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Questions and Answers
What is the initial phase of dentinogenesis characterized by the deposition of the organic matrix?
Which type of dentine formation occurs as a response to mild stimulus?
During which stage of amelogenesis do ameloblasts transition from a secretory to a maturation form?
What percentage of water is generally present in mature enamel?
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What initiates the trigger for amelogenesis during tooth development?
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Which enamel protein accounts for the majority of proteins in early enamel?
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How does the percentage of organic material change from early to mature enamel?
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What is a defining characteristic of peritubular dentine?
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What occurs during the maturation phase of amelogenesis?
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What happens to the odontoblast population over a period of four years?
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What is the composition of the enamel matrix during the maturation stage?
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Which of the following best describes the function of tuftelin in enamel formation?
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What role do ameloblasts play in the enamel maturation stage?
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What is indicated by the presence of a striated (ruffled) border in ameloblasts?
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During the post maturation stage, what characterizes the transition of ameloblasts?
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What triggers the differentiation of odontoblasts during root formation?
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What do the proteins amelogenin, ameloblastin, and enamelin primarily contribute to during enamel development?
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What happens to the population of ameloblasts during the transition phase of enamel formation?
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What effect does Hertwig's epithelial root sheath have during root formation?
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What percentage of weight does the organic material constitute in early developing enamel?
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What is the role of cementoblasts in root formation?
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What is the significance of the epithelial cell rests of Malassez in tooth development?
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During which developmental stage do nerve fibers enter the dental pulp?
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Which statement is true regarding the vascular supply around the tooth germ?
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How are fibroblasts related to the periodontal ligament?
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What is the primary role of Hertwig’s epithelial root sheath during tooth development?
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Which structure is responsible for the formation of the periodontal fiber bundles?
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What happens to the concentration of blood vessels in the dental papilla from the cap stage to the bell stage?
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What type of dentine is formed in response to more severe stimuli reaching the dental pulp?
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What defines the first stage of mineralisation in the odontoblast life cycle?
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How is the age of teeth commonly determined based on dentine structure?
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What is the main characteristic of tertiary dentine that develops under mild stimuli?
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During which phase do the cells of the internal enamel epithelium begin to differentiate?
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What main change occurs to odontoblasts as dentine continues to be deposited over time?
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Where does the mineralization of dentin primarily start during the dentinogenesis process?
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What is the relationship between amelogenesis and dentinogenesis during tooth development?
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What is the function of dentin sialoproteins (DSPs) and dentin phosphoproteins (DPPs) during dentinogenesis?
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What primary structure do the collagen fibrils form in relation to the odontoblast process?
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Study Notes
Histodifferentiation and Hard Tissue Formation
- Dentine is formed from mesenchymal tissue.
- Dentinogenesis begins during the bell stage.
- Dentine's organic matrix is composed mainly of collagen.
Dentinogenesis Stages
- Differentiation phase: Odontoblasts differentiate.
- Secretion phase: Odontoblasts secrete organic matrix.
- Mineralization phase: Organic matrix is mineralized.
- Peritubular and secondary dentine formation: Found in unerupted teeth.
- Tertiary dentine formation: Forms in response to injury.
- Mild stimulus: Original odontoblasts form reactionary dentine, which is tubular.
- Severe stimulus: Odontoblasts form reparative dentine, which is tubular and bone-like.
Life Cycle of Odontoblasts
- Odontoblasts secrete organic matrix once fully differentiated.
- Collagen fibrils form an interlacing network perpendicular to the odontoblast process.
- Peritubular dentine is not related to outside stimuli, hence found in unerupted teeth.
- The degree of peritubular occlusion can be used to determine age.
- Secondary dentine is a pre-programmed age change, not a response to external activities.
- Pulp volume decreases with continuous dentine deposition, killing some odontoblasts.
Formation of Crowns and Dentine
- Dentine matrix deposits under future cusp/incisal margin and progresses apically.
- No morphological difference between root and crown dentine.
Amelogenesis
- Occurs simultaneously to dentinogenesis.
- Both begin at the enamel-dentine junction.
- Inner enamel epithelium deposits and modifies enamel.
Life Cycle of Ameloblasts
- Differentiation phase: Ameloblasts differentiate.
- Secretion phase: Ameloblasts secrete enamel proteins.
- Maturation phase: Ameloblasts mature.
- Post maturation phase: Ameloblasts undergo post-maturation.
Stages of Enamel Formation
- Pre-secretory stage: Begins at future cusp tips/incisal margins, progresses cervically.
