When and where is hard tissue formation visible
- Late bell stage (18 weeks)
- Visible at the tips of future cusps of the developing crown
What happens to the short columnar cells of the inner enamel epithelium
They become elongated and become pre-ameloblasts
What happens to the cells adjacent to the dental papilla
Become columnar and develop into odontoblasts
What do the odontoblasts begin the formation of
Predentine and dentine
What does the presence of dentine induce
Induces the ameloblasts to further develop, deposit and then later modify the enamel
What are the 4 stages of amelogenesis
- Pre-secretory stage
- Secretory stage
- Maturation stage
- Post-maturation stage
What happens during the pre-secretory phase
- Initially, the inner enamel epithelium has a proximal end and a distal end with a central nucleus
- After differentiation into an ameloblast, nucleus shifts to the proximal end
- Distal end continues to proliferate and form the Tomes Process
What happens during the initial secretory phase
- RER form around the nucleus and translate the mRNA of enamel proteins
- Translated proteins are processed and packaged into vesicles via golgi apparatus + stored in the Tomes Process
- Tomes Process secretes organic matter made from enamel proteins
- Organic enamel matrix is STRUCTURELESS as Tomes isnt fully developed
What happens during the mineralisation secretory stage of amelogenesis
- Ameloblasts continue to grow & Tomes Process now has a proximal end and distal end
- Proximal portion forms interrod enamel whilst distal portion forms rod enamel
- Distal part secretes hydroxyapatite crystals
- Proximal part releases crystals at 65º to the long axis
What happens in the final phase of the secretory stage
- When enamel reaches max thickness, ameloblasts shorten and lose distal portion of Tomes Process
- Ameloblast secretes final layer of enamel (same structureless organic enamel as in the beginning)
- Enamel now composed of rod and interrod containing layer, sandwiched by 2 rodless layers
what happens in the maturation stage of amelogenesis
- Replace organic material with inorganic material
- Happens due to ameloblasts alternating between ruffled ends and smooth ends facing the enamel matrix
- Ruffle-ended ameloblasts degrade organic enamel proteins by secreting enzymes so enamel is in smaller proteins + easier to absorb by ameloblasts
- Creates an acidic environment so ameloblasts pump bicarbonate ions to alkaline the area
- Ameloblasts also secrete Ca+ ions - bind to hydroxyapatite, mineralising them & causing them to grow, so pushing organic enamel towards ameloblasts
What occurs in the post-maturation stage
Ameloblasts are further reduced and flattened to form a protective layer for the enamel
What are the 5 stages of dentinogenesis
- IEE cells differentiate into pre-ameloblasts, so papilla divides - 1 cell remains near basal layer + 1 cell is undifferentiated in the tooth pulp
- Due to induction from preameloblasts, growth factors stored in basal lamina cause papilla cells to differentiate into pre-odontoblasts
- Odontoblasts secrete dentine matrix & move further back leaving a cytoplasmic extension in the forming dentine called the Tommes Fibre
- During formation of organic matrix, odontoblasts secrete first layer nearest to odontoblast called pre-dentine (100% organic) so as dentinogenesis goes on, layer will always be present
- Mineralisation occurs due to hydroxyapatite crystals smaller than ones in enamel. Odontoblasts secrete vesicles into dentinomatrix, which initiate + promote mineralisation
What are 2 similarities of amelogenesis and dentinogenesis
- Forming cells in both processes are elongated columnar with visible secretory organelles
- Main mineral produces is impure hydroxyapatite crystals
What are 4 differences between amelogenesis and dentinogenesis
- Ameloblasts made from oral epithelium of ectoderm, odontoblasts made from ectomesenchyme from NCC
- In enamel the proteins in the matrix are largely degraded during maturation, in dentine theres no change in the matrix during maturation
- Enamel cannot self repair, Reparative dentinogenesis can occur due to external stimuli
- Enamel is fully formed before eruption, dentine is slowly formed throughout life
