DEVELOPMENT OF AXIAL SKELETON Dr. Ahmed Fathalla Ibrahim

VERTEBRAL COLUMN

VERTEBRAL COLUMN Mesenchymal cells from sclerotome are found in 3 areas: Around notochord Around neural tube In body wall near to notochord & neural tube

VERTEBRAL COLUMN Mesenchymal cells around notochord: form bodies of vertebrae Mesenchymal cells around neural tube: form vertebral (neural) arches Mesenchymal cells in body wall: form costal processes (give ribs in thoracic region)

FORMATION OF BODY At 4th week, each sclerotome is formed of: A cranial part of loosely arranged cells A caudal part of densely packed cells The densely packed cells of one sclerotome fuse with the loosely arranged cells of succeeding sclerotome to form centrum (primordium of body)

FORMATION OF BODY As a result: Each centrum develops from 2 adjacent sclerotomes (intersegmental) Nerves lie in close relation to discs Arteries (first intersegmental) now lie on each side of vertebral bodies Notochord degenerates where it is surrounded by vertebral bodies

INTERVERTEBRAL DISC Nucleus pulposus: notochord (between vertebral bodies) Annulus fibrosus: surrounding mesenchyme

VERTEBRAL DEVELOPMENT

VERTEBRAL DEVELOPMENT Cartilaginous stage At 6th week, chondrification centers appear At the end of embryonic period, the 2 centers in centrum fuse then fuse with those of vertebral arches

VERTEBRAL DEVELOPMENT Bony stage Primary ossific centers: Present by the end of embryonic period Three in number (one in centrum , one in each half of vertebral arch) The 2 halves of vertebral arches fuse at 3-5 years (first in lumbar, fusion proceeds cranially) Vertebral arches articulate with centrum through neurocentral joints until 3-6 years when fusion occurs

VERTEBRAL DEVELOPMENT Bony stage Secondary ossific centers: Appear after puberty Five in number (two in body , one at tip of spinous process, one at tip of each transverse process) All centers unite around 25 years

DEVELOPMENT OF RIBS Ribs develop from costal processes in thoracic region: They become cartilage during embryonic period They ossify during fetal period Site of union with vertebra is replaced by costovertebral joint Costal processes in other region fuse with transverse processes of vertebrae

DEVELOPMENT OF STERNUM Appears in the form of 2 vertical mesenchymal bars (sternal bars) Fusion of 2 bars occurs craniocaudally Chondrification occurs during fusion Ossification centers appear craniocaudally before birth EXCEPT for xiphoid process (during childhood)

DEVELOPMENT OF CRANIUM THE CRANIUM CONSISTS OF: NEUROCRANIUM: PROTECTIVE CASE FOR BRAIN VISCEROCRANIUM: SKELETON OF FACE EACH PART IS SUBDIVIDED INTO: CARTILAGINOUS PART: OSSIFY BY ENDOCHONDRAL OSSIFICATION MEMBRANOUS PART: OSSIFY BY INTRAMEMBRANOUS OSSIFICATION

DEVELOPMENT OF CRANIUM

CARTILAGINOUS NEUROCRANIUM PARACHORDAL CARTILAGE: Forms around cranial end of notochord Forms occipital bone (together with sclerotome of occipital somites) HYPOPHYSEAL CARTILAGES: Form around pituitary gland Fuse to form body of sphenoid bone

CARTILAGINOUS NEUROCRANIUM ALA ORBITALIS: Forms lesser wing of sphenoid bone OTIC CAPSULES: Form petrous & mastoid parts of temporal bones TRABECULAE CRANII & NASAL CAPSULES: Fuse to form ethmoid bone

MEMBRANOUS NEUROCRANIUM Forms the calvaria (cranial vault): frontal & parietal bones Bones are separated by fibrous joints (sutures) Six fontanelles (site of meeting of several sutures) are present

CARTILAGINOUS VISCEROCRANIUM Derived from pharyngeal arches: Dorsal end of 1st arch: malleus & incus Dorsal end of 2nd arch: stapes & styloid process of temporal bone Ventral end of 2nd arch: lesser horn & superior part of body of hyoid bone Ventral end of 3rd arch: greater horn & inferior part of body of hyoid bone

MEMBRANOUS VISCEROCRANIUM Squamous part of temporal bone Maxillary bone Zygomatic bone Mandible

POSTNATAL GROWTH OF CRANIUM At birth, calvaria is larger than face Increase in size of calvaria is greatest during first 2 years (period of most rapid postnatal growth of brain) Rapid growth of face & jaws coincide with: Eruption of teeth Development & enlargement of paranasal sinuses (which are absent or rudimentary at birth)

SPINA BIFIDA Cause: Failure of fusion of the halves of the vertebral arch Incidence: 0.04 – 0.15% Sex: more frequent in females Types: Spina bifida occulta (20%) Spina bifida cystica (80%)

SPINA BIFIDA

SPINA BIFIDA OCCULTA The closed type Only one vertebra is affected No clinical symptoms Skin over it is intact Sometimes covered by a tuft of hair

with meningomyelocele SPINA BIFIDA CYSTICA with meningomyelocele with myeloschisis

SPINA BIFIDA CYSTICA The open type Neurological symptoms are present Subdivided into: Spina bifida with meningocele: protrusion of sac containing meninges & cerebrospinal fluid Spina bifida with meningomyelocele: protrusion of sac containing meninges with spinal cord and/or nerve roots Spina bifida with myeloschisis: spinal cord is open (due to failure of fusion of neural folds)

CRANIOSYNOSTOSIS

CRANIOSYNOSTOSIS Premature closure of cranial sutures (more severe if occurs prenatally) Sex: more frequent in males Types: depend upon which suture closes prematurely Scaphocephaly (50%): sagittal suture Oxycephaly (30%): coronal suture Plagiocephaly: both coronal and lambdoid sutures on one side Trigonocephaly: frontal suture