Amelogenesis: Enamel Formation
What is the neonatal line?
A pronounced incremental line of retzius -Special, accentuated Striae of Retzius -Occurs at point of development in primary tooth when child is born -Result of marked environmental change at birth
What is the function of dentin?
Dentin provides support to enamel, preventing enamel fractures during occlusal loading. Enamel must have support of more resilient underlying dentin due to being brittle. If caries destroys dentin, enamel is fractured easily.
________ is the substance that covers the crown of the tooth.
Enamel
Describe what enamel looks like under an electron microscope.
Enamel seems like a woven fabric in the electron microscope. Enamel crystals are organized in prisms, and these extremely long and parallel-organized hydroxyapatite crystals are often organized perpendicular to each other.
T/F: All parts of the tooth, develop at the same rates of enamel formation.
False. At any given time, different parts of the tooth may be at a different stage of enamel formation.
T/F: Maturation begins after matrix has reached full thickness.
False. Maturation begins before matrix has reached full thickness.
What is tooth morphogenesis?
a process that is regulated by sequential and reciprocal interactions between the epithelial and mesenchymal tissues and during this morphogenesis the simple oral ectoderm thickens, buds, grows and folds to form the complex shape of the tooth crown.
What is the function of cementum?
Its main function is to act as an attachment layer for the periodontal ligament (PDL).
What matrix protein is mostly present during maturation?
KLK-4 (kallikrein 4)
What stage of tooth development is this?
Late bell stage
What matrix protein is mostly present during secretion?
MMP-20 (matrix metalloendoproteinase-20)
What two proteins cleave amelogenin?
MMP-20 and KLK-4 at different times during enamel formation
What protease is mostly present during secretion?
Metalloendoproteases
What is the primary component of the late maturation stage?
Mineral (95%)
What does 96% of tooth enamel consist of?
Mineral (primarily calcium phosphate = hydroxyapatite)
What constitutes as the central mechanism that regulates the morphogeneisis of all these tooth organs?
Molecular interactions between the ectoderm and the underlying mesenchyme
What are cross striations?
Periodic banding across the enamel rod Results from 4 micrometer of daily growth of the enamel -SEM shows alternating constriction and expansion of rods -Constrictions are gouges in the rods structure
What is the primary component of the secretory stage?
Protein (66%)
What protease is mostly present during maturation?
Serine proteases
What happens when cementum is exposed to environmental assaults?
Since the cementum is relatively soft, it does not wear well against environmental assaults. Therefore, it abrades away rapidly whenever it is exposed to the oral environment because of periodontal recession.
How long does the formation of enamel take?
Start to finish can be 5 years
When are amelogenin fragments removed?
during final enamel mineralization
Teeth develop as ______ appendages in the vertebrate embryo. Their early development resembles morphologically, as well as molecularly, other organs such as ______ and _______.
ectodermal; hairs; glands
The process of forming the _______ is relatively quick. The process of _______ is the reason why enamel formation takes a long time.
enamel matrix (secretion); making it hard (mineralization)
In the secretory phase, enamel mineralizes in specific patterns called _______.
enamel rods
What happens in the presecretory stage?
the ameloblasts form, polarize, and mature Cuboidal, undifferentiated cells called pre-ameloblasts go through an organizational process and differentiation to become elongated (columnar in shape). Nuclei become aligned at proximal end, next to stratum intermedium, in which they are now considered polarized cells. Cells prepare to and begin to secrete enamel proteins.
What is the hardest substance in the body?
tooth enamel **Comparable to mild steel which makes it brittle.
The ________ and _______ are associated with membrane infoldings (im) on the proximal and distal portions of the Tomes' processes, respectively.
interrod (IGS); rod (RGS) growth sites
Enamel is composed of:
large rod containing inter rod layer sandwiched by two thin roles initial and final layers
In the gradual completion of mineralization, what percentage of initially secreted protein and water are lost?
more than 90%
Amelogenin self-assembly into _______ with multiple amelogenin molecules.
nanospheres
What happens once full thickness of enamel matrix is secreted?
