DEVELOPMENT OF MANDIBLE PPT

May 25, 2012 | By | Reply More

DEVELOPMENT OF MANDIBLE PPT

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DEVELOPMENT OF MANDIBLE

  • Forms from 1st arch within the mandibular process
  •  Cartilage of the 1st arch (Meckels cartilage) forms the lower jaw in primitive vertebrates
  •  In humans it has a close relation to developing mandible but makes no direct contribution to it

MECKEL’S CARTILAGE:

  • Forms at the 6th week of development
  • It is a solid hyaline cartilaginous rod surrounded by a fibro cellular capsule.
  • Extends from otic capsule to the midline of the fused mandibular processes.
  • Two cartilages of each side do not meet in the midline are separated by a thin band of mesenchyme
  • Mandibular branch of trigeminal begins its development  2/3 along the length of the cartilage
  • At this point, it divides into 2 branches

Lingual nerve

Inferior alveolar nerve

  • Lingual runs on the medial aspect and IDN runs on the lateral aspect of the cartilage
  • Anteriorly, IDN divides to form the mental and the incisive branch

 

PRIMARY OSSIFICATION CENTRE:

  • Condensation of mesenchyme occurs lateral to Meckel’s cartilage during the 6th week
  • This condensation begins in the angle formed by the division of IDN into its incisive and mental branches
  • Intra membranous ossification begins in this condensation during the 7th week.
  • So, the primary ossification centre develops in the 7th week at the point of future mental foramen
  • From this centre bone formation spreads rapidly, anteriorly to the midline, and posteriorly to the point of division of mandibular nerve into IDN and lingual nerve (future lingula)

FORMATION OF MANDIBULAR (INFERIOR ALVEOLAR) CANAL:

  • Anteriorly spreading new bone formation along the lateral aspect of Meckel’s cartilage forms a trough consisting of lateral and medial plates that unite beneath the incisor nerve
  • This trough comes in close contact with a similar trough from the opposite side
  • The two sides fuse shortly after birth
  • Trough is converted to canal when bone forms over the nerve
  • Similarly a gutter is formed when the ossification extends backwards up to the level of future lingula
  • Later this gutter is converted to canal

FORMATION OF MANDIBULAR BODY:

  • Above the bony canal, extending from the division of mandibular nerve to the midline, medial and lateral alveolar plates develop
  • Still superiorly lies the odontogenic epithelium which has started forming the tooth germs by now.
  • Medial and lateral alveolar plates develop around these germs and they come to lie in a secondary trough
  • This trough is partitioned by the developing individual tooth germs
  • Later on alveolar plates completely enclose the tooth germs

DEVELOPMENT OF MANDIBULAR RAMUS:

  • Ossification spreads posteriorly into the mesenchyme of the 1st arch
  • It spreads away from the Meckel’s cartilage and this point of turning away is the future lingula
  • So by the 10th week primitive mandible is formed almost entirely by membranous ossification

 

FATE OF MECKEL’S CARTILAGE:

  • Has almost no direct role in formation of mandible
  • Its posterior most extremity forms the malleus (inner ear) and sphenomandibular ligament
  • Posterior to lingula the Meckel’s cartilage disappears completely and only its fibro cellular capsule persists as sphenomandibular ligament
  • From lingula to the area of mental foramen the cartilage is lost completely
  • Anteriorly it may make a small contribution to the developing mandible by endochondral ossification.

SECONDARY GROWTH CARTILAGES:

  • Further growth of mandible till birth is influenced by formation of secondary cartilages and muscle attachments
  • Total 3 in number

Condylar

Coronoid

Symphyseal

 

CONDYLAR CARTILAGE:

  • Appears during 12th week
  • Rapidly forms a cone/carrot shaped mass occupying most the ramus area
  • Converted to bone by endochondral ossification
  • By 20th wk only a thin layer of bone remains in the condylar head
  • This remnant of cartilage persists till the 2nd decade and provides a growth mechanism for mandible

CORONOID CARTILAGE:

  • Appears at about 4th month of development
  • Forms near the anterior border of ramus and the coronoid region.
  • It is a transient cartilage and disappears long before birth

SYMPHYSEAL CARTILAGE:

  • 2 in number
  • Appear in the connective tissue between the 2 ends of Meckel’s cartilage
  • Get obliterated within the 1st year of birth

SUMMARY:

  • Mandible is a membranous bone developed in relation to mandibular nerve and almost entirely independent of Meckel’s cartilage.
  • It has neural, muscular and alveolar elements
  • Its growth is assisted by the developing secondary cartilages

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Category: Dental

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