Development of the Teeth

This paragraph discusses the early stages of tooth development, known as odontogenesis, which includes the formation of both deciduous (milk) and permanent teeth. It explains that deciduous teeth start developing around the sixth to seventh week of intrauterine life, while permanent teeth begin their development during the 14th week and can continue development up to five years after birth. The paragraph highlights the diversity of teeth in humans in terms of morphology and function. It describes the two main tissue components involved in tooth development: the primitive oral epithelium (ectoderm-derived) and the dental mesenchyme. The dental mesenchyme originates from craniofacial neural crest cells that migrate from the developing midbrain and rhombomere regions. The interaction between the oral epithelium and dental mesenchyme is crucial for the precise formation and positioning of specialized teeth such as incisors, canines, premolars, and molars. The paragraph outlines the initial stages of tooth development, starting with the thickening of the primitive oral epithelium, known as primary epithelial bands, which later split into the inner dental lamina and outer vestibular lamina. Signal proteins like FGFs, BMPs, and ectodisplacin trigger cell proliferation and invagination within the dental lamina, leading to the formation of dental plecodes, which are localized projections corresponding to future teeth locations.

The second paragraph delves into the morphogenesis of teeth, detailing the processes that lead to the formation of different tooth shapes and structures. It describes how the dental placodes proliferate to form a bud-like structure known as the tooth bud, which is attached to the oral epithelium by the dental lamina. The paragraph explains the unique cell proliferation patterns within the tooth bud, where cells at the center proliferate slower than those at the periphery, leading to the formation of an invagination and the transition from bud to cap shape. During the cap stage, three types of epithelial cells form the enamel organ: the inner enamel epithelium, outer enamel epithelium, and stellate reticulum. The stellate cells secrete glycosaminoglycans, which attract water and shape the cells into a star-like form. The enamel knot, a signaling center within the enamel organ, regulates the formation of tooth cusps. The paragraph also discusses the role of ectomyson cells in the development of the dental papilla and the dental sac or follicle. It describes the formation of the bell-shaped structure that becomes the tooth crown and the late bell stage, where morpho-differentiation occurs, determining the future shape of the tooth crown. Histodifferentiation is marked by the conversion of dental organ cells into specialized cells like amyloblasts (enamel producers) and odontoblasts (dentin producers). The paragraph also touches on abnormalities that can arise from the degeneration of the dental lamina, such as epithelial pearls, supernumerary teeth, odontomas, and eruption cysts.

The final paragraph focuses on the root formation of teeth and the development of permanent teeth. It explains that root formation begins after the crown development has started, with cervical loop cells proliferating downwards to form the Hertwig's epithelial root sheath (HERS). HERS guides root formation and induces the differentiation of odontoblasts to form root dentin. The paragraph also describes the limited growth potential of HERS, which determines the root length. The disintegration of HERS leads to the formation of epithelial rests of Malassez, which allows dental follicle cells to come into contact with root dentin and differentiate into cementoblasts, depositing cementum on the root surface. The periodontal ligament, connecting the tooth to the alveolar bone, is formed by fibroblasts differentiating from dental follicle cells. The dental follicle also gives rise to osteoblasts that form the alveolar bone, with Sharpey's fibers inserting into the cementum and alveolar bone to anchor the tooth. The paragraph reiterates that permanent teeth develop similarly to deciduous teeth, starting from the successional lamina behind the deciduous tooth bud around the 14th week of intrauterine life. The exception is the molars, which begin developing directly from the dental lamina by the 20th week and continue until five years after birth.