Tooth Development: Key Cellular Interactions

• Odontogenesis refers to the complex process of tooth formation from embryonic cells, growth, and eruption into the mouth.

Early Tooth Development

• The primitive oral cavity, or stomodeum, is lined by stratified squamous epithelium called the oral ectoderm or primitive oral epithelium.
• The oral ectoderm contacts the endoderm of the foregut, forming the buccopharyngeal membrane.
• At approximately 27 days of gestation, the buccopharyngeal membrane ruptures, establishing a connection between the primitive oral cavity and the foregut.
• Connective tissue cells underlying the oral ectoderm originate from the neural crest or ectomesenchyme.
• These ectomesenchymal cells induce the overlying ectoderm to start tooth development, which begins in the future maxilla and mandible and proceeds posteriorly.
• Tooth development arises from the interaction of oral epithelial cells and underlying ectomesenchymal cells.
• From this interaction, 20 deciduous and 32 permanent teeth develop.
• Each developing tooth grows as an anatomically distinct unit.
• Dental organ, or tooth germ, constitutes the structure with the enamel organ, dental papilla, and dental follicle.

Primary Epithelial Band

• Two or three weeks after the rupture of the buccopharyngeal membrane, when the embryo is about 6 weeks old, the oral epithelium proliferates, thickens, and invaginates into the underlying mesenchyme, forming the Primary Epithelial Band.
• Arising first in the lateral region of the maxillary and mandibular processes, the band becomes continuous across the midline of the developing arches.
• It then extends posteriorly to form a horseshoe-shaped thickening in each process.
• At about the 7th week, the primary epithelial band divides into an inner lingual process called the Dental Lamina and an outer buccal process called the Vestibular Lamina.

Dental Lamina

• A band of epithelium that has invaded the underlying ectomesenchyme along the horseshoe-shaped future dental arches
• Functions as the primordium for the ectodermal portion of the deciduous teeth.
• Deciduous dentition develops directly from the dental lamina at the eighth week of fetal life.
• Along the leading edges of the lamina, 20 areas of enlargement appear, which are the forming buds of the 20 primary teeth, and the leading edge of the lamina continues to develop the 32 permanent tooth buds.
• Successional lamina refers to the lingual extension , responsible for the development of permanent incisors, canines, and premolars.
• The successional lamina is active from the fifth month in utero (for the permanent central incisor) to 10 months of age (for the second premolar).
• Successional tooth buds form the permanent dentition lingual to the buds of the primary predecessors.
• Permanent molars develop from a distal extension of the dental lamina (posterior to the primary molars).
• Initiation of the permanent first, second, and third molars occurs at the fourth month of intrauterine life, one year after birth, and four years after birth, respectively
• The dental lamina is functional in developing the 52 teeth from the sixth prenatal week until 4 years after birth.
• Activity of dental lamina extends for about five years.
• Disintegrates completely or remains as remnants in the gingiva and the jaw.
• Remnants are are called epithelial rests of Serres (Serres' pearls).

Vestibular Lamina

• Labial and buccal to the dental lamina in each dental arch.
• Another epithelial thickening develops independently and later somewhat
• Also termed the lip furrow band.
• It subsequently hollows and forms the oral vestibule between the alveolar portion of of the jaws and lips and cheeks.

Physiological Phases of Tooth Development

• Tooth development is divided into overlapping developmental phases:
• Initiation: Sites of future teeth are established with the appearance of tooth germs along an invagination of oral epithelium called dental lamina.
• Proliferation: Proliferative growth causes regular changes in the size and proportions of the growing tooth germ.
• Histodifferentiation: Differentiation of cells (begun during morphogenesis) proceeds giving rise to fully formed dental tissues (both mineralized (enamel, dentin, cementum), and unmineralized (pulp, periodontal ligament).
• Morphodifferentiation: The shape of the teeth is determined by a combination of cell proliferation and cell movement.
• Apposition: Deposition of dental hard tissue like dentin and enamel happens in the tooth.

