Life cycle of ameloblast

Life cycle of ameloblasts

• Morphogenic stage:• Organising stage:• Formative stage:• Maturative stage:• Protective stage:• Desmolytic stage:

Morphogenic phase

during the morphogenic stage – shape of the crown is determined

cells of the inner enamel epithelium are cuboidal or low columnar with large centrally located nuclei and poorly developed Golgi

separated from the dental papilla by a basement membraneIEE cells influence dental papilla cells to differentiate into

odontoblasts

Organising stage– Ameloblasts elongate. Almost become double in length– Nuclei shift toward the stratum intermedium (Reversal of

functional polarity)– Cell becomes polarized with most of the

organelles distal to the nucleus– at the distal end of the cell – extensions form called

Tome’s processes - against which enamel forms– Basement membrane fragments by the cytoplasmic

projections of the ameloblasts – during the formation of predentin• this allows contact between the pre-ameloblasts and

pre-odontoblasts– Reversal of nutritional supply.

Secretory Stage• cells acquire intense synthetic and secretory activity• enamel proteins are translated by the RER, modified by the Golgi and

packaged into secretory granules– migrate to the distal Tome’s processes

• secretion is constitutive – the secretory granules are not stored for long within the cells

• the contents of the secretory granules are released against the newly formed dentin along the surface of the Tome’s process

• little time elapses between the secretion of enamel and its mineralization• as the initial enamel layer forms – the ABs migrate away from the dentin

surface and develop a distal portion of Tome’s process – extension from the existing proximal portion of Tome’s process– the pTP extends from the distal junctional complex to the surface of the enamel layer– the dTP interdigitates into the enamel beyond the initial layer– the cytoplasm of both processes is continuous with that of the body of the AB– so once the initial enamel layer forms the AB only has a pTP– the dTP forms once the enamel forms into rods– when the dTP forms – the enamel proteins are secreted at two sites located at defined ports along

the dTP– the dTP lengthens as the enamel layer thickens – it also becomes thinner as the rod grows in diameter– eventuallyu squeezed out of existence

Maturative stage• Ameloblast bring about changes in the physiochemical properties of the

enamel• actually because the pre-existing HA crystals of the enamel grow in width

and thickness and NOT because new crystals are made• made up of a transitional phase and the maturation proper phase

– transitional phase – after the full thickness of the enamel has formed• the ABs undergo significant morphological changes that prepares them for the

maturation of the enamel• reduction of AB height and a decrease in their volume and organelle content

– maturation proper phase – Ameloblasts become involved in the removal of water and organic material• characterized by the modulation of the cells – cyclic creation, loss and recreation of

a highly ruffled apical surface and a smooth surface• occurs in waves traveling across the crown of the developing tooth – from least

mature to most mature enamel regions• the calcium ions are required for active crystal growth because the cellular

junctions at the ruffled end are more leaky• these show enhanced endocytic activity and numerous lysosomes – for the

withdrawl of enamel proteins from the maturing enamel matrix

Ruffled and Smooth ameloblasts

Ca+

Ca+

Ca+

Ca+Ca+

Ca+

Ruffled ended A Smooth ended A

Protective stage

After accomplishing of amelogenesis, the ameloblasts secrete or leave structure less material on enamel surface, known as primary enamel cuticle.

The dental organ epithelium becomes reduced in thickness (reduced enamel epithelium) and functions as a protection against contact with connective tissue to inhibit cementum deposition or enamel resorption.

In this stage the composition of Enamel can be modified.

Oral Epithelium

Reduced Enamel E

Enamel space

Dentin

Desmolytic stage

The reduced enamel epithelium induce atrophy of the mesoderm which separate it from the oral epithelium. It leads to eruption.

REE releases proteolytic enzymes which cause breakdown of collagen fibers

As tooth approaches oral epithelium both epithelium and REE proliferated

They fuse to each other forming epithelial plugThis degenerated to form a pathway for eruption

Proliferating REE

Epithelial plug

Formation of Enamel

• Enamel formation takes place in two steps– Formation of organic matrix– Mineralisation• Early incomplete mineralization• Maturation.