Histology of Enamel PDF

Histology of Enamel Lauren Stockham Oral Dental Sciences, Year 1 Intended learning outcomes By the end of the session students should be able to: Describe the composition and structure of enamel Describe how the structure of enamel relates to its function Be able to distinguish the clinic...

Histology of Enamel Lauren Stockham Oral Dental Sciences, Year 1 Intended learning outcomes By the end of the session students should be able to: Describe the composition and structure of enamel Describe how the structure of enamel relates to its function Be able to distinguish the clinical appearance of enamel over the life-course Apply this knowledge to interpret the clinical significance of enamel in health and disease Assessment Formative: (to support your learning) Workbooks, Quizzes Summative: E-assessment for Oral Dental Sciences Histology of the tissues of the teeth and supporting structures Enamel Dentine Pulp Cementum Bone (alveolar) Periodontal ligament Gingiva Let’s reflect on WHY we need to learn this… To effectively support, maintain and improve oral health for our patients, examples: Refresh your knowledge Where is enamel found? What is the embryonic origin of enamel? How does this link to the embryonic This Photo by Unknown Author is licensed under CC BY-SA-NC origin of enamel? Refer to Oral Embryology and Tooth Morphology lectures for more information. Source: Unknown CC Enamel COMPOSITION AND STRUCTURE Composition of enamel Mineral (inorganic) content: 96% inorganic minerals Majority - calcium hydroxyapatite (Ca5(PO4)3OH) Minority – carbonate, fluoride Organic content: 4% Fibrous material (collagen) Chemical structure of calcium hydroxyapatite Water (Rujitanapanich et al 2014). Some proteins Overview of structure of enamel Enamel rods (prisms) Interrods Crystallites (calcium hydroxyapatite) Link to amelogenesis Incremental lines Prismless enamel (rod-less) Dento-enamel junction and microscopic features The greenhouse in the Royal Opera House demonstrating an analogy for a strong yet brittle structure like enamel. Image credit: Lauren Stockham Enamel rods Enamel Millions of enamel rods (prisms) make up the structure of enamel. They are tightly packed and organised with a keyhole shape’ in cross-section (b). Each rod contains millions of calcium hydroxyapatite crystallites (the mineral/inorganic content of enamel). Zoomed in detail of the enamel rods from a molar tooth Enamel rods The key hole shape reflects different parts of the enamel rod – a head and a tail (A). The orientation of the head of each rod is usually towards the occlusal/incisal surface relative to the tail which is oriented towards the cervical region. The tail is also known as an interrod. Structure of enamel rods in cross-section and longitudinal sections Each rod and interrod is surrounded by a sheath (organic material). Enamel crystallites There are millions of crystallites (hydroxyapatite) in each rod tightly packed in keyhole shapes. These are extremely long, thin and ribbon-like, they may run the thickness of enamel. In orientation, the crystallites in the head = parallel with the long axis of the rod, and the tail = diverge slightly (c). The pattern of the crystallites within the rod adds to the strength of enamel. Structure of enamel crystallites within the enamel rods Enamel rods - orientation To account for the shape of the tooth, the direction of the rods varies. At the cervical margin they are directed more horizontally-apically and at the cusp tips they are almost vertical. The overall thickness of enamel also varies where it is thickest at the cusp tips and incisal edges and thinnest at the cervical margins. Why might this be? Enamel rods They run from the dento-enamel junction (DEJ) to the enamel surface. The rods traverse together bending right and left in an s- shape manner. At the DEJ the rods are positioned perpendicular (at right angles) to the dentine in general. At the cusps, the rods are twisted forming gnarled enamel. These structural features add to the overall strength of enamel. Longitudinal section of enamel rods from the DEJ to the surface Enamel rods direction and clinical significance The direction of the rods is a key consideration in cavity preparation to avoid unsupported enamel that will fracture leading to failure. Diagrams showing a posterior proximal cavity preparation Enamel rods and amelogenesis Link back to Embryology of the Crown lecture The structure of the enamel rods is formed by the ameloblasts during amelogenesis. Each enamel rod (and associated interrod) are formed by 1 ameloblast. The lifecycle of ameloblasts is significant since it means that enamel is inert – it has no cell during its