- Secretory stage: Enamel proteins are secreted to form the ECM, which is mineralized.
- Transition stage: Ameloblasts change form from secretory to maturation, enamel secretion stops, and a significant portion of the matrix is removed.
- Maturation stage: Ameloblasts move calcium, phosphate, and carbonate ions into the matrix, removing water and degraded enamel matrix proteins.
- Post-maturation stage: Ameloblasts flatten and remain in the fissures.
Mineralization
- First crystals are initiated at the enamel-dentine junction and grow outwards into the enamel matrix.
- Crystallite growth and nucleation are directed by the enamel protein tuftelin.
Root Formation
- Once crown formation is complete, the epithelial cells of the inner and outer enamel epithelium proliferate from the cervical loop of the enamel organ.
- The proliferating cells form Hertwig's epithelial root sheath.
- The inner epithelial cells of the root sheath enclose and initiate the differentiation of ectomesenchymal cells at the periphery of the pulp, facing the root sheath, into odontoblasts.
Cementum Formation
- Cementum is formed by cementoblasts that are derived from the dental follicle.
Peridontium Formation
- The dental follicle forms the periodontal ligament (PDL) cells and fiber bundle cells.
- Fibroblasts differentiate from the PDL.
Enamel Proteins
- Mature enamel contains 1% proteins; early enamel contains 25-30%.
- Amelogenin: comprises 90% of enamel proteins.
- Ameloblastin: comprises 5% of enamel proteins.
- Enamelin: Another enamel protein.
Enamel Proteases
- MMP-20 (enamelysin): An enamel protease
- KLK-4: An enamel protease.
Vascular Supply
- Clusters of blood vessels (BV) ramify around the tooth derm in the dental follicle and enter the dental papilla during the cap stage.
- The enamel organ is avascular.
- Vascular supply peaks during the bell stage when matrix deposition begins.
Nerve Supply
- Pioneer nerve fibers approach the developing tooth during the bud and cap stage.
- They target the dental follicle.
- Nerve fibers enter the pulp only when dentinogenesis begins.
- Nerve fibers never enter the enamel organ.
Root Formation: Cementum
- Some cells from Hertwig’s epithelial root sheath may transform directly into cementoblasts and may also give rise to other periodontal components.
- Cementoblasts secrete cementoid matrix which mineralises to cementum.
Root Formation: Epithelial Cell Rests of Malassez
- As the root sheath fragments, it leaves behind a number of discrete clusters of epithelial cells, separated from the surrounding connective tissue by a basal lamina, known as the epithelial cell rests of Malassez.
- They persist next to the root surface within the periodontal ligament.
- They can be the source of dental cysts.
- There is now growing evidence that these cell rests play an active role and can be activated to participate in periodontal repair and regeneration.
Root Formation: Periodontium
- The cells of the periodontal ligament and the fiber bundles also differentiate from the dental follicle.
- Fibroblasts are induced to form periodontal ligament.
- Recent evidence indicates that the bone in which the ligament fiber bundles are embedded also is formed by cells that differentiate from the dental follicle.
Vascular Supply
- Clusters of blood vessels are found ramifying around the tooth germ in the dental follicle and entering the dental papilla during the cap stage.
- The enamel organ is avascular, although a heavy concentration of vessels in the follicle exists adjacent to the outer enamel epithelium.
- The number of vessels in the dental papilla increases, reaching a maximum during the bell stage when matrix deposition begins.
Nerve Supply
- Pioneer nerve fibers approach the developing tooth during the bud-to-cap stage of development.
- Nerve fibers ramify and form a rich plexus around the tooth germ in the structure of the dental follicle.
- Nerve fibers enter the dental pulp only when dentinogenesis starts.
- Nerve fibers do not enter the enamel organ.
Initial Stage of Enamel Formation
- Ameloblasts start to differentiate and become columnar initiating the formation of enamel matrix.
The Transition Stage of Enamel Formation
- Ameloblasts change from a secretory to a maturation form.
- Enamel secretion stops and much of the matrix is removed.
- The number of ameloblasts is reduced by as much as 50% by apoptosis.
Enamel Proteins
- Proteins and peptides account for less than 1% of the weight of mature enamel but 25–30% of early developing enamel.
- The matrix of enamel comprises several proteins, of which amelogenin (90%), ameloblastin (5%) and enamelin are the most abundant.
- Enamel matrix also contains two main proteases, MMP-20 (enamelysin) and KLK-4.
Maturation Stage of Enamel Formation
- Prior to maturation, young enamel is comprised of 65% water, 20% organic material and 15% inorganic hydroxyapatite crystals by weight.
