Embryology of the tooth

• “Outside-in” theory: teeth evolved from ectoderm-derived skin denticles that folded in and integrated into the mouth (e.g. sharks have tooth-like dermal denticles on their skin)
• “Inside-out” theory: teeth originated from the endoderm, from pharyngeal teeth of jawless vertebrates and moved into the oral cavity with the evolution of the jaws (some modern animals still have teeth in their throat, e.g. lampreys)
• The lower jaw consists of epithelial cells from the ectoderm and neural crest derived mesenchymal cells, forming the future site of tooth development
• The first pharyngeal arch is where most of the tooth development takes place, except for the upper incisors which originate from the frontonasal process
• Tooth development overview: thickening of epithelium and condensation of mesenchymal cells → dental placode formation → epithelial tissues invaginate (bud stage) → enamel knot forms (cap stage) → differentiation into ameloblasts and odontoblasts (bell stage)
• Tooth development begins with the thickening of the oral epithelium, forming the primary epithelial band and then the dental lamina
• Discrete dental placodes grow from the continuous dental lamina and give rise to tooth germs
• Bud stage: once the dental placode forms, it begins to grow dramatically, invaginates and grows into the mesenchymal tissue to form a bud-like structure, which will further condensate mesenchymal tissue surrounding it
• At the cap stage, the dental epithelium is called the enamel organ and will develop into enamel tissue in the future
• At the center of the enamel organ is a growth of cells more condensed comapred to surrounding cells; it is the enamel knot, which is in charge of giving out signals to instruct tooth development
• The enamel organ has an outer enamel epithelium and an inner enamel epithelium
• The enamel organ wraps around the dental papilla, which is condensed mesenchyme that will form the dentine and pulp
• During the bell stage, the enamel organ continues to grow and the internal enamel epithelium folds to shape the future tooth crown
• At the bell stage, teeth will have multiple secondary enamel knots that also serve as signalling centers to induce folding and differentiation
• The number of secondary enamel knots = number of cusps
• During the bell stage, cells from the internal enamel epithelium and dental papilla near the secondary enamel knots begin to differentiate into ameloblasts and odontoblasts
• A basement membrane made of collagen and other external matrix separates the mesenchyme and epithelium
• Ameloblasts and odontoblasts induce the differentiation of each other to make sure that development is synchronised, but the inner enamel epithelium differentiates into ameloblasts first
• Before the ameloblasts start to secrete the matrix, they send processes through the degenerating basement membrane (Tomes’ processes) to modulate protein secretion and enamel crystal growth
• At the end of the secretory stage, Tomes’ processes will disappear, enamel reaches its full thickness, and the basement membrane reappears; ameloblasts undergo a transition and secrete different types of proteins
• Ameloblasts mature and shift between ruffled and smooth-ended phases, resulting in the removal of water and protein, and an influx of materials
• Each ameloblast’s Tomes’ process is responsible for the formation of one enamel rod
• Enamel crystallites form near the cellular junction of ameloblasts, forming interrod enamel (same composition but different crystal orientation)
• Ameloblasts send signals to instruct dental papilla cells to divide and differentiate; they increase the number and size of synthetic organelles to prepare for secretion
• Odontoblasts protrude (odontoblast process) and start to secrete the matrix; as it secretes, it moves backwards but the tip of the process remains in the same position, leaving behind a long odontoblast process (reason why caries can move rapidly into the dental pulp)
• Once crown development is complete, tooth eruption can occur and root formation continues after eruption
• When the tooth is ready to erupt, osteoclasts form and chew a tunnel for the tooth to emerge from the jaw, and at the same time the tooth root will begin to grow
• At the end of the two loops wrapping the dental papilla at the bell stage, the outer and inner enamel epithelium meet and form the epithelial root sheath
• The root sheath starts to lay down the tooth root and degenerates, but some cells are left behind, forming the cell rests of Malassez (may cause trouble such as small tumors)
• Cells in the dental follicle give rise to cementoblasts, which form cementum
• As the primary tooth develops, on the dental lamina there is also a bud for the secondary tooth (development will happen after birth)
• X-linked hypohidrotic ectodermal dysplasia (HED): mutations in the ectodysplasin pathway, which regulates enamel knot formation during tooth development
• Amelogenesis imperfecta: hypoplastic (thin/absent enamel) or hypomineralised types; hypomineralised can be divided into hypomaturation (incomplete removal of protein from enamel matrix) and hypocalcified (insufficient calcium ions into developing enamel); caused by mutations in genes in charge of the enamel matrix
• Dentinogenesis imperfecta: leads to weaker/discolored teeth, due to mutations in the DSPP gene, which is important for the dentine matrix
• Chemical exposure can cause developmental defects e.g. tetracycline, fluorides, dioxine; malnutrition, infections, etc.
• Alligators have indefinite sets of teeth as their dental lamina has a stem cell niche that is retained throughout their lives