Life Cycle of Ameloblast: Detailed Morphological and Physiological Stages

2. Amelogenesis - Process of enamel formation • Cell responsible – Ameloblast • Derived from - Inner enamel epithelium (IEE)

3. Life cycle of ameloblast. • Based on morphological & physiological changes undergone by ameloblasts during their lifetime • Directly related to function

4. LIFE CYCLE OF AMELOBLAST Pre-secretory stage • Morphogenic/ Morphogenetic stage • Organizing / Differentiating stage Secretory stage • Formative stage Post-secretory stages • Maturative stage • Protective stage • Desmolytic stage

6. Morphogenic stage • Shape of the DEJ & crown determined Cells of IEE interacts with Underlying connective tissue through differential growth Tooth Shape is determined

7. The IEE cells (ameloblasts)in Morphogenic stage Function • Determining shape of the tooth • Through differential growth helps in establishing DEJ Structure of cells • Low columnar in shape • Centrally placed nucleus • Cytoplasmic organelles are not abundant • Centrioles and golgi complex located at the apical part of cytoplasm.

9. Organizing / Differentiation stage IEE cellsdifferentiates ameloblasts (prerequisite for enamel formation) Ameloblasts exert organizing influence on Dental papilla cells differentiates Odontoblasts

10. Structure of ameloblasts • Increase in length – Tall colunar cells (40 µ) • Develops abundant cytoplasmic organelles necessary for protein synthesis. • Nucleus shifts to the apical or proximal end of cell. • Centrioles and golgi apparatus also move from apical to basal or distal part of the cell • Mitochondria becomes concentrated at the proximal end • Develop intercellular junctions at the proximal and distal ends termed as proximal & distal terminal bars • Reversal of polarity • Preparatory to secretion  organelles moved to the secretory end of cell

11. Apical or proximal end Basal or distal end

12. Apical or proximal end Basal or distal end

13. Apical or proximal end Basal or distal end

14. During the terminal phase of organizing stage • Dentin formation begins • Basal lamina supporting the ameloblast layer disintegrates • Dentin deposition plays a crucial role in life cycle of ameloblasts • Ameloblasts attain secretory function only after a layer of dentin is deposited

15. Interdependence between ameloblasts and odontoblasts is referred as Reciprocal Induction. • Derive alternate source of nutritional supply from dental sac • Dentin deposited blocks the nutritional supply from the dental papilla.

16. Differentiation of Odontoblasts Ameloblasts Enamel INNER ENAMEL EPITHELIUM Signaling molecules & GF Ectomesenchymal cells of dental papilla Preodontoblast Subodontoblast Cell Odontoblast Predentin Dentin

17. Formative / Secretory stage Function • Secretes enamel matrix and initiates partial mineralization. • Starts secretory function only after a layer of dentin is deposited. Structure • Structurally suited for synthesis & secretion of enamel proteins. • Have numerous mitochondria • Well developed golgi complex • Extensive cisterns of RER • Many secretory granules, vacuoles, free ribosomes, various types of vesicles, microtubules, etc.

18. Shows junctional specializations at basal and lateral cell surfaces. • Adjacent cells attached to each other by junctional complexes at the proximal & distal end of cell body. • to maintain organization of ameloblast layer • to control the metabolite diffusion along extracellular spaces. • Proximal junctions - Relatively leaky • Distal junctions - Permeability barrier to macromolecules (enamel proteins and calcium) • Prevents calcium from reaching the matrix through extracellular space.

19. During this stage, ameloblasts synthesize enamel protein. Steps involved are: Messenger RNA carries the message from nucleus to cytoplasm ↓ Ribosomes translate the message ↓ Protein is synthesized in rough endoplasmic reticulum ↓ Protein undergoes post translation modification in golgi complex ↓ Packing of protein into secretory granules

20. Basal portion contains numerous secretory granules packed with enamel proteins. The secretory granules move towards Basal Plasma Membrane fuse & releases • Matrix protein into the extracellular space against the newly formed dentin (exocytosis).

