Assit. Lec. Dr. Ahmed Muhsin AL-Mayali
B.D.S M.Sc. Ortho.
Dentition is defined as type; number and arrangement of teeth or it refer to all upper and lower teeth collectively. The upper and lower arches are a functional unit made up of specific components, i.e. the teeth.
The mammalian dentition goes back 75millionyears.
The dental formula of early primates consisted of 2 incisors, 1 canine, 4 premolars and 3 molars.
The jaws and teeth in the humans have undergone. Evolutionary changes over several years to reach their present form. Reptilian dentition was polyphyodont (many sets of teeth) which evolved to diphyodont (only 2 sets of teeth) in mammals. Also the dentition evolved from homodont (all teeth same) to heterodont (different types of teeth) in the present day humans. To develop a functional occlusion it became necessary for the teeth and bones to develop synchronously.
Over a period of time there was loss or fusion of cranial and facial bones, the numbers of bones have reduced and dental formula has also undergone changes.
THEORIES OF ORIGIN OF MAMMALIAN TEETH
THEORY OF CONCRESCENCE
Mammalian dentition was produced by the fusion of two or more primitive conical teeth and each tubercle with its root originated as simple reptilian tooth.
THEORY OF TRITUBERCULY
Each of the mammalian tooth was derived from a
Single reptilian tooth by secondary differentiation of tubercles and roots.
THEORY OF MULTI-TUBERCULY
Mammalian dentition is a result of reduction and
Condensation of primitive multi-tuberculate teeth.
In terms of evolution, teeth are said to have
Developed from lobes or primary central Incisors,
Premolars and maxillary molars developed from
4 lobes whereas mandibular molars developed from
CHARACTERISTIC OF HUMAN DENTITION
1-MODE OF ATTACHMENT
Teeth may be attached to the jaws in the following
• Acrodont Teeth attached to the jaw by a connective
• Pleurodont Teeth are set inside the jaws.
• Thecodont Teeth inserted in a bony socket
2-NUMBER OF SUCCESSIVE TEETH
• Polyphyodont Teeth replaced throughout Life,e.g.
• Diphyodont Two sets of teeth, e.g. humans
• Monophyodont Single set of teeth, e.g. sheep
3-SHAPE OF TEETH
• Homodont: Single type of teeth.
• Heterodont: Teeth of different shapes, e.g. incisors,
Molars, etc. as in humans.
PRENATAL DEVELOPMENT OF DENTITION
The embryonic oral cavity is lined by stratified squamous
Epithelium called the oral ectoderm, which is
Visible around 28-30 days of intrauterine life.
The first sign of tooth development appears late in
The 3rd embryonic week when the epithelial lining
Begins to thicken on the inferior border of the maxillary
Process and the superior border of the mandibular
Process which join to form the lateral margins of the oral cavity.
At 6 weeks, four maxillary odontogenic zones
Coalesce to form the dental lamina and the two mandibular zones fuse at the midline. The dental lamina is the foundation for the future dental arches. Tooth formation begins with invagination of the dental lamina epithelium into the underlying mesenchyme at specific locations.
The dental lamina gets demarcated into 20 knoblike structures namely the tooth bud/germ. A tooth bud consists of an enamel organ, which is derived from the oral ectoderm, a dental papilla and a dental sac, both of which are derived from the mesenchyme. Each of these swellings of the lamina proliferates and differentiate, passing through various histological and morphological differentiation stages namely bud, cap and bell stage.
Figure 1: Primary epithelial band at 7 week
Figure 2: stage of tooth development
BUD STAGE OF TOOTH DEVELOPMENT (Initiation)
This is the first epithelial incursion into the Ectomesenchyme of the jaw. The tooth bud is
The primordium of the enamel organ. Histologically
It consists of peripheral low columnar cells and
Centrally located polygonal cells.
Figure 3: Dental papilla & Bud stage
The area of Ectomesenchymal condensation subjacent to the Bud is the dental papilla. The dental sac surrounds
The tooth bud and the dental papilla.
Figure 4: Initiation Bud stage
Figure 5 : Bud stage picture showing Meckles cartilage
The dental Papilla later on forms the dentin and pulp whereas The dental sac forms cementum and the periodontal Ligament. Initiation takes place as follows:
• Deciduous dentition: starts from 2nd month in utero.
• Permanent dentition: starts from 5th month in utero
Growth of the free distal End of dental lamina gives rise to the successional lamina, which initiates the permanent
• Dental lamina: elongates distal to the second Deciduous molar and gives rise to the Permanent molar tooth germs.
Figure 6: Dental lamina (low magnification power)
Figure 7: Dental lamina (high magnification power)
Signals responsible for initiation of tooth bud
A great number of genes associated with signaling molecules and epithelial-mesenchymal interactions are expressed in developing teeth.
Signaling molecules from PAX9, MSX1, nerve growth factor (NGF), the sonic hedgehog (SHH) gene, fibroblast growth factor (FGF), bone morphogenetic
Protein (BMP), and distal-less (Dlx) and wingless (Wnt)* families regulate early stages of tooth morphogenesis. These gene products are organizers of odontogenesis, producing structures whose distinct dental patterns are formed by these genes.
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