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AXIAL SKELETON

AXIAL SKELETON. SKULL . VERTEBRAL COLUMN . STERNUM. RIBS. PARAAXIAL MESODERM. It is longtudinal columns on each side of the notochord. At the end of the 3 rd week, it is differentiated into: Somitomers (in the head region). Somites (from the occipital region downward). SOMITES.

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AXIAL SKELETON

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

  2. PARAAXIAL MESODERM • It is longtudinal 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. LATERAL PLATE MESODERM • It is theSomaticmesodermal layer of the body wall • It is responsible for the formation of : • 1. Pectoral and Pelvic girdles. • 2. Long bones of the limbs.

  5. 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 and Osteoblasts.

  6. NEURAL CREST CELLS • In the head region they differentiate into mesenchyme that share in the formation of the bones of the face and skull.

  7. MODE OF OSSIFICATION • (1) Membranous • In some bones,such as the flat bones of the skull , mesenchyme differentiate directly into bone. • .

  8. MODE OF OSSIFICATION • (2) Endochondral • In most bones, mesenchymal cells first give rise to hyaline cartilage models, which in turn ossify .

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

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

  11. SCLEROTOME • Each sclerotome consists of loosely arranged cells cranially and densely packed cells caudally. • Some densely packed cells move cranially opposite the center of the myotome, where they form the Intervertebral disc.

  12. CENTRUM • It is the primordium of the body of the vertebra. • It is formed from the remaining densely packed cells which fuse with the loosely arranged of the immediately caudal sclerotomes. • It is an intersegmental structure.

  13. CENTRUM • Nerves lie in close relation to the inter vertebral discs. • Arteries lie 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. INTERVERTEBRAL DISC • The notochord degenerates, between • the vertebrae, it expands to form the Nucleus pulposus. • Later this nucleus is surrounded by the Anulus fibrosus. • These together constitute the intervertebral disc.

  16. THE CARTILAGENOUS STAGE • 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 STAGE • Spinous and Transverse processes are formed from • extensions of Chondrification centers in the vertebral arch. • A cartilagenous vertebral column is formed.

  18. BONY VERTEBRAL COLUMN • Ossification begins during the embryonic period. • It ends at 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 8thweek.

  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 after puberty. • 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 of 25 years. • EXCEPTIONS • C1 (Atlas) • C2(Axis) • C7 • Lumbar,Sacrum and Coccyx.

  24. THEVETEBRAL 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.

  25. 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.

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

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

  28. SPINA BIFIDA OCCULTA • A minor, insignificant anomaly of the vertebral column that usually causes no clinical symptoms. • The skin over the bifid arch is intact • .

  29. 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.

  30. SPINA BIFIDA CYSTICA • It is a severe type of spina bifida. • The spinal cord and meninges are involved. • Associated with neurologic symptoms.

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