Oral Biology I - Enamel PDF

Faculty of Dentistry Oral Biology I Life cycle of Ameloblast I. Morphogenic stage  At the early bell stage  The IEE is Low columnar resting on basement membrane that separates them from the cell free z...

Faculty of Dentistry Oral Biology I Life cycle of Ameloblast I. Morphogenic stage  At the early bell stage  The IEE is Low columnar resting on basement membrane that separates them from the cell free zone of dental papilla.  Electron Microscopic Picture (E\M) ▪ Large nucleus that almost fills the cell body. ▪ The Golgi apparatus and centrioles are in the proximal end of the cell (adjacent to the SI). ▪ Mitochondria are evenly scattered throughout the cytoplasm. ▪ The cells have proximal junctional complex II. Differentiation or Organizing stage  At the late bell stage  Pre- ameloblast cells (still undergoing differentiation)  No mitotic activity is detected  The cells elongate to reach 40 microns  Electron Microscopic Picture (E\M) ▪ The nuclei shift proximally towards the stratum intermedium. ▪ The Golgi apparatus or complex increases its volume and migrate from proximal position to distal end. ▪ Mitochondria cluster in the proximal end. Enamel Menatalla M. Elhindawy  Faculty of Dentistry Oral Biology I ▪ The amount of rough endoplasmic reticulum (rER) increases significantly.  In this stage the ameloblast becomes a polarized cell. III. Synthesis, Secretory, Formative stage  At the late bell stage and developing tooth  Ameloblast cells have no mitotic activity  Basement membrane disintegrated and ameloblast is now in contact with first formed layer of dentin and ameloblast is now fully differentiated.  E\M ▪ Golgi complex is condensed and developed occupying the central part of the cell ▪ Mitochondria is still in the same position ▪ rER increased in number ▪ The cells developed distal junctional complex  The first layer of enamel is formed with crystals randomly oriented  Ameloblasts migrate away from dentine surface which permits the formation of Tomes’ process.  Tome’s process  Extension of the ameloblast  Marked from the cell body by distal junctional complex.  It contains primarily secretory granules and small vesicles only with no cell organelles.  The process is now embedded in a pit Enamel Menatalla M. Elhindawy  Faculty of Dentistry Oral Biology I  It creates a saw tooth appearance with the enamel surface.  Secretion of enamel is confined to two sites 1. Proximal end (close to the distal junctional complex of the ameloblast) it forms the pit’s wall, (inter rod). 2. Distal end, it forms the pit itself (enamel rod).  This explains why the crystal orientation and direction in the inter rod differs from those of the rod.  The Tomes’ processes create the unique structure of enamel and persist until just before the final few increments at the surface are formed. IV. Transitional stage  During this stage the following events take place:  Withdrawal of Tomes’ processes, reduction in the height of the ameloblasts and decrease in its volume and organelle content.  The outermost layer of enamel is formed with randomly arranged crystals V. Maturative stage  Enamel maturation (full mineralization) occurs after most of the thickness of the enamel matrix has been formed in the occlusal or incisal area.  During maturative stage ameloblast display two forms  Smooth border ameloblast, it is associated with removal of protein and water, these ameloblasts have distal leaky junction. Enamel Menatalla M. Elhindawy  Faculty of Dentistry Oral Biology I  Ruffled border ameloblast, it displays microvilli at their distal extremities. This form is associated with introduction of inorganic materials, these ameloblasts have distal tight junctional VI. Protective stage  When the enamel has completely developed and fully calcified, the ameloblasts secrete a material between distal ends of cells and the enamel surface identical to basal lamina (Hemidesmosomes) providing a firm attachment for ameloblasts and enamel surface.  Ameloblasts can’t be differentiated from the cells of the stratum intermedium, stellate reticulum and outer enamel epithelium forming reduced enamel epithelium (REE).  The function of the REE is that of protecting the mature enamel by separating it from the connective tissue until the tooth erupts. Enamel Menatalla M. Elhindawy  Faculty of Dentistry Oral Biology I  If the connective tissue meets the enamel may be either resorbed or covered by a layer of cementum VII. Desmolytic stage  The REE seems to induce atrophy of the connective tissue separating it from the oral epithelium as pathway for tooth eruption. So, teeth erupt without bleeding.  Premature degeneration of the REE may prevent the eruption of a tooth. Amelogenesis process  Two processes are involved in amelogenesis ▪ Matrix formation. ▪ Mineralization.  Keep in mind the following ▪ Enamel mineralization doesn't wait for complete matrix formation. ▪ No clear cut between matrix formation and mineralization. ▪ Matrix formation and mineralization take place almost at the same time & crystals appear almost immediately in newly secreted matrix. Enamel Menatalla M. Elhindawy  Faculty of Dentistry Oral Biology I A. Enamel matrix formation Synthesis of enamel protein occurs in rER, where it passes to the Golgi complex in which it is condensed, a polysaccharide fraction is added and packed into membrane bound secretory granules. These granules migrate to the distal extremity of the cell and contents are released against the newly formed dentine. After the first formed layer of enamel the ameloblasts move outward away from dentine surface to allow space for more secretion of matrix. As enough enamel matrix is deposited, the ameloblasts develop Tomes’ processes into the forming matrix, through which further matrix is deposited. The ameloblast interface has a saw-like shape. Tome’s process controls the direction of crystal orientation through two regions: distal end and proximal. Crystal has a perpendicular direction on the membrane of Tome’s process. This orientation gives the unite structure of enamel, rod and inter rod substance (proximal end gives inter rod; distal end gives rod). Enamel Menatalla M. Elhindawy  Faculty of Dentistry Oral Biology I B. Mineralization o Immediate partial mineralization It occurs while matrix is laid down, which may account for 25% to 30% of the ultimate mineral content of enamel. o Full mineralization Once the full width of this enamel has been laid down, enamel second maturation begins involving significant influx of additional mineral coincidence to reach 96% with the removal of organic material and water o Mineralization starts from the height of the crown and progresses cervically. Enamel Menatalla M. Elhindawy