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AXIAL SKELETON

Learn about the formation of the axial skeleton, including the skull, vertebrae, and ribs, through mesenchyme differentiation, endochondral ossification, and primary & secondary ossification centers. Understand spinal cord abnormalities like spina bifida.

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AXIAL SKELETON

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  1. AXIAL SKELETON • SKULL & MANDIBLE. • VERTEBRAL COLUMN. • STERNUM. • RIBS.

  2. PARAAXIAL MESODERM • It is longitudinal columns on each side of the notochord. • At the end of the 3rd week, it is differentiated into: • Somitomers (in the head region). • Somites (from the occipital region downward).

  3. SOMITES • They are differentiated into : • Sclerotome (ventromedial). • It forms the vertebrae and ribs. • Dermomyotome(dorsolateral). • It forms the muscles and dermis of the skin.

  4. MESENCHYME • At the end of the 4th week, the mesodermal cells form the polymorphous embryonic connective tissue (Mesenchyme). • The mesenchymal cells can migrate to different locations and are able to differentiate into : • Fibroblasts.Chondroblasts.Osteoblasts.

  5. ENDOCHONDRAL OSSIFICATION • The mesenchymal cells first give rise to hyaline cartilage models, which in turn ossify.

  6. MESENCHYMAL (Precartilagenous) Stage • In the 4th week, the mesenchymal cells from sclerotomes shift their position to be condensed around: • 1.Notochord • (around which the vertebrae develop).

  7. MESENCHYMAL (Precartilagenous) Stage • 2.Neural tube(primordium of spinal cord). • 3. Body wall.

  8. SCLEROTOME • Each sclerotome consists of: • Looselyarranged cells cranially. • Densely packed cells caudally.

  9. SCLEROTOME • Some densely packed cells move cranially opposite the center of the myotome.

  10. INTERVERTEBRAL DISC • The notochord degenerates in between the vertebrae. • It expands to form the Nucleus pulposus. • Later this nucleus is surrounded by theAnulus fibrosus(migrating densely packed cells). • These together constitute the intervertebral disc.

  11. INTERVERTEBRAL DISC • Nerveslie in close relation to the inter vertebral discs.

  12. CENTRUM • It is the primordiumofthe body of the vertebra. • It is formed from: • (1) the remaining densely packed cells which fuse with • (2) the loosely arranged of theimmediately caudal sclerotomes.

  13. CENTRUM • It is an intersegmental structure. • Arterieslie on each side of the vertebral body. • The dorsal intersegmental arteries become the intercostal arteries.

  14. VERTEBRAL (NEURAL) ARCH • It is formed from the mesenchymal cells around the neural tube.

  15. THE VETEBRAL BODY • It is a composite of the anular epiphyses and the mass of bone between them. • It includes : • Centrum • Parts of the vertebral arch • Facets for the heads of the ribs.

  16. THE CARTILAGENOUS (V. C.) • It begins in the 6thweek. • Two chondrification centers appear in each centrum . • They fuse with each other and with the centers of the vertebral arch.

  17. THE CARTILAGENOUS (V. C.) • Spinousand Transverse processes: Formed from • extensions of Chondrification centers in the vertebral arch.

  18. BONY VERTEBRAL COLUMN • Ossification begins during the embryonic period. • It endsat the age of 25years.

  19. PRIMARY OSSIFICATION CENTERS • They are Threein number • One for the centrum (dorsal and ventral centers that fuse to form one) • One in each half of the vertebral arch. • Ossification is evident in the arch in the 8th week.

  20. PRIMARY OSSIFICATION CENTERS • The bony halves of the neural arch fuse with each other in the first (3- 5) years. • The union starts first in the lumbar region then it progresses cranially.

  21. NEUROCENTRALJOINTS • They are cartilagenous joints between the vertebral arch and the centrum. • They permit growth of the vertebral arches as the spinal cord expands. • These articulations disappear during the (3rd- 6th) years of age.

  22. SECONDARYOSSIFICATIONCENTERS • They are five in number. • They appear afterpuberty. • They are : • One for the tip of the spinous process. • One for the tip of each transverse process. • Two Anular Epiphyses on the superior and inferior rami of the vertebral body.

  23. SECONDARYOSSIFICATIONCENTERS • All secondary centers unite with the rest of the vertebra around the age of25 years. • Exceptions to typical ossification of vertebrae: • C1 (Atlas) • C2(Axis) • C7 • Lumbar,Sacrum and Coccyx.

  24. SPINA BIFIDA • It is failure of fusion of the two halves of the vertebral arch.

  25. SPINA BIFIDA • It occurs more frequently in girls than boys.

  26. SPINA BIFIDA (OCCULTA) • It is a minor, insignificant anomaly of the vertebral column. It usually causes no clinical symptoms. • The skin over the bifid arch is intact • .

  27. SPINA BIFIDA (OCCULTA) • There may be no external evidence of the vertebral defect. • Sometimes the anomaly is indicated by a tuft of hair. • The spinal cord and spinal nerves are usually normal.

  28. SPINA BIFIDA (CYSTICA) • It is a severe type • The spinal cord and meninges are involved. • It is associated with neurologic symptoms.

  29. RIBS • They are formed from mesenchymal costal processes of the thoracic vertebrae. • They are united with the vertebrae at the CostoVertebral joints. • They become cartilagenous and ossified before birth.

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