Oral Histology
Oral Histology, HSTL 451
Shedding of Primary Teeth

Specific Objectives:

The student should be able to:

1. Define shedding (exfoliation) of primary teeth.
2. Describe the histological features of the tooth and its supporting tissues during shedding.
3. Describe the characteristics of the cells involved in resorption of mineralized tissues at the light and the electron microscopic levels.
4. Describe the mechanisms involved in resorption of mineralized tissues.
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 teeth.

Session Outline:

Definition: Shedding is the physiological elimination of primary teeth at specific ages.
Histological features:


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 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.

Contributing factors:

1- Increased masticatory forces.
2- Weakened supporting structures e. g. loss of cementum, alveolar bone and periodontal attachment.
3- Pressure due to erupting successor.

Clast cells: Cytological features:

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 av3 integrin.

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 submerged teeth:

-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 alveolar bone.
-A shortened tooth is a retained primary tooth which is smaller than the adjacent larger permanent teeth.
-A 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

Specific Objectives:

The student should be able to:

1. Describe the immediate changes in the periodontal ligament and alveolar bone induced by orthodontic tooth movement.
2. Describe 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.
4. Define hyalinization of the periodontal ligament.
5. Describe remodeling following hyalinization of the periodontal ligament.

Session Outline;

Periodontal changes on the tension side:

1. Alveolar bone apposition.
2. Straightening of periodontal ligament fiber bundles.

It should be stressed that periodontal fibers are collagenous fibers which do not stretch.

3. Cellular proliferation.
4. Widening of the periodontal space.

Periodontal changes on the pressure side:

1. Alveolar bone resorption.
2. Compression and disorganization of periodontal ligament fibers.
3. Increased vascularity at sites of bone resorption.
4. 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 resorption:

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.

Suggested Reading:

Nanci, A. (Ed.): Ten Cate's Oral Histology-Development, Structure and Function, Sixth edition, Mosby, 2003, Chapter 10, pp 282-295.