Dentition in Mammals
Dentition means the arrangement of teeth in the upper and lower jaws on the premaxilla, maxilla, and dentary bones.
Development of tooth:
Teeth develop over the jaw bone where certain Malpighian cells start actively multiplying forming a mass of cells called dental lamina or enamel organ. A dental papilla made of a group of dermal cells appears below the dental lamina which supplies nourishment.
Epidermal cells of the dental lamina that cover the growing dentine are called ameloblast cells. The tooth gradually grows outwards and eventually gets exposed by penetrating the skin covering the jaw bone.
Structure of teeth:
A typical mammalian tooth can be distinguished mainly into two regions i.e. crown and root.
Crown: It is an exposed part of the tooth and situated above the root. (The visible portion)
Root: The root is the hidden part of the gum. The root is anchored in the socket or alveolus of the jaw bone.
Neck: The junction of the crown and root is called the neck.
Types of tissues in typical teeth
There are three types of tissues in a typical tooth, these are enamel, dentine, and cement.
- The unworn crown is covered by a thin, hard, glistening layer named enamel.
- Enamel is the hardest tissue of vertebrates.
- It is composed of a crystal of hydroxyapatite [3(Ca3PO4)2.Ca(OH)2].
- Enamel is ectodermal in origin, without living cells. There is an absence of blood supply in the enamel.
- Dentine is found below the enamel as a hard dermal bony layer.
- Dentine is harder than bone, but it is softer than enamel.
- The ivory is specialized dentine which is (a hard creamy white substance) found in elephants, hippopotami, walruses, and narwhals’ tusks.
- The main component of human dentine is calcium phosphate and fluoride, organic matter, and calcium carbonate.
- It is mesodermal in origin.
- Cement is a thin layer that covers the root of the tooth.
- Cement is a nonvascular bone usually acellular.
- It is softer than dentine and rich in collagen fibers.
- The pulp cavity is lined by a layer of bone cells known as the odontoblast.
- Cement is also mesodermal in origin.
Some mammals lack teeth:
As mammals possess different kinds of teeth, however, some mammals lack teeth. Examples of some mammals lack teeth are given below:
- Among monotremes, the spiny anteater or echidna lacks teeth.
- In adult platypi, presence of epidermal teeth but the absence of true teeth. The embryonic teeth are replaced by horny epidermal teeth in the platypus.
- Teeth are absent in Balena (whalebone whale), Caperea (Pigmy right whale), Megaptera (Humpback whale), and Eschrichtius (Grey whale).
- Among humans, and astonishingly, males in the “Bhudas” tribe of Hyderabad Sindh in Pakistan, never grow teeth all their lives.
Types of Dentition in mammals:
Types of dentition in mammals on the basis of shape and size of teeth:
- Homodont or isodont: In this type of tooth, all of the teeth are alike in their shape and size. Example: Toothed whales, pinnipeds, amphibians, reptiles.
- Heterodont: These types of teeth are generally found in mammals. These teeth are distinguished according to their shape, size, and function. The function of these teeth is also different at different parts of the tooth row.
Types of teeth on the basis of function:
In mammals, the heterodont condition shows four functionally different types of teeth i.e. incisors, canine, premolars, and molar.
Incisors are situated anteriorly on the premaxilla in the upper jaw and tips of dentaries’ in the lower jaw.
- Incisors are conical, single-rooted, and mono-cuspid.
- These are flat and function for cutting or cropping.
- The incisors are chisel-shaped, open-rooted in rodents and they continue to grow throughout the life of the organism.
Canine teeth lie immediately behind the incisors. Canines are found single in each half of the jaw. The teeth are long and pointed, and function for piercing and tearing.
- These are mainly functional in carnivorous animals. example: dogs, tigers, lions, etc.
- In carnivores, the canine is generally used for holding and piercing in both senses of feeding and fighting.
- In rodents and lagomorphs, the canine is absent, leaving a space in between incisors and premolars, called diastema.
- The term “Diastema” used for is any gap within the dental series
Premolars or bicuspid teeth:
These teeth have two roots and two cups. Premolars are used for grinding food materials.
