Odontogenesis: Teeth Development

Odontogenesis (Teeth Development)

• Deciduous teeth begin developing during week 6-7 of intrauterine life
• Permanent teeth begin during week 14 of intrauterine life and continue up to 5 years after birth
• Tooth development begins around week 6
• The two tissue components of teeth development are primitive oral epithelium and underlying ectomesenchyme
• Primitive oral epithelium is derived from ectoderm
• Underlying ectomesenchyme is derived from cranial neural crest cells that migrate from the developing midbrain into the first branchial arch
• Tooth development starts with the thickening of the primitive oral epithelium (primary epithelial bands) on each jaw
• Each primary epithelial band then splits into a lingual dental lamina (inner) and a buccal vestibular lamina (outer)
• The vestibular lamina subsequently hollows and forms the vestibule of the mouth, which is the space between the alveolar portion of the jaws, lips, and cheeks
• Signal proteins like FGFs, BMPs, and EDA cause the cells within the dental lamina to start proliferating and invaginating in the positions of the future teeth
• The localized and enlarged projections within the dental lamina are known as the dental placodes
• As the dental placodes proliferate, the tooth undergoes morphogenesis, which results in different teeth shapes
• This is guided by interactions between the epithelial and mesenchymal tissues and progresses through distinct stages defined by morphological features of the dental epithelium

Stages of Dental Epithelium

• Bud
• Cap
• Bell

Bud Stage

• Dental placodes proliferate, developing into a bud-like structure known as the tooth bud
• The tooth bud is attached to the oral epithelium by the dental lamina
• Ectomesenchymal cells start to cluster around the tooth bud, resulting in a condensation of the ectomesenchyme
• The tooth bud is made up of an outer layer of low columnar cells and an inner bundle of polygonal cells
• The tooth bud has differential proliferation of cells, with different parts of the bud growing at different rates
• The cells at the center of the bud proliferate slower than those in the peripheries, leading to the formation of an invagination and turning the bud-shaped enamel organ into a cap shape

Cap Stage

• There are 3 different types of cells forming the enamel organ, which is the epithelial part of the tooth germ
• Types of cells forming the enamel organ include inner enamel epithelium, outer enamel epithelium, and stellate reticulum
• Inner enamel epithelium is low columnar cells
• Outer enamel epithelium is cuboidal cells
• Stellate reticulum cells are linked by desmosomes and secrete glycosaminoglycans
• The glycosaminoglycans attract water into the extracellular space, which pushes on the cell membranes of the stellate cells and turns them into the shape of a star
• There are also a number of non-dividing cells within the enamel organ that sit near the enamel epithelium, known as enamel knots
• Enamel knots are a signaling center that regulates the formation of the cusps of the tooth
• The number and location of enamel knots determines the number and location of the cusps in the developing tooth
• The ectomesenchymal cells surrounding the enamel organ continue to accumulate near the invagination of the cap, as dense condensation of the ectomesenchymal cells beneath the cap, known as the dental papilla
• Accumulation of ectomesenchymall cells encircle the dental papilla and enamel organ, known as the dental sac or follicle
• As the enamel organ continues to grow, the invagination in the cap deepens, forming the cervical loop at the tip of the invaginating epithelium where the inner and outer enamel epithelium meet

Bell Stage

• Within the enamel organ, a few epithelium cells between the stellate reticulum and the inner enamel epithelium differentiate into a layer of spindle-shaped cells known as stratum intermedium
• The stellate reticulum starts to collapse, reducing the distance between the inner and outer enamel epithelium
• The tooth germ epithelium then forms the bell-shaped structure that will become the crown of the tooth

Late Bell Stage

• Morpho-differentiation occurs, determining the future shape of the crown like incisive, canine, pre-molar, and molar
• This is conversion of cells of the dental organ into specialised cells
• Marks an important stage of histo-differentiation
• Ameloblasts become enamel
• Odontoblasts become dentine
• Following cusp formation, the inner enamel epithelial cells and the dental papilla cells continue differentiating into ameloblasts and odontoblasts respectively down the slope of the cusp and up to the cervical loop
• As ameloblasts and odontoblasts continue to secrete the mineralized matrix, the dental lamina disintegrates, which detaches the tooth germ from the oral epithelium
• Sometimes when the dental lamina degenerates, it may leave behind nests of epithelial cells called epithelial pearls, which can give rise to abnormalities in the future

Abnormalities in the Future

• Supernumerary teeth (extra teeth)
• Odontoma (benign tumor of the tooth)
• Eruption cysts (fluid-filled swelling over an erupting tooth)

Root Formation

• Begins after crown development is underway
• The cervical loop cells begin to proliferate downwards to form a two-layered structure called Hertwig's epithelial root sheath (HERS)
• HERS proliferates and migrates downward, guiding root formation and induces differentiation of odontoblasts, forming root dentine
• HERS has a limited growth potential, which determines the length of the root
• Disintegration of HERS results in the formation of an epithelial network called epithelial rests of Malassez (ERM)
• This allows cells of dental follicle to come in contact with root dentine and their differentiation into cementoblasts that deposit cementum on the root surface
• The periodontal ligament that connects the tooth to the alveolar bone is formed by fibroblasts differentiating from the dental follicle cells
• The dental follicle gives rise to osteoblasts to form the alveolar bone where the fibers of the periodontal ligament are embedded, with these fibers inserted into the cementum and the alveolar bone providing tooth anchorage and are known as Sharpey's fibers