- Upon maturation it consists of approximately 96% mineral, 3% water and 1% organic material.
- Ameloblasts move calcium, phosphate and carbonate ions into the matrix and remove water and degraded enamel matrix proteins from it.
- Ameloblasts show straited (ruffled) border.
Post Maturation Stage of Enamel Formation
- The ameloblasts become reduced further and flattened.
- They might remain columnar in the depths of fissures.
- The primary enamel cuticle together with the reduced enamel epithelium form Nasmyth’s membrane.
Mineralisation of Enamel
- The first crystals are initiated at the enamel–dentine junction and grow outwards into the enamel matrix.
- Crystallite growth and possibly nucleation are directed by the enamel protein tuftelin.
Root Formation (Hertwig’s Epithelial Root Sheath)
- Once crown formation is completed, epithelial cells of the inner and outer enamel epithelium proliferate from the cervical loop of the enamel organ.
- This forms a double layer of cells known as Hertwig’s epithelial root sheath.
- The inner epithelial cells of the root sheath progressively enclose more and more of the expanding dental pulp.
- They initiate the differentiation of odontoblasts from ectomesenchymal cells at the periphery of the pulp, facing the root sheath.
- These cells eventually form the dentin of the root.
Root Formation: Cementum
- Ectomesenchymal cells of the dental follicle penetrate between the epithelial fenestrations and become apposed to the newly formed dentin of the root.
- Peripheral ectomesenchymal cells divide, with some daughter cells migrating below the root sheath.
Life Cycle of the Odontoblast: Secretion Phase
- Nucleus moves basally as the cell becomes polarised.
- A number of odontoblast processes begin to form, one odontoblast process becomes enlarged and begins to secrete matrix.
Life Cycle of the Odontoblast: Mineralisation
- The odontoblast retreats as matrix is laid down, leaving behind a single main process.
- Mineralisation commences once a narrow layer of matrix is laid down.
- The collagen fibrils form an interlacing network perpendicular to the odontoblast process.
- The surface of predentine is a boundary between dentin and pulp.
Dentin Formation
- The odontoblast begins to secrete its characteristic organic matrix once fully differentiated.
- Type I collagen fibrils that are laid down initially lie at right angles to the future dentine–enamel junction.
- DSPs and DPPs are expressed not only by odontoblasts but also during the early stages of dentinogenesis by the preameloblasts of the internal enamel epithelium.
- DPP is involved in signalling during epithelial–mesenchymal interactions.
Formation of Crown and Root Dentin
- Dentin matrix starts to deposit under what will become the cusp tip or incisal margin and progressing rootwards.
- The basic process of root dentinogenesis does not differ fundamentally from coronal dentinogenesis.
- There is no morphological difference between root and crown dentin.
Peritubular (Intratubular) Dentin
- Peritubular dentin formation does not seem to be related to outside stimuli as it is found in unerupted teeth.
- The degree of tubular occlusion can be used to determine the age of teeth and is applied in forensic circumstances.
Secondary Dentin
- Seems to be a pre-programmed age change rather than a response to external activity.
- This could be due to apoptosis as the pulp volume decreases with continuing dentin deposition, odontoblasts die.
- Over a 4-year period, the odontoblast population may be reduced by 50%.
Tertiary Dentin Formation in Response to Injury
- The nature and severity of stimuli that reach the dental pulp vary over a considerable range.
- If the stimulus is mild and the original odontoblasts remain alive, they will lay down a tubular form of tertiary dentine, reactionary dentine.
- If the stimulus is more severe reparative dentin forms which is atubular and bonelike.
Cap Stage of Tooth Development
- The other components of the enamel organ play important supportive roles.
- The innermost cell layer of the enamel organ, the internal enamel epithelium, deposits and later modifies the enamel.
Enamel Formation: Amelogenesis
- Amelogenesis and dentinogenesis occur almost but as distinctly different processes.
- The site where they both begin is the enamel–dentine junction.
Life Cycle of the Ameloblasts: Differentiation Phase
- The cells of the internal enamel epithelium start to differentiate, beginning at the future enamel–dentine junction of the cusp tip.
- The differentiating cell is characterised by a reversed polarity; the cell becomes columnar and the nucleus moves to that part of the cell furthest from the enamel–dentine junction.
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Description
Explore the complex stages of dentinogenesis and the critical role of odontoblasts in hard tissue formation. This quiz covers the differentiation phases, secretion and mineralization processes, and the life cycle of odontoblasts. Test your knowledge of dental histology and the responses of dentine to stimuli.
