Oral Biology I: Amelogenesis Lecture Notes PDF

Course Title: Oral Biology I Course Code: BDS123 Topic of Lecture: Amelogenesis. Lecturer: Prof Samah Mehanny F A C U L T Y O F D e n t i s t r y T h e F u t u r e S t a r t s H e r e F a l l 2 0 2 4 Prepared b...

Course Title: Oral Biology I Course Code: BDS123 Topic of Lecture: Amelogenesis. Lecturer: Prof Samah Mehanny F A C U L T Y O F D e n t i s t r y T h e F u t u r e S t a r t s H e r e F a l l 2 0 2 4 Prepared by Samah Mehanny  Amelogenesis: Human enamel forms at a rate of 2.5-6 microns per day, beginning at the future location of the cusps or incisal edges and proceed cervically. Prepared by Samah Mehanny Enamel formation is complex, but can generally be divided into two stages. The first stage called the secretory stage involves secretion of proteins (organic matrix) and some crystals forming partially mineralized enamel. The second stage, called the maturation stage, completes enamel mineralization. Matured enamel Enamel matrix Prepared by Samah Mehanny  Amelogenesis 1 – Formation of 2 – Maturation of enamel matrix. enamel. The beginning of the enamel matrix mineralization doesn’t await the completion of its formation. Prepared by Samah Mehanny  Different stages of ameloblasts 1-Pre-secretory stage: a-Morphogenetic phase. (at the beginning of early bell stage) b-Differentiation phase. (at the end of early bell stage) 2-Secretory stage. 3-Maturative stage. a-Transistional phase. b-Maturative phase. Prepared by Samah Mehanny Note: Ameloblasts perform protective and desmolytic function when share in the formation of Reduced enamel epithelium. Prepared by Samah Mehanny B and C are the ultrastructure pictures of the Secretory stage. D maturative stage smooth ended. E maturative stage ruffled ended. Prepared by Samah Mehanny Prepared by Samah Mehanny  1-Pre-secretory ameloblast. A-Morphogenetic phase (pre-secretory ameloblast). Light microscopic features The inner enamel epithelium are short ( low) columnar with centrally placed nuclei. EM features: 1-Large central nucleus. 2-Golgi 3-Dispersed Mitochondria 4-Proximal Junctional Complex 5-Cells rest on basal lamina Prepared by Samah Mehanny The arrangement of the inner enamel epithelium maps out the shape of the crown and is maintained by the development of junctional complexes. B-Differentiating phase Prepared by Samah Mehanny  B-Differentiating phase Light microscopic picture: -Occurs at the end of the early bell stage. -The cells of the inner dental epithelium will change from a cuboidal to columnar and become polarized, with the nucleus sitting close to the proximal end of the cell that is facing the dental organ. -Induction of odontoblasts Prepared by Samah Mehanny EM features: 1-Proximally located nucleus. 2-Increase Golgi and RER. 3-Mitochondria around the nucleus. 4-Distal Junctional Complex 5-Disruption of BL due to projections from Ab and MMP secretion from odontoblasts. Prepared by Samah Mehanny Morphogenetic to Differentiating phases: preameloblasts. Inner dental epith. Proximal end Distal end Odontoblasts Centriol Golgi apparatus Nucleus Mitochondria  Preameloblasts IEE Dental papilla Ameloblasts Preameloblasts Odontoblasts Odontoblasts Prepared by Samah Mehanny 2- Secretory stage: responsible for the formation of: A-Initial rodless (aprismatic) enamel layer B-Rod (prismatic) enamel layer Prepared by Samah Mehanny a-Initial rodless (aprismatic) enamel layer (20-30 microns): -The secretory ameloblasts synthesize enamel proteins and 20-30 % mineral content. This initial enamel layer does not contain enamel rods as the Crystals are parallel to each other. Ameloblasts ← Odontoblasts Prepared by Samah Mehanny -Little (if any) time elapses between the secretion of enamel proteins and its mineralization. -The first hydroxyl-apatite crystals formed interdigitate with the crystals of dentin achieving chemical bond between enamel and dentin Prepared by Samah Mehanny -When this first layer is formed, the ameloblasts (70 micron) move away from dentin allowing for the development of a distal portion that is called Tome’s process. Filled with secretory Prepared by Samah Mehanny granules. b-Rod (prismatic) enamel layer: -The ameloblasts in this stage have Tome’s processes which result in the presence of different secretory surfaces that lead to the formation of prism, inter-prismatic region and prism sheath. Prepared by Samah Mehanny EM features: Protein forming criteria Large open-faced nucleus Abundant cytoplasm in no. of mitochondria in no. of rough endoplasmic reticulum in size of Golgi apparatus Presence of secretory granules Prepared by Samah Mehanny **Steps Enamel proteinprotein of enamel formation (How)? formation: 1- Nucleus send mRNA for enamel protein which translated by ribosomes on the membrane of rough endoplasmic reticulum. 2- Ribosomes synthesize the enamel proteins. 3- The formed enamel proteins move to the Golgi complex through transport vesicles. Prepared by Samah Mehanny 4- In Golgi, the proteins are glycosylated, packed and then released in granules (secretory granules) from the concave surface of Golgi apparatus. 