-Ameloblasts become shorter and lose the Tome's processes/resemble original shape. -Since Tome's processes dictates the rod shape, the outer-most layer of enamel does not have a rod structure (aprismatic).
What happens in the late bell stage of tooth development?
-Dental lamina is disintegrating. Tooth formation is not connected to oral epithelium. -Dentin and enamel is beginning to form at the crest of the inner enamel epithelium, which is the site of future cusp tip.
In what direction do the hydroxyapatite crystals run?
-Head: parallel to the longtudinal axis of the rod in the head -Tail: Flaring of crystals -Crystal pattern results in many different patterns seen when enamel is sectioned in different planes.
What is the Striae of Retzius?
-Incremental weekly rhythmic growth lines -Most common in human permanent teeth -Identified using ground sections of calcified teeth: --Longitudinal section: appear as series of dark lines from DEJ to tooth surface --Cross-section: concentric rings
What is the function of amelogenin?
-Structural protein that binds calcium (hydroxyapatite) -Helps direct enamel crystal formation -Many alternatively spliced forms of amelogenin exists (the different forms have different functions)
What are enamel rods?
-Tightly packed mass of hydroxyapatite crystals -Cylindrical-with special relation to interrod region cervical to it
What are lamellae?
-extend varying lengths from enamel surface (extends from DEJ to enamel surface) -are filled with organic material (protein) -arise from abrupt changes in the direction of groups of rods
What are enamel tufts?
-small, dark brushes that project from DEJ a short distance -are hypomineralized areas with greater protein -arise from abrupt changes in the direction of groups of rods
What happens during amelogenin processing?
1. Amelogenin gets secreted as a full protein. 2. It gets cleaved by MMP-20. 3. As the enamel matures, KLK-4 is what will degrade it later.
What makes up the protein/organic component of the enamel?
1. Enamel Matrix Proteins -Amelogenin -Ameloblastin -Enamelin -Amelotin -ODAM 2. Dentin/Bone Proteins -Dentin Sialo-Phospho Protein (DSPP-DSP/DPP) -Dentin Matrix Protein-1 (DMP-1) -Bone Sialo Protein (BSP) 3. Proteinases (removes the proteins secreted in the matrix by degradation): -Matrix Metallo -Proteinase-20 (MMP-20) -Kallikrein-4 (KLK-4
What are the three stages of ameloblast development?
1. Presecretory 2. Secretory 3. Maturation
Describe the maturation process of enamel cells into rods.
1. The cell (tome's process of the ameloblast) secretes amelogenin. 2. Assembly into nanospheres. 3. Nanospheres assemble together and bind to the calcium-phosphate that is in the environment there. As they do this, it allows them to make these rods. As the enamel rod grows in thickness, the ameloblast will begin to secrete Proteinase-1 (MMP-20) which starts to degrade amelogenin and clearing it out of the developing rod. 4. When it gets to the point that the rod is fully developed, the ameloblast will secrete Proteinase-2 which will be KLK-4, and that will remove the rest of the amelogenin. 5. Once the rest of the amelogenin is removed, you will have a fully mature rod.
What is the thickness scale of enamel?
2.5mm to feather edge
______% to _____% of final mineral content is achieved during initial mineralization of enamel.
25; 30
What happens during the transitional phase from secretory to maturation?
Ameloblasts change -reduce height and volume -25% of the cells die (apoptosis = programmed cell death)
Why is the self-assembly of amelogenin into nanospheres important?
Amelogenin is an inhibitor and organizer of crystal growth. In the absence of amelogenin, crystal growth is present but they are not in a pattern that we expect. In the presence of amelogenin, calcium phosphate and amelogenin bind to form nanospheres which allowed it to become rod shaped that we expect.
_________ directs enamel crystal growth.
Amelogenin nanospheres
The cementum meets the enamel in a line that surrounds the tooth called ________.
