Dentition in mammals

Hard and usually pointed structure connected to jaw bones in buccal cavity of vertebrates are known as teeth and the arrangement of teeth is termed as dentition.

• Functions of teeth -

1. Helps in grasping and holding the prey.
2. Help in chewing the food.
3. Serve as an organ for affense and defense .

• shape of teeth -

= morphologically, teeth can be distinguished as Homodont or Heterodont .

1. Homodont - if all the teeth present are similar in shape and size. They are said to be homodont. Example - dolphins, porpoises and armadillos
2. Heterodont — if the teeth are dissimilar in shape and size. They are distinguished into several types known as incisors, canines, premolars and molars.

• attachment of teeth -

= The manner of attachment of teeth at their bases with the jaw bones varies throughout Vertebrates. These attachments are as follow-

1. Acrodont - in this condition teeth are attached to the free surface of the jaw bones. Example- shark or frog.
2. Plurodont - in this condition, teeth are attached to the inner side of the jaw bone by their base as well as one side. Example - lizards.
3. Thecodont - in this condition, teeth have well developed roots in deep sockets in jaw bones. Example - crocodiles, some tooth birds and fishes.

• succession of teeth (occurrence) -

= on the basis of their occurance, they are of 3 different types :

1. Polyphyodont - in this, teeth can be replaced an indefinite number of times during life. Example- lower vertebrates.
2. Diphyodont- in most mammals teeth develop during life in two successive sets, a condition known as diphyodont. Teeth of the first set are called deciduous. Lacteal or milk teeth. They usually errupt after birth. Milk dentition has no molars included. Later, milk teeth are replaced in the adult by the permanent teeth which last throughout the life, if lost are not replaced.
3. Monophyodont -in some mammals such as platypus, marsupials, moles, Sirenians, toothless whales etc, only one set of the teeth develops, known as monophyodont condition.

•kinds of teeth -

= 4 types of teeth occur in mammals -incisors, canines, cheek teeth or molars, and premolars.

1. Incisors- these are front teeth. They are single-rooted, Monocuspid And long, curved and sharp -edged. They are adapted for seizing, cutting, and bitting.
2. Canines- a single canine tooth occurs in each half of each jaw, just outside the incisors. These are elongated, single-rooted and with a conical sharp monocuspid crown. They are meant for piercing, tearing and offence and defence.
3. Cheek teeth - canines are followed by premolars followed by molars. Both types are collectively called the cheek teeth, their crowns have broad surfaces with ridges and tubercules meant for crushing , grinding and chewing.
4. Premolars - usually have 2 roots and 2 cusps and molars have more than 2roots and several cusps.

* cheek teeth are of various types depending upon the number ; shape; and arrangement of cusps —

(A) Triconodont : found in fossils mesozoic mammals in which 3 cones are arranged in a straight line or linear series .

(B) Trituberculate : in this, 3 cones or tubercles are arranged in the form of a triangle.

(C) Bunodont : their crowns bear small, seperate blunt and rounded tubercles meant for crushing.

(D) Secodont : this condition is found in carnivore in which teeth have sharp cutting edges for tearing and cutting flesh.

(E) Selenodont : this condition occurs for grinding in herbivorous grazing cusps of hard enamel enclosing softer areas of dentine.

(F) Lophodont : in this condition,there is a folding of enamel and dentine like in elephants.

• Dental formula-

= In mammals, no of teeth varies in different species. However no of teeths is constant and characteristic for every species. Therefore, no of kinds of teeth in a species of mammals can be represented by a sort of equation, which is called as Dental Formula.

Since 2 halves of each jaw are identical, only the teeth of one side are recorded. Teeth of the upper and lower jaws are separated by a horizontal line.

Kinds of teeth are denoted by their initial letters — i, c, pm, m representing incisors, canines, premolars, and molars respectively.

Number of teeth shown in the formula multiplied by 2 — gives the total number of teeth in a species.

For example -

a) dental formula of human beings is as —

=( i 2/2 ; c 1/1; pm 2/2; m 3/3 ) × 2 = 32

Or to simply further :

2. 1 . 2 . 3 / 2 . 1 . 2 . 3 = 32

b) dental formula of horse is as — 3 . 1 . 4 . 3 / 3 . 1 . 4 . 3 = 44

c) dental formula of kangaroo is as -

3 . 1 . 2 . 4 / 1 . 0 . 2 . 4 = 32 (when a certain type of teeth is lacking it is indicated as 0)

d) dental formula of rabbit is as —

2 . 0 . 3 . 3/ 1 . 0 . 2 . 3 = 30

•structure of teeth -

In mammals, tooth is differentiated into 3 parts : crown, neck, and root. crown is exposed part above the gun and out of the jaw bone. Root is the Basal part above the gum and out of the jaw bone. Root is the basal part of the jaw bone, root is Embedded in a socket of jaw bone. Junction of crown and root is called neck.

Tooth is made largely of a hard substance i.e, dentine. In the crown it is covered externally by a thin and shiny layer called enamel, the root of the tooth is surrounded and fixed to jaw bone by a layer of cement . Inside the tooth a pulp cavity is present, which is filled with gelatinous connective tissue containing blood vessels and nerves. Lining of the the pulp cavity is a layer of bone cell called ‘odontoblast’ that send Protoplasmic process into dentine….