Embryology of Exfoliation and Eruption DTH21 PDF

Embryology of tooth eruption and exfoliation Learning Outcomes  Describe the three phases of tooth eruption  Describe how the dento-gingival junction originates  Describe the process of tooth exfoliation  Outline the theories of tooth eruption  Be able to link the processes to the d...

Embryology of tooth eruption and exfoliation Learning Outcomes  Describe the three phases of tooth eruption  Describe how the dento-gingival junction originates  Describe the process of tooth exfoliation  Outline the theories of tooth eruption  Be able to link the processes to the developmental timeline of teeth and to eruption and exfoliation ages for each tooth 2 Select the single best answer: The eruption process of teeth: a) Begins in early childhood around 6 months of age b) Begins before birth and continues throughout life c) Is completed when all teeth are present in the mouth d) Is completed once all the primary teeth erupt 3 Select the single best answer: The eruption process of teeth: a) Begins in early childhood around 6 months of age – this is part of the active eruption phase, there are eruptive processes that start much earlier b) Begins before birth and continues throughout life c) Is completed when all teeth are present in the mouth – continues after the teeth have appeared in the mouth d) Is completed once all the primary teeth erupt – the permanent dentition erupts after the primary dentition 4 Eruption of teeth  The eruption process of teeth is a continuous process that occurs throughout life that begins during early embryological development  It involves three phases  It begins once the tooth crown development is complete during the bell stage and alongside root development so although the first tooth is visible in the oral cavity around 6 months of age, the process begins much earlier A histological section of an erupting lower incisor and the developing permanent  It is a multi-factorial process not successor – what stage of development is entirely understood where there are the successor based on its shape? various theories that explain how it occurs 5 Lets reflect on why we need to know this? Provides the physiological foundation of oral health advice for:  ‘teething’ for parents  wobbly teeth for children and parents  ‘wisdom’ teeth erupting Be able to clinically distinguish Photo showing the lower normal and abnormal tooth jaw of a child with a eruption and exfoliation for wobbly front tooth. example:  Identify a supernumerary tooth 6  Identify a missing tooth Definitions  Eruption is the physiological movement of teeth from their developmental position in the alveolar bone through the soft tissues to its position of function in the oral cavity. The process of eruption has three phases.  Exfoliation is the physiological Gif showing eruption and resorption of primary teeth until exfoliation of a mandibular they are lost (they exfoliate). permanent premolar and its primary predecessor respectively. 7 Embryological links to eruption and exfoliation  Begins during the bell stage  Ameloblasts, cells of the enamel organ, osteoclasts, odontoclasts and oral epithelium play a role  Underpinning knowledge explains the various theories on the mechanisms of the erupting tooth 8 Clinical links to eruption and exfoliation The developmental timeline during embryology follows into life with the eruption timeline that is ESSENTIAL to identify the presence and position of:  Missing teeth  Supernumerary teeth  Crowding of teeth and Clinical image showing an ectopically orthodontic referral erupting UL3 with an over-retained ULC 9 Workbook activity  Complete Section 1, Question 1-2, to recap your existing knowledge of embryology relevant for tooth eruption. 10  Three phases of the eruption process A outlines the phases of eruption, B shows the histological perspective of each 11 1. The pre-eruptive phase  The movement of the developing tooth within the alveolar bone until crown formation is complete  This movement starts during the bell stage remodelling the bony crypt to allow more space for the developing tooth 12 2. Active eruption phase  The movement of teeth through the alveolar bone (intraosseous) then the soft tissue (supraosseous) to the oral cavity  This phase starts around the similar time as root formation and continues until the tooth reaches occlusion (remember root 13 3. Post-eruptive phase  The movement of teeth after active eruption to maintain occlusion and compensate for occlusal and proximal tooth wear as well as growth  This movement occurs through out life such as when an opposing tooth is removed 14 Single best answer: The eruption process of teeth… a) Starts during the bell stage of tooth development b) Starts after birth for all the primary teeth c) Starts once the root formation of a tooth is complete d) Starts at the same time as the crown formation begins 15 Single best answer: The eruption process of teeth… a) Starts during the bell stage of tooth development b) Starts after before birth for all the primary teeth c) Starts once alongside the root formation of a tooth is complete which continues after the active eruption phase d) Starts at the same time as the after