AMELOGENSIS Mean the Process of Production & Development (Mineralization) of Enamel, and Begins When the Crown Is Forming During the Bell Stage of Tooth Development

Home , Ameloblast, Amelogenesis, Inner enamel epithelium, Outer enamel epithelium

Lec. 6 Dr. Ali H. Murad

AMELOGENSIS Mean the process of production & development (mineralization) of enamel, and begins when the crown is forming during the bell stage of tooth development.

A- Life cycle of the ameloblast: The life span of the cells of the inner enamel epithelium can be divided into six stages. 1-morphogenic stage: the inner enamel epithelium interacts with the adjacent mesenchymal cells of dental papillae, determining the shape of the dentinioenamel junction & the crown. During this morphogenic stage the cells are short columnar, with large oval nuclei. Terminal bars appear represent points of close contact between cells. The inner enamel epithelium is separated from the C.T of dental papillae by basal lamina.

2-organizing stage: the inner enamel epithelium cells become longer & come into close contact with C.T. cells of the pulp which differentiate into odontoblasts. The 1st appearance of dentin is a critical phase in the life cycle of the inner enamel epithelium as it’s in contact with the C.T. of dental papillae; it receives nutrient material from the blood vessels of this tissue. When dentin forms, it cuts off the ameloblasts from their original source of nourishment, then they are supplied by the capillaries that surround & penetrate the outer enamel epithelium.

3-formative stage: the ameloblasts enter their formative stage after the 1st layer of dentin has been formed. During formation of the enamel matrix the ameloblasts retain the same length & arrangement. The earliest change is the development of cell process on the ameloblast surface, which penetrate the predentin & known as Tome’s processes. 1

4-maturative stage: enamel maturation (full mineralization) occurs after most of the thickness of the enamel matrix has been formed in the occlusal or incisal area. During enamel maturation the ameloblasts are slightly reduced in length & closely attached to enamel matrix & display microvilli at their distal ends & cytoplasmic vacuoles. These 2 structures indicate an absorptive function of the ameloblasts

5- Protective stage: when the enamel has completely developed & has fully calcified, the ameloblasts can no differentiated from the cells of the stratum intermedium & outer enamel epithelium which fuse together to form the reduced enamel epithelium. The function of reduced enamel epithelium is to protect the mature enamel by separating it from the C.T, until the tooth erupts.

6- Desmolytic stage: the reduced enamel epithelium proliferates & elaborate enzymes that atrophied & destroyed the C.T, fibers by desmolysis separating it from the oral epithelium so that fusion of the two epithelium can occur then the tooth erupted into the oral cavity, in this time the reduced enamel epithelium contribute to form the junctional epithelium.

B-formation of the enamel matrix Ameloblasts begin enamel deposition after a few amount of dentin have been deposited at the dentinoenamel junction. The ameloblasts maintains cell-to-cell attachments at both the proximal & distal ends of the cell. Short conical processes (Tome’s processes) develop at the apical end of the ameloblasts during the formative or secretory stage.

Junctional complexes called the terminal bar appear at the junction of the cell bodies & Tome’s processes & maintain contact between adjacent cells. As the ameloblast influenced by dentin, the matrix is synthesized & deposited first along the dentin & establishes the dentinoenamel junction. As the enamel matrix develops, it forms in continuous rods from the dentinoenamel junction to the surface of the enamel. With synthesis of enamel, substances needed for enamel production arrive via the blood vessels & pass through the stellate reticulum to the stratum intermedium & ameloblasts. In this manner the protein amelogenin is produced. Only a few ameloblasts at the tip of the cusps begin to function initially. As the process proceeds, more ameloblasts become active & the increments of enamel matrix become more prominent.

C- Mineralization & maturation of enamel matrix As amelogenin is deposited & enamel matrix formed, the matrix begins to mineralize. As soon as the small crystals of mineral are deposited, they begin to grow in length & diameter. The initial deposition of mineral amounts to approximately 25% of the total enamel. The other 71% of mineral in enamel is a result of growth of the crystals (4% of enamel is water & organic materials). The time between enamel matrix deposition & its mineralization is short. Therefore the pattern of mineralization closely follows the pattern of matrix deposition (the 1st matrix deposited is the 1st enamel mineralized, occurring along the dentinoenamel junction. Matrix formation & mineralization continue peripherally to the tips of the cusps & then laterally on the sides of the crown, finally the cervical region mineralized During this process, protein of the enamel changes or matures & is termed enamelin.

As the ameloblast completes the matrix deposition phase, their terminal bars disappear & the surface enamel become smooth.

This phase is signaled by a change in the appearance & function of the ameloblast. The apical end becomes ruffled along the enamel surface. The length of the ameloblast decreases & becomes more active in absorption of the organics matrix & water from enamel, which allows mineralization to proceed. Then the ameloblasts secrete the organic cuticle in the protective stage. The ameloblast shorten & contact the stratum intermedium & stellate reticulum & outer enamel epithelium to form the reduced enamel epithelium. The reduced enamel epithelium remains on the enamel surface until the tooth erupts into the oral cavity.