life. The pattern of amelogenesis results in incremental lines. Source: dent-wiki.com Zoomed in longitudinal section of enamel rods associated with the ameloblast. Incremental lines Incremental lines represent the pattern of amelogenesis that occurs in waves reflecting active and rest phases of growth Similar to growth rings (lines) of a tree, the rings of growth in teeth are called Stria of Retzius (line of retzius) These lines may be detected clinically on the surface of enamel as normal subtle Tree in cross-section features and as more distinct features Incremental lines Stria of Retzius - are visible under a microscope in ground sections of enamel as growth rings/lines Perkymata – The edge of the stria of retzius that is visible as a shallow furrow on the enamel surface showing where the incremental lines reach the surface on the labial/buccal surfaces. These are most marked when newly erupted and gradually wear over time. Visible clinically. Diagram of the incremental lines in enamel Looking at the pattern of the Stria of Retzius - can you remember where amelogenesis starts?  1. Identify the teeth present in each clinical image Take a look… 2. Identify the perkymata in each – note that the teeth have been air-dried to enable visibility of these features 3. Identify the morphological features indicated by the arrows at the incisal edges 4. Consider why there might be a difference in the size of the features between 3. both images. 2. ‘Prismless’ Enamel Although enamel rods (prisms) are the main structural unit of enamel, there are some area of enamel where it is unstructured known as prismless/aprismatic. Surface – The very first and last formed enamel shows no usual prism no prisms structure where the crystals are parallel with the surface. Body – prisms/ It is 30 microns wide at the surface, highly radio-opaque, rods harder and less soluable – includes more fluoride and carbon thus key for demineralisation/ remineralisation. Light microscopy image showing the distinction between the surface enamel (no rods/prisms) and body (rods) of enamel It is seen in the primary dentition and 70% of the permanent dentition, greatest in the cervical regions. Clinical significance  may interfere with optimal etching. Dentine-Enamel Junction (DEJ) The junction between enamel and dentine that forms once dentinogenesis and amelogenesis have started. Scalloped appearance under a microscope thought to strengthen the bond between the two materials almost locking them together. The dento-enamel junction between enamel and dentine Structural features at the DEJ Enamel Tuft Thought to result from abrupt changes in direction of the enamel rods This is because of the scalloped boundary of enamel at DEJ Possibly supports bond between dentine and enamel? No known clinical significance Seen in traverse sections of enamel Cross-section of DEJ Structural features at the DEJ Enamel Spindles extension of dentine tubules into enamel May result from odontoblast processes extending into the ameloblast layer becoming trapped since dentine starts to form before enamel Possibly contribute to minor sensitivity Cross-section of DEJ Structural features visible under a microscope Features are only visible under a microscope, they help to grasp the histological structure light and dark bands under light microscope = Hunter Schreger Bands longitudinal section run upwards from dentine – see image cross-section appear as growth rings Cross-section of enamel and dentine Structural features at the enamel surface Lamella Appear as cracks in enamel - developmental defects appear as jagged lines in surface of crown clinically extend inwards maybe as far as dento- enamel junction result of ameloblast ceasing production of enamel Can be mistaken for cracks in enamel and vice versa Cross-section of enamel and dentine Link to structure FUNCTIONS OF ENAMEL Soft boiled egg analogy to explain the functions and link with structure Functions Inability to Ion Protection Eating repair or Smile exchange feel injury How the functions of enamel link to the structure Functions Structure Protection of the tooth/pulp Thickest at cusp tips, occlusal and Eating: chewing, biting etc incisal surfaces Inability to repair or feel injury Covers the entire tooth crown Able to remineralise and demineralise Inert tissue (no living cell due to – ion exchange ameloblasts’ limited lifecycle Smile - Aesthetically appealing ‘pearly Hardest biological tissue whites’ Highly mineralized tissue White translucent crystallite Permeable ‘micropores’ Link to functions and clinical appearance CHANGES IN ENAMEL OVER THE LIFE- COURSE Enamel changes over the life-course Over time enamel is subject to tooth wear including: Attrition