Dentition in Mammals

Definition: The arrangement of teeth in the upper and lower jaws, mainly on premaxilla, maxilla and dentary bones is called dentition.

Teeth evolved from denticles.

Absence of Teeth:

• Reptiles: Turtles
• Aves: Birds
• Mammals: Echidna, Spiny ant-eater (Tachyglossus sp.), Right Whale (Balaena sp.); Rorqual whale (Balaenoptera sp.); Humpback Whale (Megaptera).

A typical mammalian tooth can be distinguished mainly into two regions: Crown and Root.

• Crown: Crown is the exposed part of the tooth covered by enamel, situated above the root.
• Root: The root is the hidden part in gum which is anchored in the socket of jaw bone, containing nerves and connective tissue.
• Neck: The junction between the crown called neck.

 There are three kinds of tissues in a typical tooth. They are Enamel, Dentine, Cementum

• Enamel: The crown is a thin, very covered by a hard, glistening layer composed of crystals of hydroxyapatite [Ca₃PO₄.Ca(OH)₂] is called enamel.  Enamel is ectodermal in origin and totally, acellular.
• Dentine: Below enamel, the hard dermal, bony layer is found called dentine. Dentine is is harder than bone but softer than enamel.
• Cementum: Cementum is a non-vascular bony thin layer covering the root of the tooth.

Pulp cavity is lined by a layer of bone cells called Odontoblast.

Classification according to the shape and size of the teeth:

1. Homodont: The condition where the teeth are all alike/similar in their shape and size called homodont type of teeth.

Example: Toothed whales, fishes, amphibians, reptiles and in the extinct toothed birds.

2. Heterodont: The condition where the teeth are different according to shape, size and function is called heterodont type of teeth

Example: Mammals, Port Jackson Shark

According to the attachment of the teeth:

1. Acrodont: Teeth fused to surface of underlying jaw bone, having no roots, called acrodont.

Example: Fishes, Amphibians & some reptiles.

2. Pleurodont: Teeth are attached with the inner side of the jaw bone, touching the bone only with the outer surface of its root, called pleurodont.

Example: Necturus, Iguana, Varanus, many snakes. baroques pristus,

3. Thecodont: Teeth are lodged in bory socket of the jaw bone and blood capillaries & nerves enter the pulp cavity through open tips of hollow roots called thecodont.

Example: Mammals, Crocodiles, Haddock, Garpike.

According to replacement of teeth:

1. Monophyodont: Only one set of teeth develops in their lifetime, this condition is called monophyodont.

Example: Platypus, sirenians, toothed whales etc.

2. Diphyodont: In most mammals two sets of teeth develop in their lifetime, called diphyodont. The first temporary set of teeth, called deciduous teeth, milk teeth or lacteal teeth. These teeth are replaced by a second set of teeth termed permanent teeth.

Examples: Bats, guinea pigs, Humans.

3. Polyphyodont: When teeth are replaced continuously throughout life, the condition is called pleurodont.

Example: Lower vertebrates, Dogfish (Shark), few snakes.