LECTURE NOTES ON DENTITION IN MAMMALS

 Teeth are the dermal derivatives of integument They are developed as a result of calcification in the mucous membrane of the buccal cavity.. The teeth are present in almost all the mammals except in a few mammals In whale, the teeth are fused into plates and lost in the adult stage of Ant eaters. But in Echidna (spiny ant eater) the teeth are absent even in the embryo.

Structure of tooth: Each typical mammalian tooth is placed in the socket over the jaw bone. It is distinguished into three main parts.

1) Root - It is the basal part embedded in the bony socket of jaw.

2) Neck - it is the part above the root enclosed by the gum.

3) Crown - It is the upper part beyond the surface of the gum.

The toot is separated from the socket by a vascular pridontal membrane. The vertical section tooth consists of the following parts.

I) Pulp cavity: The entire tooth encloses a central pulp cavity surrounded by a layer of odontoblast cells filled with soft pulp. It is made up of connective tissue, blood vessels and nerve fibers.

II) Dentin: A substance chemically similar to bone- dentine forms the major part of the tooth. But the dentine is permeated by numerous thin canaliculi.

III) Enamel: It is present over the de in the crown and neck regions of the tooth. It is hardest and contains only traces of living matter.

IV) Cement: It surrounds the denting of the root portion of the tooth. It is bony in nature.

Types of Dentition

 Based on the attachment of teeth in the jaw bone, the teeth are classified into 3 types, namely Acrodont Pleurodont Thecodont

Based on the types of teeth, teeth are classified into two types, namely Homodont Heterdont .

Based on the number of sets of teeth that develops during the life time, the teeth are classified into 3 types. Monophyodont ,Diphyodont  and Polyphyodont

1. Acrodont :In this type of dentition, the teeth have no roots and they are fused to the edge of the jaw bone. Acrodont dentition is seen in lower vertebrates like amphibians and reptiles but not in mammals. 2. Pleurodont :In pleurodont dentition, the teeth are attached to the rim of jaw. This type of dentition is found in lizards and not in mammals.

3. Thecodont :In thecodont dentition, the teeth have roots and the roots are embedded in sockets of jaw bone. Thecodont dentition is seen in all mammals.

 4. Homodont :Homodont type of dentition is a condition where the teeth are similar in shape. Eg. Dolphins. 

5. Heterodont:In heterodont, the teeth are different in shape and function and are present in the same animal. In heterodont type, teeth are differentiated into four types, namely incisors, canines, premolars and molars.

 6. Monophyodont: In certain mammals, only one set of teeth develops during the life time and this condition is called monophyodont. Eg. Moles and squirrels.

7. Diphyodont:In diphyodont condition, two sets of teeth develop during the life time of an animal. The first set of dentition is called milk dentition or lacteal dentition. This milk dentition is later replaced by the permanent dentition. In bats and guinea pigs, the milk dentition is lost even before birth. In Homo sapiens (man), the milk teeth are replaced by the permanent teeth around the age of 7 years. 8. Polyphyodont In lower vertebrates, many sets of teeth develop during the life time. This kind of dentition is called polyphyodont.

 Types of Teeth In heterodont dentition, there are four different types of teeth:

1. Incisors They are flat chisel-shaped teeth having a single root. They are used for cutting.

2. Canines Canines are sharp pointed teeth having a single pointed crown and a single root. They are used for piercing and tearing the flesh of the prey.

3. Premolars and Molars (cheek teeth) They are collectively called grinders. They have two or more roots and the crowns bear complex cusps or tubercles. The cheek teeth are used for grinding the food.

 Cusp Patterns of Molars

 The molars show many cusps on their surface. Based on the number and shape of the cusps different types of teeth have been distinguished.

They are: 1. Monocuspid: This is characterized by the presence of only one cusp.

 2. Bicuspid: When two cusps are present, the tooth is called bicuspid.

3. Tricuspid: These are molars having three cusps. Tricuspid teeth are also called triconodont and trituberculate. 

4. Bunodont: In bunodont type, the crown has a number of blunt or pointed cusps. Eg. Man.

5. Lophodont: When the cusps are arranged in the form of ridges, it is called lophodont. Eg. Indian elephant.

6. Selenodont: These are molars with cusps in the form of crescent. Eg. Cow and sheep.

7. Secodont: In secodont type of molars, the crown has cusps having sharp cutting edges. Eg. Carnivorous mammals.

8. Hypsodont: These are molars with high crowns and short roots. The cusps are crescent-shaped.

9.Brachydont:Molar teeth with short crown and with neck at the surface of the gum are called brachydont.They have also crescent shaped cusps.

Mostly the number of teeth is fixed in each mammalian species. Mammalian heterodont dentition is expressed by a ‘dental formula’. The number and arrangement of teeth in each half of the upper and lower jaws is constant and identical. Hence the teeth can be expressed by using the initials - I, C, Pm and P4. The number of teeth differs in the various orders of mammals and is closely related to their ‘feeding habits. Typical number of teeth in the mammals is 44. This can be observed in Horse and pig and their dental formula is 

The numerator indicates the number of teeth on one side of upper jaw. The denominator indicates the number of teeth on one side of the lower jaw. As the two halves of each jaw have same number and kind of teeth, the number of teeth on one side alone is usually ex pressed in the formula.

The dental formulae of some common mammals are given below