21. In the initial stage of secretory phase, the ameloblasts have a flat basal region. • After initial layer of enamel matrix deposition, ameloblasts develop a conical process at the base, called as Tomes’ process. • Tomes' process is partially separated from cell body by an incomplete septa formed by the microfilaments and tonofilaments extending from the distal terminal bars. • Cytoplasm of cell body is in continuation with that of Tomes' process.

22. Cytoplasm of Tome's process does not contain any organelles other than secretory granules, microtubules, microfilament and fewer mitochondria. • After the Tome's process is formed, the secretion of enamel takes place from two different sites and is responsible for the rod structure of enamel. • Tomes' process is lost before the last phase of secretory stage, before the surface layer of enamel is deposited.

24. Tomes’ process • Is the conical process appear at the basal region of secretory ameloblasts. • This process is formed after initial layer of enamel matrix is deposited and is lost before the last phase of secretory stage • Cytoplasm of cell body of ameloblast is in continuation with that of Tomes' process . • Tomes' process is partially separated from cell body by an incomplete septa formed by the microfilaments and tonofilaments extending from the distal terminal bars. • Cytoplasm of Tome's process does not contain any organelles other than secretory granules, microtubules, microfilament and fewer mitochondria. • After the Tome's process is formed, the secretion of enamel takes place from two different sites and is responsible for the rod structure of enamel. • Enamel formed before formation of Tomes’ process and after it is lost are rodless enamel

25. Maturative stage Function • Helps in the mineralization and maturation of enamel • Enters maturative phase only after the desired thickness of enamel matrix is laid down. • Ameloblasts perform dual function • Introduce inorganic material necessary for maturation • Reabsorb proteins & water, to provide space for the minerals

27. Ameloblasts in maturation stage Structure • Slight reduction in height • Decrease in volume and organelle content • Excess synthetic organelles removed • Remaining organelles are shifted to the distal end of the cells. • The basal plasma membrane • Ruffle ended - when introducing inorganic components • Smooth ended- when they are reabsorbing proteins and water • The cytoplasmic vacuoles contains material resembling enamel matrix • Indicates absorptive function.

28. Protective stage • Secretes protein similar to basal lamina onto the surface of newly formed enamel. • develop hemidesmosomal attachments to these basal lamina structure which help in holding these firmly to the tooth surface. • Formation of REE (IEE + S. Intermedium + S reticulum +OEE) • Covers & protects the newly formed enamel, till the tooth erupts. • Plays an important role in establishing the dento-gingival junction. • If not protected, • Enamel resorption • Cementum deposition on enamel surface.

29. Desmolytic stage

30. REE secretes Collagenase enzyme destroys CT between oral mucosa and erupting tooth REE Proliferates fuses with Oral epithelium Desmolytic stage form Solid plug of epithelial cells central cells degenerate Form a canal Through which the tooth erupts

31. Amelogenesis • Two process • Organic matrix deposition • Mineralization • Immediate partial mineralization • Maturation

32. Organic matrix deposition • Enamel formation begins in the cusp tips and incisal edges → progresses outward and cervically. • As the matrix deposition progresses the ameloblasts move outward, away from the matrix • In early stages of amelogenesis, the enamel matrix consists of 20-30% proteins (amelogenin and non-amelogenin) and the proportion gradually decreases during mineralization

33. Enamel proteins • 80% - Amelogenin ( rich in proliene & help in crystal growth) • 5-20 % - Non amelogenin (rich in glycene & help in nucleation and controls crystal growth) • Enamelin • Tuftelin • Ameloblastin(ameilin & sheathlin) • Amelotin & enzymes like protienases

35. Mineralization • Mineralization takes place in two steps • Immediate partial mineralization • Maturation.

36. Mineral deposition in enamel matrix occurs in four phases. • Primary mineralization corresponds to immediate partial mineralization (30 % of mineral deposition) • Secondary stage starts at the surface and proceeds toward DEJ • In tertiary stage, mineral rebounds from inner layer of enamel outward. • Fourth stage is responsible for further deposition of minerals in the surface enamel which makes it hyper- mineralized than the rest of enamel.

38. Maturation • This starts from the height of the crown and then gradually proceeds cervically. It begins at the dentinal end of the enamel rods. • Begins even before the matrix has formed to its full thickness. • Crystals grow in size rapidly, and the organic matrix becomes thin to make room for the growing crystals.

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