By Assit. Lec. Dr. Ahmed Muhsin al-mayali B. D. S m. Sc. Ortho. 2014 introduction



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By
Assit. Lec. Dr. Ahmed Muhsin AL-Mayali

B.D.S M.Sc. Ortho.
2014
INTRODUCTION

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.


EVOLUTION

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


  1. 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.



  1. THEORY OF TRITUBERCULY

Each of the mammalian tooth was derived from a

Single reptilian tooth by secondary differentiation of tubercles and roots.




  1. 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

5 lobes.
CHARACTERISTIC OF HUMAN DENTITION

1-MODE OF ATTACHMENT

Teeth may be attached to the jaws in the following

Ways:

Acrodont Teeth attached to the jaw by a connective



Tissue.

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.

Shark

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

Dentition.

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.




Reference

  • Text book of orthodontics , Gurkeerat Singh 2nd Edition 2007.

  • Singh G. :Textbook of Orthodontics ,2007.

  • Nanci A.: Ten cate's Oral Histology, development, structure & function, 7th edition, Mosby Elsevier, 2008.

  • Cawson RA. : Oral medicine, 2008.

  • Berkovitz BKB, Holland GR, Moxham BJ. : Oral anatomy, histology &embryology, 4th edition, Mosby Elsevier, 2009.

  • Proffit WR, Fields HW, Server DM. : Contemporary orthodontics, 4th edition, Reed Elsevier , 2013.


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