CEJ (cemento-enamel junction)
What are perikymata?
Circumferential, horizontal shallow furrow that run on the surface of the enamel Result from the Striae of Retzius extending onto the surface of the tooth
The gradual completion of mineralization begins at the _______, and moves in what direction?
DEJ; moves out to incisal edge then starts at occlusal/incisal region of tooth and moves towards CEJ.
What happens when the tooth erupts into the oral cavity?
The ameloblast cell layer is lost. Thus, enamel cannot be regenerated.
What makes enamel the hardest and toughest material in the human body?
The hierarchical microscale-architecture and the fact that a given enamel rod contains more than 10,000 hydroxyapatite crystallites (nanoscale ceramic particles) are the reasons that enamel of mouse (or any mammalian, including the human) is the hardest and toughest material in the body.
What do the membrane infoldings represent?
These infoldings represent the sites where secretory granules (sg) release enamel matrix proteins extracellularly for growth in length of enamel crystals that result in an increase in the thickness of the enamel layer.
What are Tome's processes?
Tome's processes (TP) extend into enamel: -TPs are cytoplasmic extensions of ameloblasts and gives a "saw tooth" appearance to the enamel/ameloblast junction. -TPs are the interdigitation of ameloblasts and enamel rods. -TP lie perpendicular to the enamel rods. Function: create more SA from which the ameloblasts can secrete the enamel matrix proteins
What is the color scale of enamel?
Translucent light yellow to gray-white in color
T/F: Enamel and dentin are made at the same time.
True
T/F: The formation of enamel occurs over an extended period of time.
True
What happens in the secretory phase?
ameloblast produce the enamel matrix proteins, including amelogenin At what will be known as the DEJ, ameloblasts actively secret enamel proteins that accumulate and participate in the formation of a partially mineralized initial layer of enamel. This layer does not have rods and is called aprismatic enamel. Ameloblasts move away from the dentin surface. Thus, the enamel rods do not directly contact dentin. Ameloblasts deposit enamel primarily at a mineralization front very near the ameloblast cell membrane.
What happens in the maturation phase?
ameloblasts remove degraded proteins (by proteases) which matures the enamel mineral Maturation proper: -Bulk removal of water and organic material from enamel -Additional inorganic material is introduced -Modulation: cyclic between ruffled- and smooth- ended ameloblasts Gradual completion of mineralization: -Mineral is added (hydroxyapatite) -Begins at DEJ, and moves out to incisal edge -Starts at occlusal/incisal region of tooth and moves towards CEJ (out and down)
Enamel is made by cells called _______ in a process called ______.
ameloblasts; amelogenesis
What is the major enamel matrix protein?
amelogenin
What cells form the cementum?
cementoblasts
What cell type are pre-ameloblasts?
cuboidal, undifferentiated
The ______ signifies the first signaling center of tooth development.
dental placode
Dentin is made through a process called ______.
dentinogenesis
What is cementogenesis? When does it occur?
formation of cementum; late in the development of teeth
Interrod (IR) enamel surrounds the _______ and the _______; this portion is the continuation of the ______ into the enamel layer.
forming rod (R); distal portion of the Tomes' processes (dpTP); proximal portion (ppTP)
When does initial mineralization of enamel occur?
immediately as it is laid down
Prior to enamel formation, cells of inner enamel epithelium are ________.
pre-ameloblasts
Electron micrographs of enamel crystals are organized in bundles called _______.
prisms
In initial mineralization of enamel, some organic material is removed by _______.
proteases
What are enamel spindles?
short, dentinal tubules produced by disoriented odontoblast and are now in enamel -form from odontoblast processes that become trapped in the developing enamel -extend from DEJ -Found most frequently at the cusp tips
What is cementum?
soft bony tissue that covers the tooth surface in a thin layer
The color of enamel is effected by...?
underlying dentin
What does 4% of tooth enamel consist of?
water and organic material (proteins)
When do ameloblast cells die?
when enamel formation is complete and tooth erupts into oral cavity
