6 - Morphology of the tooth enamel. Morphilogical units of mature enamel. Characteristics of the enamel of primary and permanent teeth in childhood

Morphological units of mature enamel

• rods

• striae of retzius

• neonatal line 

• cross striation 

• bands of hunter - schreger

• DEJ

• enamel tufts and spindles 

• enamel lamellae 

• enamel surface

rods

• extend from DEJ to enamel outer surface

• each rod is formed by 4 ameloblasts, 1 for the head, 2 for the neck and another 1 for the tail

• Keyhole shape and almost the same size as a red blood cell

• they are interlocked to prevent fracture and splitting of the tooth

Striae of Retzius

• illustrate changes of metabolism during matrix secretion, maturation and mineralisation

• Less mineralised and thin compared to enamel

Neonatal lines

• enlarged striae of Retzius reflecting great physiological changes occurring at birth

• found in primary teeth and 1st permanent molar

Cross striations

• periodic bands at 4 um intervals across rods (enamel forms at a rate of 4um daily) 

bands of hunter - schreger

• Optical phenomenon produced by changes in directions between adjacent groups of rods 

• Alternating dark and light strips of varying width 

DEJ

scalloped shape prevents shearing of enamel during fucntion

enamel tufts and spindles

• Tufts - occur due to abrupt changes in direction of groups of rods arising from different groups of DEJ

• Spindles - odontoblast processes extending to ameloblast layer and become trapped forming spindles once enamel formation begins

enamel lamellae 

• faults in development of enamel and extend from varying depths to surface of enamel 

• Type A - poorly calcified rod segments 

• Type B - degenerated cells 

• Type C - organic matter 

enamel surface

• Perikymata - shallow furrows

• Cuticle - after tooth eruption the tooth is covered in a cuticle which is debris from enamel organ that is lost rapidly

Characteristics of enamel of primary and permanent teeth in childhood

• higher amount of organic material

• incomplete mineralisation

• small crystals with thicker hydrate layers

• larger intercrystalline spaces

• more intense ionic exchange

• greater opportunities for preventative procedures