Enamel (Part 2) Lecture Notes PDF

Enamel (Part 2) Amelogenesis By the end of this lecture, you should be able to: Summarize the dynamic lifecycle of the ameloblast during enamel formation (amelogenesis) Describe the role of the Tomes’ processes in enamel formation Identify the major components of the enamel matrix Identify the major...

Enamel (Part 2) Amelogenesis By the end of this lecture, you should be able to: Summarize the dynamic lifecycle of the ameloblast during enamel formation (amelogenesis) Describe the role of the Tomes’ processes in enamel formation Identify the major components of the enamel matrix Identify the major features of the enamel organ at different stages of tooth crown formation The development of the dental tissues Enamel development involves the following cell layers and types. By the end of this lecture, you should know where these structures are located at different stages of tooth development, their roles in tooth development, their appearance, etc. Ten Cate’s Oral Histology (8th Edition); Fig. 5-35 Amelogenesis- the long road to making enamel Important to remember: It takes up to 5 years to complete the crown of a permanent tooth! Amelogenesis is accomplished by ameloblasts: these epithelial cells do not produce mature enamel, but instead produce a special protein scaffold within which hydroxyapatite crystals will grow. Up to two thirds of the time it takes to produce a full crown of enamel can be taken up by the maturation stage: ameloblasts remove water, proteins, and other organic materials to increase the mineral content of enamel. Tooth development- a quick reminder Bud stage (~8th week of embryonic development) Cap stage (~9-10th week of embryonic development) Bell stage (~11th week of embryonic development) Enamel development (Amelogenesis) Bud stage (~8th week of embryonic development) Enamel organ Enamel development (Amelogenesis) Enamel organ Cap stage (~8th week of embryonic development) Enamel development (Amelogenesis) Outer enamel epithelium Enamel organ Stellate reticulum Inner enamel epithelium Cap stage (~8th week of embryonic development) Enamel development (Amelogenesis) Inner enamel epithelium Cap stage (~8th week of embryonic development) Amelogenesis Enamel organ Early bell stage (~11th week of embryonic development) Amelogenesis Outer enamel epithelium Enamel organ Stellate reticulum Inner enamel epithelium Early bell stage (~11th week of embryonic development) Amelogenesis Inner enamel epithelium Early bell stage (~11th week of embryonic development) Amelogenesis- presecretory stage 1. Morphogenetic phase The cells forming the innermost lining of the inner enamel epithelium are low, cube-shaped cells. Nuclei are centrally placed in the cell, organelles are not very numerous. Cells of the inner enamel epithelium help shape the future crown. Ten Cate’s Oral Histology (8th Edition); Fig. 7-14 Amelogenesis- presecretory stage 2. Histodifferentiation phase Inner enamel epithelium cells differentiate into ameloblasts. Nuclei shift to proximal sides of cells; cell bodies elongate into columnar ameloblasts. The Golgi complex increases in volume and migrates distally, amount of rough endoplasmic reticulum increases (this is for protein synthesis and secretion) Neighbouring ameloblasts are tightly joined by junctional complexes. Ten Cate’s Oral Histology (8th Edition); Fig. 7-14 Amelogenesis Outer enamel epithelium Stellate reticulum Inner enamel epithelium Late bell stage (~18th week of embryonic development) Amelogenesis Inner enamel epithelium Late bell stage (~18th week of embryonic development) Amelogenesis- secretory stage 3. Initial secretory phase The large Golgi complex and rough endoplasmic reticula synthesize and package enamel matrix proteins into secretory granules, which are then secreted on the distal end of the ameloblasts. The secretory surfaces of the ameloblasts are flat à produce aprismatic enamel matrix protein scaffold at this point. Ten Cate’s Oral Histology (8th Edition); Fig. 7-14 Amelogenesis- secretory stage 4. Main secretory phase Ameloblasts migrate away from dentine surface and develop their Tomes’ processes. This outgrowth of the ameloblast makes the protein secretion staggered into two parts: one forms the prisms, the other forms the interprismatic regions. This phase is when most of the enamel is formed. Calcium and phosphate ions are transported into the protein matrix to form the initial hydroxyapatite crystals. This hardens enamel to about 30% of its final hardness. Ten Cate’s Oral Histology (8th Edition); Fig. 7-14 Amelogenesis- secretory stage 4. Main secretory phase Ameloblasts migrate away from dentine surface and develop their Tomes’ processes. This outgrowth of the ameloblast makes the protein secretion staggered into two parts: one forms the prisms (the Tomes’ process does this), the other forms the interprismatic regions. Tomes’ process This phase is when most of the enamel is formed. Calcium and phosphate ions are transported into the protein matrix to form the initial hydroxyapatite crystals. This hardens enamel to about 30% of its final hardness. Ten Cate’s Oral Histology (8th Edition); Fig. 7-14 Amelogenesis- secretory stage Stellate reticulum Inner