Oral histology Preliminary

• Root development:

  • Dentin covered by cementum and embedded in the alveolar bone
• Root formation begins after enamel and dentin formation has reached the future cementoenamel junction (CEJ)
• Hertwig’s epithelial root sheath (HERS) outlines the future root and is responsible for the shape, length, size, and number of roots
• Cementogenesis:

  • After the first root dentin is deposited, the cervical portion of Hertwig’s epithelial root sheath breaks down
  • New dentin comes in contact with dental sac, stimulating cells to differentiate into cementoblasts which produce cementum
  • Cementum is connected to dentin by collagen fibers and to the bony alveolus by Sharpney’s fibers
  • Thinner acellular layer of cementum around the root neck, with thicker cellular cementum covering the lower part of the root up to the apex
• Epithelial rests of malassez:

  • Remnants of Hertwig’s epithelial root sheath found as a group of epithelial cells in the periodontal ligament
  • Some degenerate and others become calcified (cementicles)
• Tooth enamel:

  • Hardest calcified matrix of the body consisting of 96% hydroxyapatite
  • Organic matrix makes up 1%, water 3%
  • Semitranslucent, yellow to grayish white in appearance
• Amelogenesis:

  • Formation of enamel in 3 functional stages: presecretory, secretory, maturation
  • Presecretory phase starts at the bell stage of tooth development
  • Secretory phase begins when a small amount of dentin has been laid down
  • Maturation phase involves hardening of enamel as the tooth erupts in the oral cavity
• Enamel rods:

  • Fundamental morphologic unit of enamel covered by prism sheath or rod sheath
  • Enamel rods are oriented at right angles to the dentin surface and built up of segments separated by dark lines
  • Incremental lines of Retzius illustrate the incremental pattern of enamel formation
• Neonatal line:

  • Marks the division between enamel formed before birth and after birth
  • Found in deciduous teeth and cusps of permanent first molars
• Hunter-Schreger bands:

  • Alternating light and dark lines seen in dental enamel that begin at the DEJ and end before reaching the enamel surface
• Enamel spindles:

  • Elongated odontoblastic processes that traverse the DEJ from the underlying odontoblast and may serve as pain receptors
• Dentinoenamel junction:

  • Surface of the dentin at the dentinoenamel junction is pitted, fitting rounded projections of the enamel
  • Scalloped line
• Important facts about enamel:

  • Enamel has no power of regeneration
  • Enamel has no nerve supply
  • Enamel is a good thermal insulator
  • Acid solubility of enamel is reduced by fluoride
• Mineralization occurs during both primary and secondary dentin formation.
• Odontoblasts are responsible for producing primary dentin.
• Secondary dentin begins forming shortly after tooth eruption and increases with age.
• Amelogenesis: Presecretory Phase

  • Starts on bell stage of tooth development - shape of the crown is established
  • Cells of the inner enamel epithelium differentiates to ameloblasts

  Ameloblast - enamel forming cell
• Amelogenesis: Secretory Phase

  • Ameloblasts begin their secretory activity when a small amount of dentin has been laid down.
  • Enamel deposition proceeds, a thin, continuous layer of enamel is formed along the dentin

  Amelogenin - major component of enamel matrix proteins, hydrophobic 90%

  Tomes' processes - short extensions towards the dentinoenamel junction
• Amelogenesis: Maturation Phase

  • As the tooth erupts in the oral cavity, enamel hardens
  • Principal activity of ameloblasts is the bulk removal of water and organic material form the enamel to allow introduction of additional inorganic material.
  • Process of maturation starts from the height of the crown and progresses cervically.
• Nasmyth's Membrane

  : Primaran covering the crown of a newly erupted

  tooth

  • Produced by ameloblasts after producing enamel rods
• Incremental lines of Retzius

  • Illustrate the incremental pattern of the enamel, that is, the successive apposition of layers of enamel during formation of the crown.
  • Brownish bands in ground sections of the enamel
• Perikymata

  • Transvers, wave-like grooves, believed to be the external manifestations of the striae of Retzius
  • Tiny valleys on the tooth surface that level circumferentially around the crown