Amelogenesis - October University (PDF)

Amelogenesis Prof. Dr. Hala Zaatar Prof. Dr. Sahar Shawkat Dr. Lougine Elkhousht LIFE HISTORY OF AMELOBLASTS STAGES 6 –5 1 - MORPHOGENIC FORMATIVE – 3 PROTECTIVE 2 - ORGANIZING MATURATIVE - 4...

Amelogenesis Prof. Dr. Hala Zaatar Prof. Dr. Sahar Shawkat Dr. Lougine Elkhousht LIFE HISTORY OF AMELOBLASTS STAGES 6 –5 1 - MORPHOGENIC FORMATIVE – 3 PROTECTIVE 2 - ORGANIZING MATURATIVE - 4 DESMOLYTIC - 6 DEAL WITH INNER FUNCTIONS OF FUNCTIONS OF DENTAL DIFFERENTIATED THE REDUCED EPITHELIUM AMELOBLASTS DENTAL EPITHELIUM https://www.youtube.com/watch?v=B2QlVeer_cE https://www.youtube.com/watch?v=Lw3VEVt_tpU 1-Morphogenic stage: (I.D.E) In the beginning of early bell stage I.D.E are arranged on the BM. To determine the morphology of the future crown The cells are short columnar with large nuclei ,distributed mitochondria, and Golgi apparatus and centriols situated proximally The cells are separated from dental.papilla by BM. and cell free zone 2-Organizing stage: (I.D.E) At the end of early bell stage :I.D.E induce UMC of the dental papilla to differentiate into odontoblasts The following occurs: 1-Elongation of I.D.E leads to disappearance of cell free zone. 2- Alteration of functional polarity. Golgi apparatus and centeriols are shifted distally , nucleus and mitochondria become at the proximal end, the nucleus occupies 1/2 of the cell. 3-Histodifferentiation of ODONTOBLASTS ( INDUCTION) At the end of this stage dentine will  3- Formative stage: Dentin will induce I.D.E to differentiate into Ameloblast (AB) by the process of reciprocal induction AB. will form enamel matrix and excrete it through Tomes process. TOMES’ PROCESS Responsible for the rod ,sheath, interrod regions Distal terminal bar picket-fence appearance Ten Cate's Oral Histology Formation of Tomes’ processes and terminal bars Orban’s Oral Histology and Embryology AB are characterized by : Long cells (40 microns ) with: 1-large open face nucleus situated proximally 2-abundant cytoplasm. 3-increased mitochondria, Golgi apparatus, RER, secretory granules. The ameloblasts are closely aligned together by proximal and distal terminal bars. 4- Maturative stage: AMELOBLAST AB. will calcify enamel matrix. The cells of stratum intermedium will be active and show alkaline phosphatase activity which is necessary for calcification The AB are reduced in length and have less organelle's content. They contain phagocytic vacuoles to digest excess organelles by lysosomal enzymes. Increased number of mitochondria are present distally. The distal end of the cell membrane will show folding to form striated border (ruffled border )which will increase the surface area for active transport of material across the plasma membrane). They are characterized by the presence of tight distal terminal bar. 5- Protective stage: REDUCED DENTAL EPITHELIUM AB and other layers of dental organ will form 3-4 layers of stratified epithelium covering the enamel called reduced.dental epithelium This epith. protects the newly formed enamel surface from the surrounding connective tissue. If the CT. comes in contact to the enamel before tooth eruption; the enamel may be resorbed or afibrillar.cementum will be formed on it Desmolytic stage:-6 REDUCED DENTAL EPITHELIUM The cells of the reduced dental epithelium secretes desmolytic enzymes that cause degeneration of the connective tissue above the developing tooth to.facilitate its eruption  AMELOGENESIS 1 - FORMATION OF 2 – MATURATION ENAMEL MATRIX (calcification ) OF ENAMEL A-Matrix formation 1-Formation of Rodless enamel The ameloblasts will form enamel protein in the R.E.R. then condensed, receive CHO content and packed in Golgi apparatus into secretory granules. These granules will migrate to the distal end of the ameloblasts and then released extracellularly against the first dentine layer. This first enamel matrix contains some enamel hydroxyapatite crystals 2- Development of Tomes’ process. After formation of the first layer of enamel (rodless E), the ameloblasts will move away from the dentine (proximally) and develops a short conical projection at its distal end called Tomes' process. The Tomes' process contains cytoplasm & secretory granules but no cell organelles. The secretory granules in Tomes' process will be secreted with perpendicular angle to the cell membrane of Tomes' process. This direction of enamel protein secretion and the arrangement of crystals on it will be responsible for the appearance of enamel rod, rod sheath and inter-rod substance. There are distal terminal bars at the distal end of the ameloblasts delineating the Tomes' process from the ameloblasts cell body. They prevent the cell organelles from entering Tomes' process and prevent leakage of the secretion in between the ameloblasts.  Rod Rod region Rod Rodless B-MATURATION STAGE  1- Primary (initial) maturation The first secreted enamel matrix contains 20-30 % of the mineral content  :Secondary Maturation -2 20 -30 % To reach 96 % Crystal growth The sequence of enamel maturation starts from the cusp tip or the incisal edge and progress cervically. So, the incisal and the occlusal regions will reach maturity a head to the cervical regions. Maturation occurs by the growth of the primary secreted enamel crystals The protein content of the organic matrix in between the crystals will become thinner to create room for the growing crystals. :Secondary Maturation -2 The loss of volume of the organic matrix will be due to the withdrawal of 90% of the pre-secreted water and protein by the ruffled border of ameloblast. The remaining organic matrix will form sheath around the crystals. The prism sheath has higher content of organic matrix because of the abrupt change of crystal orientation. The distal end of the ameloblasts at the maturative stage is responsible for the active transport of the organic components from the enamel matrix during maturation {The process of transport is not only a quantitative but also a qualitative one}. I.e. not all the amino acids are absorbed in the same ratio as they were secreted (selective removal of organic matrix).  3- Tertiary maturation After tooth eruption and exposure of enamel to saliva maturation of the enamel continues by deposition of ions from the saliva to reach 98% of its weight inorganic ions material.