Enamel Flashcards
Composition of enamel by weight
95% hydroxyapatite, 4% water, 1% organic matrix
What forms the small (1%) organic component of enamel?
Proteins - amelogenins, enamelins, peptides, amino acids
What is the basic structural unit of enamel?
enamel prism/rod
Cross-sectional shape of enamel prisms
keyhole shaped
Longitudinal shape of enamel prisms
sinusoidal
How does the enamel rod/prism structure/boundary arise?
Sudden changes in crystallite orientation
Structure of amelodentinal junction
Scalloped - bowl appearance with edges pointing towards enamel (convexities) and the concavities on the dentinal surface.
Which cell type does the enamel originate from?
Ectodermal origin
Which dental tissues are of ectomesenchymal origin?
Dentine, cementum, pulp
Which sections are used to study enamel?
Ground sections - hard mineralised tissues remain intact but soft CT and epithelia are lost.
In which sectioning technique is enamel not visible?
Demineralised section - only soft CT and organic matrix of mineralised tissue remains.
How does the thickness of enamel vary?
Thickest over cusps and incisal edges, thinnest at the cervical margin.
Why is it important that enamel resists abrasion?
Wears down slowly - enamel cannot be repaired/replaced.
What is the hardest tissue in the body?
Enamel
How is the possibility of enamel fracture reduced despite being brittle?
Flexible support of underlying dentine
How does hardness / density of enamel vary?
Surface enamel is harder and more mineralised than deeper enamel. Hardness also decreases from the cusp tip / incisal edge to the cervical region.
How does translucency of enamel vary?
Translucency increases with age as mineralisation increases. This may lead to transmission of the yellow colour of underlying dentine.
What are enamel rods/prisms made up of?
Several million crystallites
What is the principal component of enamel?
calcium hydroxyapatite crystallites (95% weight, 90% volume of enamel) which run from the amelodentinal junction to the surface
How does the mineral content of the enamel rods/prisms vary from the ADJ to the surface?
Mineral content increases from ADJ to surface
Shape of hydroxyapatite crystals
Hexagonal
Molecular arrangement within each crystallite
Central hydroxyl group surrounded by 3 calcium ions which are surrounded by 3 phosphate ions that are evenly spaced. 6 calcium ions in a hexagonal shape enclose the 3 phosphate ions.
Possible ions which may substitute into the hydroxyapatite structure
Mg2+, CO3 2-, F-
Importance of water and distribution within enamel
Ions (e.g. F-) can travel through it
How are the regions of the keyhole-shaped enamel rods described?
Head and tail - tail lies between heads of adjacent prisms.
Which directions do crystals run in enamel prisms?
In the head of the prism, the crystals run parallel to the long axis of the prism. In the tail, crystals diverge to become angled 60-70 degrees to the long axis.