Author: Sanketh DS, MDS
Amelogenesis is the process of enamel formation, which starts with deposition of enamel matrix by ameloblasts, followed by mineralization of the matrix. Enamel matrix essentially consists of organic material composed of two groups of proteins, namely amelogenins and non-amelogeins. Non-amelogenins consist of proteins enamelin, tuftelin and ameloblastin. The amelogenins form bulk of the enamel proteins, comprising at least 90%. Once enamel is fully formed, organic material makes up 4% of enamel. The inorganic portion of enamel is composed of hydroxyapatite and comprises 96% of enamel.
Amelogenesis is divided into 3 stages referred to as pre-secretory, secretory and maturation stages.
1. The pre-secretory stage is the phase in which, inner enamel epithelial cells differentiate to become ameloblasts ready for secretion.
2. In the secretory stage, ameloblasts secrete organic matrix comprising various enamel proteins forming the entire enamel thickness.
3. Ameloblasts carry out activities in the maturation stage that help in degradation of the enamel matrix and subsequent replacement by minerals or inorganic material, thus completing the process of enamel formation.
This stage is further subdivided into two phases referred to as the Morphogenetic phase and the Differentiation phase.
The enamel organ is in the bell stage of tooth development during this phase and the shape of the crown is established. The inner enamel epithelial cells are low columnar or cuboidal cells with a central nucleus. Golgi apparatus of the cell is located in the proximal portion of the cell, i.e. towards the stratum intermedium and the other cytoplasmic components are scattered across the cell.
During this phase, the inner enamel epithelium starts to differentiate to become an ameloblast. The cell elongates and becomes tall columnar and the nucleus that was previously at the centre shifts proximally. Golgi complex shift distally i.e. towards the dental papilla and mitochondria that were scattered begin to cluster proximally. Rough endoplasmic reticulum also begins to increase in the cell.
When these changes to the inner enamel epithelial cells are happening, there is simultaneous differentiation of the cells of the dental papilla to odontoblasts, thanks to signalling molecules from the differentiating inner enamel epithelium. Odontoblasts begin to form dentin at the end of differentiation phase.
The inner enamel epithelial cell, at the end of the pre-secretory stage, differentiates to become an ameloblast with the nucleus polarized away from the dental papilla and most organelles located distally. Ameloblasts are attached to each other with the help of junctional complexes both proximally and distally.
Once the predentin or the first layer of dentin is formed, it stimulates the ameloblasts to begin enamel formation. It has to be understood that, this initial layer of dentin is necessary for stimulation of enamel formation just as it was necessary for the inner enamel epithelium to help in differentiation of dental papilla cells to odontoblasts!
In the secretory stage, ameloblasts develop a structure called the Tome’s process. Now, a fully developed tome’s process is a distal extension or projection of the ameloblast consisting of a proximal portion and a distal portion. However, at the beginning of the secretory stage, tome’s process has only a proximal portion.
Ribosomes help in synthesising proteins which are then packaged into secretory granules in the golgi complex. These secretory granules containing the enamel proteins are then secreted via the tome’s process. The initial layer of enamel matrix is laid on the mantle dentin, and is mineralized immediately. Hence, this initial layer of enamel, does not contain enamel rods. Now, as more enamel matrix is laid down, the ameloblast moves away from the enamel, with the tomes process developing an extension from the proximal portion. This extension is the distal portion of the tome’s process. This extension keeps elongating as more enamel matrix is secreted, with the distal portion, penetrating the enamel.
Now, it has to be understood that rod and inter-rod enamel is a result of the tome’s process. Let’s understand how this happens. When the distal portion of the tome’s process is developed, secretion of enamel happens from two sites. One is from the proximal portion of the tome’s process and the other from the distal portion. The enamel matrix secreted by the proximal portion, encircles or surrounds, that enamel matrix secreted by the distal portion of the tome’s process. The enamel matrix secreted by the distal portion of the tome’s process forms the enamel rod, and that which surrounds the rod is called inter rod enamel.
As the enamel matrix continues to be secreted, not only, does the distal portion of the tome’s process elongate, but also gradually becomes thinner and gets squeezed out of existence. Once this happens, there is a thin space at this junction between the rod and inter-rod enamel that is filled with organic material to form the enamel sheath.
Once the, distal portion of the tome’s process disappears, the ameloblast again resembles how it was initially and thus the last increment of the enamel formed, has no enamel rods! Hence, it has to be understood that, finally enamel is composed of rod/inter-rod containing enamel squeezed between two layers of rod-less enamel!
The maturation phase consists of a “transitional” phase and a “maturation proper” phase.
Before the enamel matrix could mineralize and mature, there is a brief transitional phase where ameloblasts reduce in height and volume. Also, ameloblasts undergo programmed cell death or apoptosis and there is approximately 25% reduction in the number of ameloblasts in this phase!
This phase of the maturation stage is when the bulk of the proteins and water in the organic matrix are removed to be replaced by inorganic material. This is accomplished by a process called modulation. Modulation is the process of cyclic creation, loss and recreation of an invaginated ruffle-ended apical surface i.e. the ameloblasts cyclically alternate between having a ruffle-ended border and a smooth ended border at the apical surface facing the enamel matrix. A ruffle ended ameloblast possess a tight distal junction and the smooth ended ameloblast possess a leaky distal junction.
Ruffle-ended ameloblasts possess enzymes like lysosomes, metalloproteinases and serine proteases, that they secrete via their ruffle ended borders, to degrade in bulk, the proteins in the enamel matrix. Once the proteins degrade to small fragments of peptides, they escape and diffuse into the ameloblasts via the leaky junctions of the smooth ended ameloblasts. Ruffle-ended ameloblasts show endocytic activity, which help in endocytosis of degraded peptides into the cell.
When the enamel matrix is being degraded, the calcium binding proteins and calcium ATPases present in the ruffle ended ameloblasts, help in pumping calcium ions to the degrading enamel matrix in order to mineralize it and help in active crystal growth.
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