Molars lie behind the premolars. Molars have two or more roots and several cusps. These are used for crushing food.
Types of teeth on the basis of replacement:
In this type of dentition, the teeth are replaced continuously throughout life, so that the jaws are never left without teeth.
In lower vertebrates the attachment of teeth is loose, so they usually lose teeth so the teeth grow to gain to replace the lost ones. Examples of organisms with these teeth are – Dogfish, snakes, etc.
This type of tooth is characteristic of mammals in which two sets of teeth are found. IN this type of dentition, the first set of teeth called milk teeth or lacteal teeth is replaced by the second set of teeth called permanent teeth.
- Molar teeth are absent in milk teeth.
- In organisms like bats and guinea pigs, the milk teeth are lost before birth.
In this type of dentition, the teeth appear only once in the lifetime of the organism. In monophyodont, the replacement of teeth is not found.
- This type of teeth may not erupt or, if they develop are usually shed afterward.
- Marsupials retain all their milk teeth except the last premolars
- The toothed whales (Odontoceti), some rodents (squirrels), few insectivores (moles).
Types of teeth/dentition on the basis of attachment to the jaw bone:
These types of teeth are found among mammals. In this condition, the teeth are fixed in the bony socket of the jaw bone and capillaries and nerves enter the pulp cavity through the open tips of the hollow roots.
- This type of dentition is also found in crocodiles, and some fishes (e.g. Haddock, Garpike, and Barracuda).
These teeth are attached to the top surface of the jaw bone as in fish and amphibians. They are fused to the surface of the underlying jawbone, and their absence of roots, though they are attached to the edge of the jawbone by a fibrous membrane.
- Some reptiles possessing acrodont teeth are Sphenodort and Caltoes. Draco, Agama, Uromastix, and some snakes.
In this type of dentition, the teeth are attached to the inner side of the jaw bone.
- Roots are absent in this type of dentition.
- The nerves and blood vessels do not enter the pulp cavity at the base.
- g. Necturus, and some reptiles.
- Examples of some reptiles possessing pleurodont are Iguanas (Iguanidae), Xenosauraus, Zonuridae, and several snakes.
Some other types of teeth:
- In this type, the cusps in the cheek teeth remain separate and rounded.
- These teeth are used in grinding the food material.
- Bunodont types of teeth are found in man and some omnivore mammals.
- In this type, the cusps are joined to form ridges or lophs.
- the lophodont teeth are found in the cheek teeth of elephants.
- These teeth are used for grinding plants and grasses.
These teeth are with sharp cutting edges present in terrestrial carnivores.
- The Secodonts are sharp and function like scissors to cutting and shearing the flesh
These are cheek teeth with crescent-shaped cups known as selenodont.
- These types of teeth are found in ruminants, and horses, and are used for grinding.
It is a kind of tooth possessing a low crown and comparatively long root, e.g. man.
In this type the teeth’s crown is high and the roots are short, and found open, e.g. Horses, incisors of elephants.
The dental formula shows the system of summarizing the number of different types of teeth i.e. incisor, canine, premolar, and molar in each quadrant of the mount.
The dental formula shows variation between species. The number of teeth varies in different species.
- In the dental formula, the count teeth of each half jaw, and the teeth of the upper jaw are placed as numerators and those in the lower jaw as denominators.
- These different kinds of teeth are indicated by initial letters as such i-for incisor, c-for canine, Pm-for premolar, and m-for molar.
- If there is an absence of a certain type of teeth, then it is indicated by zero.
- The maximum number of teeth in heterodont mammals is 44.
The dental formula of some mammals:
The dental formula in Dog: the dental formula of a dog is 42 teeth = 3142/3143 *2
The dental formula in humans:
- Milk teeth 20 = 2102/2102 *2
- Permanent teeth 32 = 2123/2123 *2
The dental formula of Rabbit: number of teeth 28 = 2033/1023 *2
The dental formula of Sheep or cow: number of teeth 32= 0033/4033*2
The dental formula of some mammals is given in the table below:
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