5- The secretory granules are transported to the secretory pole of the cells. Prepared by Samah Mehanny Prepared by Samah Mehanny 3-Maturative stage: A-Transitional stage: responsible for the formation of Final rodless (aprismatic) enamel layer Prepared by Samah Mehanny Changes in transitional ameloblasts: -The ameloblasts reduce in number (death of about 25% of the cells). -The ameloblasts become shorter and Tomes' processes are lost and accordingly the last formed layer of enamel dose not contain any enamel rods resembling the initial layer of enamel. -The crystals are of uneven thickness and are arranged parallel to each other and perpendicular to the surface. -Reduction in protein synthesizing organelles. - Excess organelles are enclosed in autophagic vacuoles and digested by lysosomes Prepared by Samah Mehanny -The enamel layer is thus composed of a prismatic layer sandwiched between thin aprismatic initial and final layers. Prepared by Samah Mehanny -Within this organic matrix, the initial hydroxyl-apatite crystals of the enamel (20-30 % of the total mineral content of enamel) appear almost immediately (primary maturation), So that a distinct zone of unmineralized matrix similar to predentin or osteoid tissue is never seen in enamel. Prepared by Samah Mehanny  b-Maturation phase (secondary maturation): responsible for the maturation of previously secreted enamel. - It is characterized by gradual completion of mineralization to reach the 96 % of the total weight of enamel. - After the entire thickness of enamel matrix is secreted the maturation begins. Prepared by Samah Mehanny -In the maturation stage, a selective removal of enamel proteins and water occurs resulting in a change in the ratio of the proteins of the newly secreted enamel matrix and the final matrix of mature enamel. Prepared by Samah Mehanny Ameloblasts undergo programmed cell death (apoptosis) 25%............in transitional stage 25%.............in maturative stage Thus during the process of maturation, A total loss of about 50% of the amelablasts population. Prepared by Samah Mehanny Recently formed crystals Mature crystals Prepared by Samah Mehanny Maturative ameloblasts are of two types: 1- Smooth ended. 2- Ruffle ended. Prepared by Samah Mehanny  Types of enamel maturation 1ry maturation: 1st secreted matrix contain 25-30% mineral. 2ry maturation: complete mineralization to reach 96% mineral. 3ry maturation: after tooth eruption & exposure to saliva, mineral precipitate in the outer layer of enamel leading to increase in mineral content to 98%, which increase the hardness of enamel. Prepared by Samah Mehanny kindly figure out the effect of higher fluoride intake during teeth-forming years. Prepared by Samah Mehanny Dental fluorosis Prepared by Samah Mehanny What are the diagnostic features of dental fluorosis? The clinical manifestation of mild dental fluorosis is mostly characterized by a snow flaking appearance that lack a clear border, opaque, white spots, narrow white lines following the perikymata or patches as the opacities may coalesce with an intact, hard and smooth enamel surface on most of the teeth. Prepared by Samah Mehanny Mechanism of fluorosis 1.Inhibition of protein secretion during the secretory stage of ameloblasts: High levels of fluoride can disrupt the normal functioning of ameloblasts, leading to decreased enamel matrix secretion. This can result in enamel defects and changes in enamel thickness. 2.Altered enamel crystal formation: Fluoride can also affect the mineralization process of enamel crystals. The maturation will be subsurface leaving hypomeneralized enamel underneath. Prepared by Samah Mehanny DIRECTIONS OF MATURATION ENAMEL DENTIN PREDENTIN ENAMEL DENTIN PREDENTIN ASG Prepared by Samah Mehanny  DIRECTIONS OF MATURATION 1- The maturation starts from the cusp tip or incisal edge directed cervically. 2- The maturation starts from the Dentino-enamel junction directed to the surface. i.e. maturation starts away from the cell. Prepared by Samah Mehanny kindly figure out what’s meant by biomimetic remineralization. Prepared by Samah Mehanny Before going through the enamel structure, kindly figure out if the defective enamel can regenerate or not and why. Prepared by Samah Mehanny Protective stage: responsible for the protection of the previously secreted and matured enamel. -At this stage, when the ameloblasts complete enamel formation and maturation, they lose their differentiation and become short cuboidal cells which together with the remnants of the other layers of the dental organ form a multilayered structure, namely the reduced enamel (dental) epithelium that covers the enamel. The last product of ameloblast is primary enamel cuticle. This cuticle is a basal lamina that adheres tightly to the enamel surface. Prepared by Samah Mehanny  REE Prepared by Samah Mehanny *Desmolytic stage Prepared by Samah Mehanny - Reduced enamel epithelium has the function of secreting desmolytic enzymes which cause degeneration or desmolysis of the connective tissue that separate the tooth from the oral epithelium, so it has a role during eruption of the tooth. Prepared by Samah Mehanny Prepared by Samah Mehanny All these stages of amelogenesis happened first in the incisal or occlusal and proceed in cervical direction i.e older events near the incisal edge or cusp tip and recent events near the cervical region. Prepared by Samah Mehanny Differentiation Gradient From cusp tip  cervically Prepared by Samah Mehanny  1- Inner dental epithelium. (Differentiating stage) 2- Pre-ameloblast. (Differentiating stage) 3- Secretory ameloblast. (Without Tome’s process) 4-Secretory ameloblast. (With Tome’s process) 5 &6- Maturative ameloblast. 7- Protective ameloblast. (Reduced dental epithelium. Remember the presence of Transitional Ameloblast Prepared by Samah Mehanny between 4 &5 AGE CHANGES 1 - ATTRITION Prepared by Samah Mehanny  2 - PERMEABILITY Recently Main path Old enamel Erupted teeth Prepared by Samah Mehanny Prepared by Samah Mehanny