crown formation begins is complete 16 Active eruption phase in more detail Diagrams showing the active eruptive phase 17 The role of the Reduced Enamel Epithelium Once amelogenesis is complete, the ameloblasts shrink and combine with the outer enamel epithelium, stratum intermedium and any residual stellate reticulum forming the reduced enamel epithelium It functions to:  Protect the developing tooth crown  Fuse with oral epithelium creating an eruption pathway Vertical cross-section of the cusp tip of an  Form the dento-gingival incisor during the pre-eruptive phase 18 junction – an essential seal The eruption pathway  The bone overlying the developing crown is resorbed by osteoclasts (and odontoclasts of predecessor teeth)  This allows a pathway for movement that is initiated by complex signaling of the cells  The reduced enamel epithelium protects the tooth crown from the osteoclasts and the odontoclasts 19 The eruption pathway  The reduced enamel epithelium fuses with oral epithelium to create an eruption pathway  There are no blood vessels or nerves present yet  Stimulation of and trauma occurs to ectomesenchyme to enable to fusion  This fusion forms a seal that Fusion of the oral epithelium prevents exposure of the and reduced enamel epithelium 20 underlying ectomesenchyme Formation of the dento-gingival junction and sulcus  As the tooth breaks through the oral epithlium, the reduced enamel epithelium and oral epithelium (now combined) form the dento-gingival junction sealing the external oral cavity off from the rest of the body  It also forms a shallow trough that will create the gingival sulcus A tooth erupting through the gingiva and forming the dento-  This junction has clinical gingival junction 21 Rate of eruptive movement  Movement through bone is slow = 1-10um/day  Movement through soft tissue is faster = 75um/day until occlusion reached  Muscular forces of the tongue, cheek, lips guide the tooth into position  Sustained force of 4-5g is required – any habits that you can think of? 22 Single best answer The reduced enamel epithelium: a) Forms a protective layer over the tooth root during eruption b) Forms from ameloblasts during amelogenesis c) Surrounds the developing tooth crown during eruption d) Fuses with the oral epithelium to form the periodontal attachment with the tooth 23 Single best answer  The reduced enamel epithelium: a) Forms a protective layer over the tooth root crown during eruption b) Forms from differentiated ameloblasts and the outer enamel epithelium, stratum intermedium and any residual stellate reticulum during after amelogenesis c) Surrounds the developing tooth crown during eruption d) Fuses with the oral epithelium to form the periodontal gingival attachment with the tooth 24 Exfoliation (shedding) of primary teeth  As the permanent successor teeth develop (positioned lingually), they increase in size and start the eruptive phases  This signals the exfoliation of the primary teeth where odontoclasts slowly resorb the roots of the primary teeth up to its crown that remains largely intact  Masticatory forces also contribute to the exfoliation process by applying pressure  The pattern is usually the same thus A ground section of an exfoliating any variations are a key indicator of tooth and an erupting permanent 25 abnormalities clinically successor So how does the tooth actually erupt? We do not really know... There are various theories that attempt to explain this process. Research is ongoing in this area. 26 Root formation theory  The tooth crown is elevated by the thrust of root development  Refuted since eruption occurs throughout life and root development does not Rocket launching analogy for root formation pressure 27 Bone re-modelling  It is unsure whether bone resorption and deposition cause teeth to erupt OR whether this is an effect  It is not the only mechanism, it is believed that it is modulated by the dental follicle Histological section of alveolar bone surrounding the developing tooth 28 Dental follicle theory  Signals between the dental follicle and the reduced enamel epithelium have been found to induce bone re- modelling  This may explain the  A – enamel organ consistency of eruption  B - dental papilla times as linked to the  C – dental follicle lifecycle of ameloblasts A histological section of a tooth in the bell 29 stage Periodontal ligament theory  The power of the formation of the periodontal ligament by fibroblasts is thought to contribute to the movement in tooth eruption  This is refuted similar to the root formation theory 30 Formation of the periodontal ligament Molecular determinants of tooth eruption Various molecules are thought to be involved in the complex process of tooth eruption each playing a different role. There are also more recent theories under investigation including bite forces on the soft tissues and neuromuscular forces 31 Summary The eruption and exfoliation processes are complex and multi-factorial, we have discussed:  The three phases of eruption  The active phase in detail  How the phases link to embryological development  Outline of the theories of eruption More detail and the clinical application will be discussed further in Oral Biology. 32