ORAL HISTO PRELIMS ALL (4)

• Origin of human tissues:

  • Sperm and ovum fertilization leads to the formation of a zygote
  • Zygote develops into a morula (ball of cells) and then into a blastocyst
• Gastrula development involves:

  • Two cavities divided by an embryonic disk
  • Three distinct cell layers: ectoderm (inner cavity), endoderm (outer cavity), and embryonic disk (mesodermal/mesenchymal cells)
• Development of the oropharynx:

  • At the 4th week of development, the human embryo appears like a flat disk with the brain expanding and enlarging
  • The heart is pushed beneath the brain, creating a pit called the stomodeum or primitive oral cavity
  • The oral pit deepens to form the oral cavity, with the deepest extent known as the oropharyngeal membrane/buccopharyngeal membrane, which ruptures along the 5th week opening the oral cavity to the tubular gut
• Pharyngeal arches/branchial arches formation:

  • 1st arch: mandibular arch (bony mandible, muscles of mastication, nerves, and blood vessels)
  • 2nd arch: hyoid (facial muscles, vessels, and hyoid bone)
  • 3rd arch: pharyngeal muscle
  • 4th arch: muscles of larynx and pharynx
• Branchial grooves/pharyngeal grooves and pouches:

  • 1st branchial groove leads to the external auditory canal
  • Pharyngeal pouches/branchial pouches separate the arches within the pharynx, with different pouches giving rise to structures like the middle ear, eustachian tube, tonsils, parathyroids, thymus, and more
• Development of the face and palate:

  • At the 4th week, frontal processes bulge forward and laterally, while the maxillary process develops as swellings of tissue from the mandibular arch below it
  • By the 5th week, nasal placodes thicken and develop into nostrils, and the frontonasal process is formed
  • The 6th week sees broadening of the face due to lateral growth of the brain, with the median nasal process and maxillary process fusing to form the upper lip
• Development of the teeth:

  • Tooth development begins around the 6th week with the formation of a dental lamina and tooth bud
  • The tooth germ undergoes stages like initiation, bud stage, cap stage, bell stage, apposition, and maturation, involving processes like induction, proliferation, histodifferentiation, morphodifferentiation, and apposition
• Enamel characteristics:

  • Enamel is the hardest tissue in the body, acellular, and permeable
  • It consists of 96% inorganic material (hydroxyapatite) and 4% organic material, including water, amelogenins, and enamelins
• Enamel is a tissue in the body that is brittle, acellular, permeable, with a thickness ranging from 10 microns to 2.5 mm, and a color that can be yellowish white or grayish white
• Chemical properties of enamel include:

  • 96% inorganic material (hydroxyapatite) and trace minerals (strontium, magnesium, lead, fluoride)
  • 4% organic material including water, amelogenins, and enamelins
• Main structures of enamel:

  1. Enamel rods

  2. Rod sheath

  3. Interrod substance
• Enamel rods or enamel prisms may vary from 5-12m in number and have three parts: the body or head, the neck, and the tail
• Rod sheath is the boundary of enamel rod and interrod substance, while the interrod substance cements or holds together the enamel rods
• Gnarled enamel refers to the intertwining or twisting of enamel rods at the incisal or cuspal region, aiding in resisting high masticatory loads without fracture
• Hunter-Schreger bands are a series of alternating dark and light bands in the enamel due to changes in the direction of the enamel rods, with dark bands called diazones and light bands called parazones
• Incremental lines of Retzius are brownish bands illustrating the incremental pattern of laying down enamel, comparable to growth rings of a tree
• Enamel lamellae are thin, leaflike structures that extend from the enamel surface towards the DEJ and even towards dentin, being organic in nature and hypomineralized
• Enamel tufts appear as tufts of grass projecting into the enamel, originating from the DEJ to at least 1/3 of the enamel, being less mineralized and creating weakened planes
• Perikymata are wavelike transverse grooves, the external manifestation of incremental lines of Retzius, present only in postnatal enamel
• Enamel cracks are narrow fissure-like structures which are actually the outer edges of the enamel lamellae
• Enamel cuticle, also known as primary enamel cuticle or Nasmyth's membrane, is a delicate membrane covering the entire crown of a newly erupted tooth, derived from the ameloblasts
• Dentinoenamel junction is the junction between dentin and enamel, appearing scalloped or with a pitted appearance, representing a hypomineralized zone
• Enamel pearls, also known as enamel droplets, are HERS that become detached and differentiate into ameloblasts, forming enamel in the bifurcation or trifurcation of teeth
• Age changes in enamel include:

  1. Incapable of repair and replacement

  2. Undergoes attrition, abrasion, and erosion

  3. Loss of rod ends and flattening of perikymata

  4. Darkening in color

  5. Becoming less permeable with advancing age
• Clinical considerations for enamel:

  1. The course or direction of enamel rods is important in cavity preparation

  2. Enamel is brittle and must be supported by sound dentin

  3. Deep pits and fissures predispose teeth to dental caries

  4. Dental lamellae and enamel tufts are predisposing locations for dental caries

  5. Enamel is permeable to fluoride and other substances

  6. Keeping the cervical surface of enamel well polished and smooth is essential

  7. The process of etching produces pitting and irregularities aiding in microretention of the restoration
• Amelogenesis involves the epithelial enamel organ and the life cycle of ameloblasts, including stages like morphologic, organizing, formative, maturative, protective, and desmolytic