Enamel Histology: Composition and Structure

• This presentation is about the histology of enamel
• The presentation is by Lauren Stockham, Oral Dental Sciences, Year 1

Learning Outcomes

• Students should be able to describe the composition and structure of enamel
• Students should be able to describe how the structure of enamel relates to its function
• Students will be able to distinguish the clinical appearance of enamel over the life-course
• Student can apply this knowledge to interpret the clinical significance of health and disease

Assessment

• Formative assessment includes workbooks and quizzes for learning support
• Summative assessment: E-assessment for Oral Dental Sciences

Structures of the Teeth

• Enamel is one of the tissues of the teeth
• Dentine is one of the tissues of the teeth
• Pulp tissue is inside the tooth
• Cementum is around the root of the tooth
• Bone (alveolar) supports the tooth
• Periodontal ligament connects the cementum with the bone
• Gingiva is the gum around the tooth

Composition of Enamel

• Enamel is 96% inorganic minerals
• Majority of the inorganic part is a calcium hydroxyapatite (Ca5(PO4)3OH)
• Minority of the inorganic part is carbonate and fluoride
• Enamel is 4% organic material
• The organic materials are fibrous material (collagen), water and some protiens

Structure of Enamel

• Enamel rods, also called enamel prisms
• Space betwen rods that contain crystallites (calcium hydroxyapatite) called interrods
• Link to amelogenesis
• Incremental Lines
• Prismless enamel (rod-less)
• Dento-enamel junction

Enamel Rod Details

• Millions of enamel rods (prisms) make up the structure of enamel
• Enamel rods are tightly packed and organised with a keyhole shape' in cross-section
• Each rod contains millions of calcium hydroxyapatite crystallites (the mineral/inorganic content of enamel)
• Key hole shape reflects different parts of the enamel rod - a head and a tail
• The head of each orientation of each rod is towards the occlusal/incisal surface
• The tail of each orientation of each rod is towards the cervical region
• Tail=interrod
• Crystallites (hydroxyapatite) are tightly packed inside keyhole shapes
• Extremely long, thin and ribbon-like crystallites may run the thickness of enamel.
• Crystallites in the head parallel with long axis, and tail crystallites diverge slightly
• The pattern of the crystallites within the rod adds to the strength of enamel
• Direction of the rods varies to account for tooth shape
• At the cervical margin the rods are directed more horizontally-apically
• At the cusp tips the rods are almost vertical
• Overall thickness of enamel also varies where it is thickest at the cusp tips and incisal edges and thinnest at cervical margins
• Enamel rods run from the dento-enamel junction (DEJ) to the enamel surface
• Enamel 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
• The direction of the rods is a key consideration in cavity preparation

Amelogenisis

• 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

Incremental Lines

• Incremental lines represent the pattern of amelogenesis that occurs in waves that reflect active and rest phases of growth
• Stria of Retzius are rings of growth in teeth
• Stria of Retzius are visible under a microscope in ground sections of enamel as growth rings/lines
• Perkymata are 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
• Lines in the enamel most marked when newly erupted and gradually wear over time
• The lines in the enamel are visible clinically

Prismless Enamel

• Prismless/aprismatic enamel have unstructured areas
• The very first and last formed enamel shows no usual structure
• The crystals parallel with the surface
• Prismless enamel 30 microns wide at the surface and highly radio-opaque, harder and less soluble and key for demineralisation
• Prismless enamel appears in the primary dentition at 70% in the permanent dentition greatest in the cervecal regions

Dentine-Enamel Junction (DEJ)

• Dentine-Enamel Junction(DEJ) is he junction between enamel and dentine that forms once dentinogenesis and amelogenesis have started
• Dentine-Enamel Junction(DEJ) has a scalloped appearance under a microscope
• Scalloped appearance strengthens bond between the two materials locking them together

Structural Features Visible Under a Microscope

• Enamel Tuft are the result from abrupt changes in direction of the enamel rods
• Boundary of enamel at DEJ is scalloped
• Possibly supports bond between dentine and enamel
• No known clinical significance
• Enamel Spindles is an extension of dentine tubules into enamel
• May result from odontoblast processes
• Odontoblast processes extend into the ameloblast layer becoming trapped
• Process become trapped as dentine starts to form before enamel
• Enamel Spindles can possibly contribute to minor sensitivity
• Hunter Schreger Bands appear as light and dark bands under a light microscope
• Hunter Schreger Bands run upwards from dentine in longitudinal section
• Hunter Schreger Bands appear as growth rings in cross-section

Structural Features on Enamel Surface

• Lamella appear as cracks in enamel
• Lamella are developmental defects
• Lamella appear as jagged lines in surface of crown clinically
• Lamella extend inwards maybe as far as dento-enamel junction
• Lamella can be a result of ameloblast ceasing production of enamel
• Lamella can be mistaken for cracks in enamel and vice versa

Functions of Tooth Enamel

• Protection of tooth
• Help with eating
• Ion exchange
• Smile
• Enamel is inert tissue and does not repair or feel injury
• Thickest at cusp tips, occlusal and incisal surfaces
• Covers the entire tooth crown
• Inert tissue (no living cell due to ameloblasts' limited lifecycle
• Hardest biological tissue, highly mineralized and a crystalline
• White translucent crystallite and permeable with ‘micropores'

Changes in Enamel

• Over time, enamel is subject to tooth wear including attrition, abrasion, and erosion
• Perkymata are worn away, scratches and cracks develop
• Reduced translucency
• Underlying dentine makes enamel appear yellower – normal aging process
• Reduced permeability’ - exchange of ions Ca, PO, F
• Younger tooth clinically significant for exposure in a progression of early enamel lesions

Mineralisation

• Enamel is a mineralised structure and is subject to demineralisation (loss of mineral) and remineralisation (uptake of mineral)
• Acidic favours demineralisation
• Alkaline favors remineralisation
• Fluoride / calcium phosphate are used in remineralisaiton
• The critical pH of enamel is 5.5
• Enamel does not contain a 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

Clinical Application Details

• Enamel that has fluoride incorporated (fluorapatite) has a critical pH of 4.5, more resistant to acids
• Acid removes minerals from the enamel surface creating ‘tags’

Significance of the DEJ and Caries

• Caries cause breakdown of enamel leading to pulp
• Note the difference in size of caries lesion in enamel compared to the lesion in dentine at the DEJ
• Radiopaque structures of enamel, dentine and alveolar bone the whiter it is, the more mineralized it is
• Radiolucent area of enamel consistent with interproximal caries

Structural Abnormalities with Incremental Lines

• Neo-natal line has distinct clinicals
• Neo-natal line is an exaggerated line representing distinction between enamel that has formed before birth and after birth
• Neo-natal reflects disturbance amelogenesis at time of birth
• Exaggerated lines reflect disturbance ie fever, tetracycline stains
• Local disturbances affect teeth in tooth formation
• Systemic disturbances affect all teeth exposure to tetracycline, nutritional deficiencies, molar-incisor hypomineralisation
• Genetic factors may affect teeth

Enamel Structural Abnormalities

• Hypoplasia is when not enough enamel is formed
• Hypomineralisation poorly formed enamel-known as molar-incisor hypomineralisation
• Structural abnormalities’s impact range patients can range from minimal to significant clinical
• Look for colour changes and morphology discrepancies