Tooth structure and evolution
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Tooth Structure and Evolution. Tooth Anatomy. Enamel – Composed of crystals of hydroxyapatite [3(Ca 3 PO 4 ) 3 * Ca(OH) 2 ] Enamel is the hardest tissue in the body

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Tooth anatomy l.jpg
Tooth Anatomy

  • Enamel – Composed of crystals of hydroxyapatite [3(Ca3PO4)3 * Ca(OH)2]

    Enamel is the hardest tissue in the body

  • Dentine – Softer than enamel. The inorganic salt composition is the same as enamel but the content of organic fibers is approx. 30% compared to 3% for enamel.

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Anatomy (Continued)

  • Cementum – Cementum is a nonvascular bone that is usually acellular.

    Cementum is rich in collagen fibers and is softer than dentine.

  • Pulp Cavity – Blood vessels and nerves pass into the pulp cavity

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Diagram of Tooth Structure


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Rooted versus Rootless Teeth

  • Rooted Teeth – In most mammals the opening to the pulp cavity constricts reducing the blood supply to the tooth and growth stops.

  • Rootless Teeth – In other mammals the opening to the pulp cavity does not constrict and the tooth continues to grow throughout the life of the mammal.

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Examples of Rootless Teeth

Incisors of rodents

Cheek teeth of some rodents

All teeth of rabbits except 2nd pair of incisors

Upper incisors of elephants – all dentine tusks

Upper canines of the walrus

Left incisor of male narwhal

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Development of Teeth

  • Dental Lamina

  • Enamel Organ

  • Dermal Papilla

  • Ameloblasts

  • Odontoblasts

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Types of Teeth

  • Incisors

  • Canines

  • Premolars

  • Molars

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Dental Formulas

  • Primitive Placental Dental Formula

    • I 3/3, C 1/1, P 4/4, M 3/3 = 44

      Reductions in this number are common. Increases are rare. The giant armadillo, African bat eared fox, most toothed whales have more than 44 teeth.

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Dental Formulas (Continued)

  • Primitive Marsupial Dental Formula

    • I 5/4, C 1/1, P 3/3, M 4/4

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How to Distinguish Types of Teeth

  • Incisors in upper jaw are in premaxilla bone

  • In most mammals you cannot distinguish premolars from molars

    • Unless the animal is a carnivorous member of the Placental Order Carnivora and has a Carnassial Pair of teeth (fourth upper premolar and first lower molar)

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Sets of Teeth

  • Most mammals have two (2) sets of teeth

  • Toothed whales and a few other mammals have only one set of teeth

    • Diphyodont, Deciduous milk teeth and a permanent second set of teeth

    • Monophyodont, this is a secondary condition

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Sets of Teeth (Continued)

  • In Diphyodont mammals the Incisors, Canines, and Premolars have deciduous precursors. The molars do not have deciduous precursors.

  • Some people consider the molars to be the first set of teeth retarded in their development.

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Evolution of Teeth

  • Problem – to go from a single cusped reptilian tooth to a multicusped mammalian tooth.

    Two Theories:

    Concrescence Theory

    Differentiation Theory (Cope-Osborn Theory)

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Conscrescent Theory

  • States that a multicusped mammalian molar is formed by the fusion of a number of simple conical teeth.

    • Greatest support came from Multituberculates in which molars had distinct cusps. Assumed that each cusp represented a reptilian tooth.

    • Later Multituberculaltes had more cusps than older species.

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Cope-Osborn Theory

  • Even the most complex mammalian molar originated from a single cusped reptilian tooth.

    • Formation of additional cusps in front of and behind the original cusp

    • Accessory cusps change position relative to the main cusp to form a triangle.

    • A heel is added to triangle in lower jaw

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Specialization of Cheek Teeth

  • Hypsodont = high crown

  • Brachydont = low crown

  • Bunodont = usually brachydont with 4 major rounded cusps, omnivores

  • Lophodont = usually hypsodont with a fusion of cusps to form elongated ridges termed lophs, herbivores

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Specializations (Continued)

  • Selenodont = usually hypsodont with each ridge formed by the elongation of a single cusp, ridges are crescent shaped, herbivores

  • Secodont or Carnassial = scissor action of upper and lower teeth, carnivores of the Order Carnivora