neighboring teeth and increased height of the alveolar bone, the ankylosed tooth
may be either “shortened” or submerged in the alveolar bone. Submerged
deciduous teeth prevent the eruption of their permanent successors .Submerged
deciduous teeth should therefore be removed as soon as possible