Morphological Stages of Tooth Development

• Tooth germs are classified into Bud, Cap, and Bell stages according to the degree of morphodifferentiation and histodifferentiation of their epithelial components (enamel organs).
• Leading up to the late bell stage, the tooth germ changes rapidly both in its size and shape, and cells divide.

Bud stage

• Round or ovoid swellings at ten different points in each jaw arise from the dental lamina, corresponding to the future position of deciduous teeth.
• Represent the primordial of enamel organs (the tooth buds).
• The development of the tooth germ is initiated, and cell proliferation is faster than in adjacent ectomesenchymal cells.
• Epithelial condensation is poorly morpho-differentiated and histodifferentiated.
• Cells of the tooth bud have a higher RNA content than those of the overlying oral epithelium, a lower glycogen content, and increased oxidative enzyme activity.
• Enamel organ histology consists of peripherally-located low columnar cells and centrally-located polygonal cells.
• The enamel organ is separated from the adjacent ectomesenchyme by a basement membrane.
• Many cells of the tooth bud and surrounding mesenchyme undergo mitosis.
• Increased mitotic activity and neural crest cell migration into the area causes condensation of the ectomesenchymal cells surrounding the tooth bud.
• Area of ectomesenchymal condensation immediately subjacent to the enamel organ is the dental papilla. The condensed ectomesenchyme that surrounds the tooth bud and dental papilla is the dental sac.
• Dental papilla and dental sac becomes more well-defined as the enamel organ grows into the cap and bell shapes.

Cap Stage

• As the epithelial bud continues to proliferate into the ectomesenchyme, morphogenesis has progressed, and the deeper surface of the enamel organ invaginates, forming a cap-shaped structure.
• Although the enamel organ appears relatively poorly histodifferentiated, a greater distinction develops between the more rounded cells in the central portion of the enamel organ and the peripheral cells which are becoming arranged to form the outer and inner dental epithelium.
• In the late cap stage of tooth development, the central cells of the enlarging enamel organ become separated (through maintaining contact by desmosomes).
• Intercellular spaces contain glycosaminoglycans, resulting in the stellate reticulum; formation is not fully developed until the late bell stage.
• Outer enamel epithelium cells remain cuboidal.
• Inner enamel epithelium cells become more columnar and increase their RNA content, hydrolytic and oxidative enzyme activity.
• Adjacent ectomesenchymal cells continue to proliferate and surround the Enamel organ.
• Part of the ectomesenchyme lying beneath the inner enamel epithelium is called dental papilla, and the ectomesenchymal tissue surrounding both the enamel organ and dental papilla is called the dental sac or dental follicle.

Transitory Structures

• During the early stages of tooth development, three structures may be seen:
• Enamel knot
• Enamel cord
• Enamel niche

Enamel Knot

• Localized mass of cells in the center of the inner E. epith.
• It was once thought it played a role in crown pattern formation by outlining the central fissure, but it soon disappears.
• Its recent studies suggest it may represent an important signaling center during tooth development.
• Unlike adjacent cells, the cells within this knot are non-proliferative and produce molecules associated with signaling in different sites.
• Molecules include bone morphogenic proteins and fibroblast growth factor.

Enamel Cord

• Strand of early bell stage development
• It arises from the increasingly high enamel organ as a vertical extension of enamel knot.
• Termed enamel septum when it extends from the enamel knot to outer enamel epithelium.

Enamel Niche

• Apparent structure in the histologic section
• Created because the dental lamina is a sheet rather than a single strand, and often contains a concavity filled with connective tissue.
• A section through this arrangement creates a double attachment to the oral epithelium by two separate strands.

Explore the cellular and tissue interactions driving tooth development. Learn about inner enamel epithelium changes, dental sac contributions, and the enamel knot's role. Understand the origins of connective tissue cells and the sequence of early developmental events.