Tooth Development and Eruption Part 2

Root Development

  • Occurs after the crown is shaped and the tooth starts erupting.

  • The tooth forms from crown to apex.

  • Cervical loop:

    • Responsible for root development.

    • Most cervical part of the enamel organ.

    • Bilayer rim (IEE and OEE).

    • Grows into ectomesenchyme, encloses dental papilla tissue.

    • Forms Hertwig epithelial root sheath (HERS).

  • HERS:

    • Shapes the root(s).

    • Induces dentin formation.

Root Dentin Formation

  • Outer cells of dental papilla become odontoblasts.

  • HERS induces odontoblastic differentiation but doesn't differentiate into ameloblasts.

  • Absence of enamel in roots.

  • Odontoblasts secrete predentin.

  • Basement membrane disintegrates, HERS cells may become epithelial rests of Malassez (ERM).

Cementum and Pulp Formation

  • Cementogenesis occurs after HERS disintegrates.

  • Cementoblasts lay down cementum matrix (cementoid).

  • Cementoblasts become cementocytes.

  • Cementoid mineralized into cementum.

  • Dentinocemental junction (DCJ) forms.

  • Central cells of dental papilla form the pulp.

Clinical Considerations: Concrescence

  • Excess cementum formation.

Multirooted Tooth Development

  • Premolars and molars originate as a single root trunk.

  • Root trunk divides into root branches.

  • Differential growth of HERS divides the root trunk.

  • Horizontal extensions divide the cervical opening.

  • Dentin formation starts after odontoblast induction and HERS disintegration.

  • Cementoblasts form cementum.

Clinical Considerations with Root Formation Disturbances

  • Enamel Pearl:

    • Misplaced ameloblasts form enamel over cementum.

  • Dilaceration:

    • Distorted root(s) or crown angulation due to HERS distortion.

  • Accessory Roots:

    • Extra roots due to trauma or metabolic disease.

Periodontal Ligament and Alveolar Process Development

  • Supporting tissues develop from the dental sac.

  • Ectomesenchyme forms the periodontal ligament (PDL).

  • Sharpey fibers support the tooth.

  • Ectomesenchyme mineralizes to form alveoli.

Primary Tooth Eruption and Shedding

  • Eruption in chronologic order.

  • Active eruption: vertical movement.

  • Passive eruption: gingival recession.

  • Ameloblasts place acellular dental cuticle.

  • Layers form reduced enamel epithelium (REE).

  • REE fuses with oral epithelium, forming an eruption tunnel.

  • Fused tissue remains near CEJ as initial junctional epithelium.

  • Primary tooth is lost as permanent tooth develops lingual to it.

  • Osteoclasts absorb alveolar process, odontoclasts resorb root.

Permanent Tooth Eruption

  • Erupts lingual to primary tooth roots (except maxillary incisors).

  • Process similar to primary tooth eruption.

  • Nonsuccedaneous eruption occurs without primary tooth shedding.

Clinical Considerations with Eruption Process

  • Spacing