enamel epithelium Enamel organ Outer enamel epithelium Amelogenesis- secretory stage 4. Main secretory phase The stellate reticulum collapses, bringing the inner and outer enamel epithelia closer together. This brings surrounding vascular tissue closer to the ameloblasts, which nourishes them during this phase. Ameloblasts are also supported by cells from the stratum intermedium. Ameloblasts Stratum intermedium Enamel Enamel matrix Amelogenesis- secretory stage What are ameloblasts secreting? NOT hydroxyapatite! Enamel matrix proteins (EMPs): noncollagenous proteins that are helping initiate and guide the formation of hydroxyapatite crystals outside of the cells. Main EMP: Amelogenin. Ameloblasts Enamel Enamel matrix Amelogenesis- secretory stage Matrix vesicle Enamel matrix proteins: Amelogenin Secreted in matrix vesicles by the ameloblasts Self-aggregating, gel-like matrix Creates nanospheres that aggregate into chains and ribbons These prevent lateral growth of HA crystals. The shape of the amelogenin scaffold dictates the shape of the Amelogenin hydroxyapatite crystals. nanosphere HA crystals Amelogenesis- secretory stage Enamel matrix proteins: Ameloblastin ~10% of the enamel matrix. Has calcium-binding properties. May assist ameloblasts in adhering to the forming enamel surface during the secretory stage. Amelogenesis- secretory stage Enamel matrix proteins: Enamelin ~5% of the enamel matrix. Appears to bind strongly to mineral and inhibit crystal growth. May help promote crystal elongation in the earliest stages of enamel formation. Amelogenesis- secretory stage Enamel matrix proteins: Others: Enamelysin (MMP20) à cleaves amelogenin and ameloblastin to make active form Enamel matrix serine proteases à degrades residual EMPs during enamel maturation Tuftelin à residual protein found in enamel tufts (see Enamel Part 1 lecture) Amelogenesis- maturation stage 5. Maturation phase-early Ameloblasts shorten and lose their Tomes’ processes. This leads to the production of the outer layer of aprismatic enamel. Calcium and phosphate ions are actively pumped into the enamel to increase the sizes of the hydroxyapatite crystals. Ten Cate’s Oral Histology (8th Edition); Fig. 7-14 Oral Anatomy, Histology and Embryology (5th Edition); Fig. 7.26 Amelogenesis- maturation stage 6. Maturation phase-late Ameloblasts secrete enamel matrix proteindigesting enzymes to degrade and remove the protein scaffold from the immature enamel. Ameloblasts also actively remove water from the developing enamel, increasing the mineral content of the maturing tissue. Ameloblasts seem to cycle between the early and late maturation stages repeatedly. Ten Cate’s Oral Histology (8th Edition); Fig. 7-14 Amelogenesis- maturation stage 6. Maturation phase-late Ameloblasts secrete enamel matrix proteindigesting enzymes to degrade and remove the protein scaffold from the immature enamel. Ameloblasts also actively remove water from the developing enamel, increasing the mineral content of the maturing tissue. Ameloblasts seem to cycle between the early and late maturation stages repeatedly. Amelogenesis- maturation stage 6. Maturation phase-late Enamel is most susceptible to fluoride and tetracycline at this stage. Dental fluorosis Wang et al. (2020) J. Int. Med. Res. Tetracycline staining Sanchez et al. (2004) Int. J. Dermatol. Amelogenesis- maturation stage 7. Protective stage After the completion and full maturation of the enamel, the enamel organ collapses into a dense covering of epithelium. This is called the reduced enamel epithelium, which then protects and covers the outer surface of the enamel until tooth eruption occurs. Ten Cate’s Oral Histology (8th Edition); Fig. 7-14 The fate of the reduced enamel epithelium The reduced enamel epithelium eventually merges with the oral epithelium as a tooth erupts. This leads to the destruction of most of the reduced enamel epithelium, except for the cervical portions. The cervical portions of the reduced enamel epithelium become the junctional epithelium, which acts as a seal from the oral environment. The fate of the reduced enamel epithelium The reduced enamel epithelium eventually merges with the oral epithelium as a tooth erupts. This leads to the destruction of most of the reduced enamel epithelium, except for the cervical portions. The cervical portions of the reduced enamel epithelium become the junctional epithelium, which acts as a seal from the oral environment. The fate of the reduced enamel epithelium The reduced enamel epithelium eventually merges with the oral epithelium as a tooth erupts. Enamel gap This leads to the destruction of most of the reduced enamel epithelium, except for the cervical portions. The cervical portions of the reduced enamel epithelium become the junctional epithelium, which acts as a seal from the oral environment. Junctional epithelium Periodontal tissues By the end of this lecture, you should be able to: Summarize the dynamic lifecycle of the ameloblast during enamel formation (amelogenesis) Describe the role of the Tomes’ processes in enamel formation Identify the major components of the enamel matrix Identify the major features of the enamel organ at different stages of tooth crown formation