Development of mandible


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Development of mandible

  2. 2. Mandible ??? • From Latin “mandibula” meaning jawbone. • It is the largest and strongest bone of the face. • It houses the lower teeth. • Consists of a curved horizontal portion: the body, and two perpendicular portions: the rami. • The body and rami unite at the angle of mandible nearly at right angles.
  3. 3. Development of Mandible • Forms from the 1st arch within the mandibular process. • Meckel’s cartilage (cartilage of 1st arch) forms lower jaw in primitive vertebrates. • In humans, it has a close relation to the developing mandible but makes no direct contribution to it.
  4. 4. Meckel’s Cartilage • Forms at 6th week of development. • It is a solid hyaline cartilage surrounded by a fibro-cellular capsule. • Extends from otic capsule to the midline of the fused mandibular processes. • Cartilages of each side do not meet at midline; they are separated by a thin band of mesenchyme.
  5. 5. Meckel’s Cartilage Cont’d • Formation of the mandibular division of trigeminal nerve begins two- thirds along the length of the cartilage. • It divides into two branches at this point:  Lingual nerve  Inferior alveolar nerve (IAN) • Lingual nerve runs along the medial aspect of the cartilage. • Inferior alveolar nerve runs along the lateral aspect of the cartilage. • Anteriorly, IAN divides to form mental and incisive branches.
  6. 6. Primary Ossification Centre • Condensation of mesenchyme occurs lateral to Meckel’s cartilage in the 6th week gestation. • This condensation begins at the angle formed by division of the IAN into incisive and mental branches. • Intra-membranous ossification begins in this condensation during the 7th week. Ossification begins in the membrane covering the outer surface of Meckel’s cartilage. • From this centre, bone formation spreads rapidly, anteriorly to the midline, posteriorly to the point of division of the mandibular nerve into lingual nerve and inferior alveolar nerve.
  7. 7. Formation of Mandibular Canal • Ossification grows medially beneath the incisive nerve, then spreads beneath this nerve and Meckel’s cartilage. • Incisive nerve is contained within this trough or groove of bone formed by lateral and medial plates which are united below the nerve. • This trough comes in close contact with a similar trough from the opposite side. The two fuse shortly after birth. Trough converted into canal when bone forms over the nerve. • Spread of ossification in the backward direction produces at first a trough- like gutter of bone in which lies the inferior alveolar nerve up to the level of the future lingula. • Later this gutter is converted to bone.
  8. 8. Development of Body and Ramus BODY OF MANDIBLE • Above bony canal, medial and lateral alveolar plates develop. • Odontogenic epithelium forming the tooth germs lies superiorly. • Medial and lateral plates develop around these germs and they come to lie in a secondary trough. • Trough is partitioned by developing individual tooth germs. • Alveolar plates completely enclose the tooth germs. RAMUS OF MANDIBLE • Ossification spreads posteriorly into mesenchyme of the 1st arch. • Spreads away from Meckel’s cartilage and this point of turning away is the future lingula. • By 10th week, primitive mandible is formed almost entirely by membranous ossification.
  9. 9. Secondary Growth Cartilages Further growth of mandible till birth is influenced by formation of three secondary cartilages. Secondary cartilages Condylar Coronoid Symphyseal
  10. 10. Secondary Growth Cartilages Cont’d CONDYLAR CARTILAGE • Appears during 12th week. • Rapidly forms cone/carrot shaped mass occupying most of ramus area. • Converted to bone by endochondral ossification. • By 20th week, only a thin layer of cartilage remains in the condylar head. • This remnant of cartilage persists till the 2nd decade of life. • Provides a growth mechanism for the mandible in the same way as an epiphyseal cartilage does in the limbs. CORONOID CARTILAGE • Appears at about 4th month of development. • Forms near anterior border and top of the coronoid process. • It is a transient cartilage. • Disappears long before birth. SYMPHYSEAL CARTILAGE • Two in number. • Appear in the connective tissue between two ends of the Meckel’s cartilage. • Get obliterated within the first year of birth.
  11. 11. Fate of Meckel’s Cartilage • Its posterior-most extremity forms the incus and malleus (ear ossicles) of the inner ear, and the spheno-malleolar ligament. • From the sphenoid to the division of mandibular nerve into its inferior alveolar and lingual branches, the Meckel’s cartilage disappears completely. Only its fibro-cellular capsule persists as sphenomandibular ligament. • From lingula to area of mental foramen, Meckel’s cartilage degenerates.
  12. 12. Summary • Mandible is a membrane bone, developed in relation to the nerve of the first arch. • Almost entirely independent of Meckel’s cartilage. • Its growth is assisted by the development of secondary cartilages.