Enamel Formation (Amelogenesis) PDF

Enamel formation (Amelogenesis) Dr: Abdelatif Galal Lecturer of oral Medicine & Periodontology Enamel formation(Amelogenesis) Amelogenesis begins shortly after dentinogenesis at the advanced or late bell stage. Ameloblasts fully differentiate at the growth center...

Enamel formation (Amelogenesis) Dr: Abdelatif Galal Lecturer of oral Medicine & Periodontology Enamel formation(Amelogenesis) Amelogenesis begins shortly after dentinogenesis at the advanced or late bell stage. Ameloblasts fully differentiate at the growth centers located at the cusp tips or incisal edges and progressing along the sides of the crown toward the cervical margin. Amelogenesis is a complex process involves 2 stages: 1- Enamel matrix deposition. 2- Mineralization of enamel matrix. 1- Enamel matrix deposition It means the secretion of the enamel matrix by ameloblasts The freshly secreted enamel matrix contain 30% minerals as hydroxyapatite crystals and 70% waters and enamel proteins which include 90% amelogenin protein and 10% nonamelogenins protein. 2- Mineralization of enamel When the full thickness of enamel matrix has been deposited , mineralization will be started This process involved additional minerals with the removal of organic material and water to reach 96% mineral content. The source of minerals during maturation are from : ▪ Ameloblast. ▪ Another cells of enamel organ like stratum intermedium. ▪ Capillaries which are approach to the outer enamel epithelium. Life cycle of ameloblasts : include the following stages: 1- Presecretory. 2- Secretory. 3- Transitional. 4- Maturative. 5- Protective. 6- Desmolytic. 1-pre secretory stage It includes: A- Morphogenic phases. B-Differentiation phases. A-Morphogenic stage During early bell stage; IEE cells can undergo mitosis at the cervical region. During this stage the cells are short columnar with large oval nuclei that almost fill the cell body.  B-Differentiation stage IEE cells elongate and their nuclei shift toward the stratum intermedium. Thus the ameloblast becomes polarizes cell, with the majority of its organelles situated in cell body at distal portion..  2-Secretory stage The cells of ameloblasts develop blunt processes which penetrate the basal lamina and protrude into the predentin. During enamel formation, the cell organelles of ameloblasts increased in number with the initiation of enamel secretion.  The hydroxyapatite crystals are deposited more or less parallel to each other and interdigitate with the crystals of dentin. As the first layer of enamel is formed, the ameloblast migrate away from dentin surface which permits the formation of a conical projection called Tomes’ process  When the tomes process is established, the enamel matrix is formed a rods. As the secretion of enamel protein becomes confined to two sites.  First site involves secretion from the proximal sites of Tomes process and forms interrod enamel. Second site involves secretion from the distal sloping portion of Tomes process and is called enamel rods.  3-Transitional stage Withdrawal of Tomes’ processes. Reduction in the height of the ameloblasts and decrease in its volume and organelle content. During this period most probably the outer structureless (rodless) enamel is formed. 4- Maturative stage occurs after most of the thickness of the enamel matrix in the occlusal or incisal area has been formed. while in the cervical parts of the crown, the enamel matrix formation is still progressing.  During this stage, the following changes occur: The ameloblasts undergo significant reduction in the height and a decrease in its volume and organelle content. Next the water and organic materials are selectively removed from the enamel while the additional inorganic material is introduced.  The ameloblasts modulate their morphology between ruffle- ended and smooth- ended forms.  Ruffle-ended ameloblasts add mineral to the enamel. Smooth-ended ameloblasts allow removal of water and degraded matrix proteins. incorporation of inorganic exit of protein material fragments & water Ruffle-ended Smooth-ended  5-Protective stage After complete formation and mineralization of enamel, the ameloblasts form 3-4 layers of stratified epithelium that cover the enamel and called reduced enamel epithelium. This reduced enamel epithelium protects the enamel by separating it from the connective tissue until the tooth erupts.  6-Desmolytic stage Reduced enamel epithelium has the function of secreting desmolytic enzymes which cause degeneration of the connective tissue that separates the tooth from the oral epithelium.  Defects of amelogenesis Tetracycline It is an antibiotic which incorporated in mineralizing tissues result in band of brown pigmentation or even total pigmentation.  Fluoride ion chronic ingestion of F. concentration in excess of 5 parts per million, result in mottled enamel as patches of hypomineralized and altered enamel. Clinical consideration Enamel is incapable of repairing itself once it is destroyed because the ameloblast cell degenerates following the formation of the enamel rod. The final act of the ameloblast cell is secretion of a layer covering the end of the enamel rod. Diseased, fractured, or otherwise damaged enamel can only be repaired through operative procedures.