Embryology Series (5) The Embryology of Tooth Development 2 PDF

The embryology of tooth development 2 Dr Wai Ling Kok Learning Objectives Consolidate knowledge on the structure of the dental hard tissues Detail the role of epithelial-mesenchymal interactions in the induction of developmental processes Illustrate this with a discussion of...

The embryology of tooth development 2 Dr Wai Ling Kok Learning Objectives Consolidate knowledge on the structure of the dental hard tissues Detail the role of epithelial-mesenchymal interactions in the induction of developmental processes Illustrate this with a discussion of the epithelial- ectomesenchymal interactions important in the induction of odontogenesis Describe the stages classically used to describe the histological progression of tooth development Emphasise how a thorough knowledge of normal tooth developments essential to understanding what happens Developmental stages of teeth Hard tissue formation The next step in the development of the tooth is terminal differentiation of ameloblasts and odontoblasts This leads to formation of the two principal hard tissues of the tooth, the enamel and dentin, respectively This process is called histodifferentiation Cap stage of tooth development Dentine develops from mesenchymal tissue: Ectomesenchyme of the dental papilla and are of neural crest origin Dentine formation: Dentinogenesis Dentine formation begins when the tooth germ has reached the bell stage The organic matrix of dentine is principally collagen Dentinogenesis is a continuous process 1. Differentiation of the odontoblasts 2. Deposition of organic matrix 3. Mineralisation and modification of the organic matrix 4. Peritubular and secondary dentine formation 5. Tertiary dentine formation in response to injury  Life cycle of the odontoblast Ameloblast Odontoblast  Life cycle of the odontoblast: Differentiation phase Ameloblast begins to differentiate first Peripheral ectomesenchymal Peripheral ectomesenchymal cells cells divide Life cycle of the odontoblast: Differentiation phase Acting on a signal from the ameloblast, the pre-odontoblasts begin to differentiate Differentiated cell Undifferentiated cell Life cycle of the odontoblast: Differentiation phase Synthetic organelles increase in size and number, especially the Golgi apparatus and rough endoplasmic reticulum. Peripheral ectomesenchymal cells divide, with some daughter cells migrating below the 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. Once a narrow layer of matrix is laid down mineralisation commences. Life cycle of the odontoblast: Mineralisation Once the first layer of dentine is laid down the differentiated ameloblast begins to deposit matrix.  The collagen fibrils form an interlacing network perpendicular to the odontoblast process Collagen fibrils Odontoblast Right angle process The surface of predentine  The odontoblast begins to secrete its characteristic organic matrix once fully differentiated The 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 dentine 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 not any morphological difference between root and crown dentine Peritubular (intratubular) dentine Peritubular dentine 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 dentine 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 dentine deposition, odontoblasts die Over a 4-year period, the odontoblast population may be reduced by 50%  Tertiary dentine 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 dentine 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 simultaneously but as distinctly different processes The site where they both begin is the enamel– dentine junction Life cycle of the Ameloblasts  Life cycle of the Ameloblasts: Differentiation phase The cells of the internal enamel The differentiating cell (2) is characterised by a epithelium (1) start to differentiate, reversed polarity; the cell becomes columnar and beginning at the future enamel– the nucleus moves to that part of the cell furthest dentine junction of the cusp tip. from the dentine Life cycle of the Ameloblasts: Differentiation phase At stage (3), the cell secretes the initial enamel component of the enamel–dentine junction. This thin layer will be continuous with the inter-rod enamel of the later formed tissue Life cycle of the Ameloblasts: Secretion phase At the beginning of secretion, half the cells are in each form. In this secreting phase, two appearances of ameloblasts can be distinguished by the position of the nuclei within the cell: high (4a) and low (4b). Crystallites are formed at both surfaces of the Tomes process process As the cell retreats, the secreting pole becomes morphologically distinct as a pyramidal  Life cycle of the Ameloblasts: Secretion phase Up to 50% of them die and are phagocytosed by others in the layer. At the end of secretion, most of the high nuclei When the full thickness of enamel have moved to a low position, effectively has formed, ameloblasts lose the increasing the areas of the ameloblast cells as the secretory extension, the Tomes surface of forming enamel increases. process  Life cycle of the Ameloblasts: Maturation Phase The maturation phase lasts two to three times longer than the secretory phase. During the maturation phase there is a regular, repetitive modulation of cell morphology between a ruffled (5a) and a smooth (5b) surface opposed to the enamel. Life cycle of the Ameloblasts: Post Maturation Phase Once the maturation changes are complete, the cells regress in height (6). At this stage, they serve to protect the enamel surface during eruption and later will contribute to form the junctional epithelium.  Stages of enamel formation 1) Pre-secretory 2) Secretory 3) Transition 4) Maturation 5) Post maturation  Pre-secretory Stage Initiation of dentine matrix formation is the trigger for the beginning of amelogenesis Differentiation of the pre-ameloblasts and subsequent resorption of a basal lamina This differentiation begins at the future cusp tips or incisal margins and progresses cervically The ameloblasts differentiate from cuboidal into columnar cells The pre-ameloblasts secrete enamel proteins on top of the dentine matrix  Secretion Stage Enamel proteins are secreted to The shape of Tomes process form an extracellular matrix that is responsible undergoes the initial stages of for the prismatic mineralisation structure of Secretory stage enamel contains enamel 25–30% mineral by weight and is soft and translucent The Tomes process forms at the distal secretory end of the Tomes process ameloblasts, in which they contains Enamel no organelles besides secretory granules Initial stage of enamel formation A=? B=? F=? G=? Enamel Initial stage of enamel formation A = odontoblasts B = ameloblasts C = stratum intermedium D = stellate reticulum E = external enamel epithelium F = developing enamel G = developing dentine Enamel  The transition stage Is the period in which the ameloblasts change from a secretory to a maturation form During this phase, 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 that is soft and translucent 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 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  Ruffled border of ameloblast during the maturation stage Striated (ruffled) border Enamel space  Post maturation stage 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 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 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 As 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 The root is beginning to form as an extension of the inner and outer enamel epithelia in the cervical loop region, which form a bilayered structure called Hertwig’s epithelial root sheath The root sheath will induce differentiation of odontoblasts from the radicular pulp  Root formation The differentiation of odontoblasts and the formation of root dentin  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 sheath the root. fragments after dentin formation These cells differentiate into cementum-forming cells (or cementoblasts) Root formation: Cementum Another possibility is that 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 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 Some 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 Their number in the 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 The target of nerve fibers clearly is the dental follicle; nerve fibers ramify and form a rich plexus around the tooth germ in that structure Nerve fibers enter the dental pulp only when dentinogenesis starts At no time, do nerve fibers enter the enamel organ Summary of tooth formation  References/Reading list Nanci, Antonio. Ten Cate's Oral Histology-e-book: development, structure, and function. Elsevier Health Sciences, 2017 Berkovitz, Barry KB, Graham Rex Holland, and Bernard J. Moxham. Oral anatomy, histology and embryology E- book. Elsevier Health Sciences, 2017 3D modeling : Root tooth development (youtube.com) Acknowledgment Dr Araz Ahmed