Abrasion Erosion For example, perkymata are worn away, scratches and cracks develop. Colour changes – reduced translucency and increase in underlying dentine makes enamel appear yellower – normal aging process. Enamel changes over the life-course Over time enamel is subject to: Reduced ‘permeability’ – exchange of ions such as Ca, PO, F- Clinical significance for exposure to topical fluoride as a younger tooth and in the progression of early enamel lesions. Demineralisation-Remineralisation cycle As a mineralised structure, enamel is subject to Link to Aetiology of Caries Lecture demineralisation (loss of mineral) and remineralisation (uptake of mineral) In acidic conditions the balance favours demineralisation In alkaline conditions the balance favours remineralisation enabling uptake of fluoride and calcium phosphate The critical pH of enamel is 5.5 Acid Alkaline What substance in the mouth is alkaline and thus favours remineralisation? Demineralisation-Remineralisation cycle Since enamel does not contain any living cell, it is not able to repair itself with the immune system and therefore it can not feel injury This allows the progression of the early stages of dental caries to occur unnoticed to the host The composition and structure of enamel is relevant for the clinical prevention and treatment of dental caries Cross-section of DEJ Clinical application – preventive and restorative Fluoride – enamel that has fluoride incorporated (fluorapatite) has a critical pH of 4.5, lower than hydroxyapatite thus more resistant to acids and demin. Acid etch – acid removes minerals from the enamel surface creating ‘tags’ that enable the bond to fill in and stick the composite to. Link to Histopathology of Enamel Caries lecture in Year 1. The significance of the DEJ and caries Take close look at: The breakdown of enamel The progression into dentine and to the pulp Note the difference in size of the lesion in enamel compared to the lesion in dentine at the DEJ… Clinical image showing the progression of caries Radiographic view of dental caries in enamel Take a close look at the radiopaque structures of enamel, dentine and alveolar bone – the whiter it is, the more mineralized it is. The circle identifies a radiolucent area of enamel that is consistent with interproximal caries clinically although not breached the DEJ. Radiograph showing the LR56 Enamel STRUCTURAL ABNORMALITIES Structural abnormalities - Incremental Lines Clinically distinct: Neo-natal line an exaggerated line representing the distinction between enamel that has formed before birth and after birth It usually reflects a disturbance in the amelogenesis at birth (perinatal) Other exaggerated lines Reflect systemic disturbances during amelogenesis ie fever, tetracycline staining Primary dentition showing the distinction between when enamel forms in the crown. Structural abnormalities in enamel Link to Embryology of the Crown lecture As a result of disturbances during amelogenesis, defects in the enamel structure may result from: Local disturbances – affect individual teeth such as trauma systemic disturbances – affect all the teeth forming at the time such as fluorosis (too much fluoride), or exposure to tetracycline, nutritional deficiencies, molar-incisor hypomineralisation Genetic factors – may affect all teeth such as amelogenesis imperfecta These defects may result in significant clinical Tetracycline staining of the primary dentition. implications. Structural abnormalities in enamel Defects during amelogenesis can result in enamel hypoplasia or enamel hypomineralisation collectively known as molar-incisor hypomineralisation. The impact for patients can range from Showing enamel hypomineralisation (Patel et al minimal to significant clinical 2019). implications. Further information in articles. Link to Developmental Anomalies lecture. Structural abnormalities in enamel Can you identify: Any colour changes in specific teeth? Any morphology discrepancies with specific teeth? Image showing Molar incisor-hypomineralisation in an 8 year old child. (Seow 2014). Summary Enamel The clinical Composition Structure Functions significance How functions organic and structural enamel rods link to inorganic abnormalities structure changes over caries the life-course prevention Knowledge and understanding of enamel structure is essential to promote the prevention of dental caries - most prevalent chronic condition globally. Key area of research focus to understand how enamel can be regenerated. Reading Highly recommend reading – included in post-session on moodle. First article gives detailed and succinct explanation of amelogenesis and dentinogenesis Both articles discuss the clinical impact