Embryogens Axial Skeleton The Axial Skeleton The axial
Embryogens Axial Skeleton
The Axial Skeleton The axial skeleton consists • of : 1 - Cranium (skull) 2 - the Vertebral column 3 - the Ribs 4 - Sternum
• The skeletal system develops from mesodermal is and • Each neuralcolumn crest cells continuous with • As the notochord and intermediate neural tube are formed mesoderm • Embryonic mesoderm on each side of them proliferates Forming a thick longitudinal columns of paraxial mesoderm
Somites • by the end of 3 rd week , Paraxial mesoderm differentiates and begins to divide into cuboidal bodies called somites • These blocks of mesoderm are located on each side of developing neural tube • About 38 pairs of somites form during the somite period of human development (20 -30 days)
• Each somite differentiates into two parts : 1 - The ventromedial part called sclerotome , Its cells form the vertebrae and ribs 2 - The dorsolateral part is the dermomyotome , Cells from myotome form myoblasts and Cells from dermatome form the dermis
Vertebral Column • During the precartilaginous or mesenchymal stage , mesenchymal cells are found in three main areas: 1 - Around the notochord 2 - Around the neural tube 3 - In the body wall • Mesenchymal cells around notochord forms bodies of vertebrae • Mesenchymal cells around neural tube forms vertebral )neural) arches • Mesenchymal cells in body wall forms costal processes (give ribs in thoracic region)
• • At. As the 4 th week a result : , each sclerotome is formed of: 1 - Each centrum develops from 2 1 adjacent - A cranial part of loosely arranged sclerotomes cells (intersegmental ) 2 2 - A caudal part lie of densely – The nerves in close packed relation cells to intervertebral disc 3 – The arteries (first intersegmental) now lie on each side of vertebral bodies 4 – The Notochord degenerates where it is surrounded by vertebral • bodies Later on , the densely packed cells of one sclerotome fuse with the • Nucleus pulposus theimmediately only part of loosely arranged cells: of notocord that doesn’t degenerates caudal sclerotome to form centrum ( between theofvertebral (primordium body) bodies ) • Annulus fibrosus: the mesenchyme that surround the Nucleus pulposus and together they form the IV disc
Cartilaginous Stage • At 6 th week , chondrification centers appear • At the end of embryonic period , the two centers in each centrum fuse with each other to form the cartilaginous centrum • Then the centers in the vertebral arches fuse with each other and the centrum • Chondrification spreads until a cartilaginous vertebral column is formed
Bony Stage • There are 3 primary Ossification centers present by the end of embryonic period • Vertebral arches articulate with • Onecentrum in the centrum and one in each half of vertebral arch through neurocentral joints • At birth consists of three bony parts untileach 3 -6 vertebra years when fusion occurs connected by cartilage • The 2 halves of vertebral arches fuse at 3 -5 years (first in lumbar region , fusion proceeds cranially)
Bony Stage • Ther are 5 secondary ossification centers appear in the vertebrae after puberty • two in the body (two anular epiphysis ) , one at the tip of spinous process and one at the tip of each transverse process • All centers unite around 25 years
DEVELOPMENT OF RIBS • The Ribs develop from costal processes of the vertebrae in the thoracic region • They become cartilage during embryonic period and ossify during fetal period • The site of union of the costal processes with the vertebra is replaced by costovertebral joints • Costal processes in other region fuse with transverse processes of vertebrae
DEVELOPMENT OF STERNUM • A pair of vertical mesenchymal bands ( sternal bars ) develop ventrolaterally in the body wall • Fusion of 2 bars occurs craniocaudally at median plane • Chondrification occurs during the fusion • Ossification centers appear craniocaudally before birth EXCEPT for xiphoid process (during childhood) ste ba rn rs al
Development of Cranium • The cranium develops from the mesenchyme around the • developing Both neurocranium and brain viscerocranium are subdivided • into The: cranium consists of: 11 -NEUROCRANIUM cartilaginous part , (brain ossify box ) the part of the skull that by endochondral ossification encloses the brain 2 - membranous part , ossify 2 - intramembranous VISCEROCRANIUM by (facial bones ) the part of the ossification skull that encircle the face and derived from the embryonic pharyngeal arches
Cartilaginous Neurocranium 3 - ala orbitalis • Consist of : - : 1 - parachordal cartilage • Give rise to lesser wing : of sphenoid • Formed bone around cranial end of notochord , give rise to occipital 4 - otic(together capsuleswith : sclerotome bone of occipital somites) • Give rise to petrous & 2 - hypophyseal mastoid parts of cartilages temporal : bones • Formed around pituitary gland , they Fuse to form body of 5 sphenoid - trabeculae bonecranii & nasal capsules : • They fuse to form ethmoid bone
Membranous Neurocranium • The membranous neurocranium forms the calvaria )cranial vault (or skull roof • it’s consist of : 1 - frontal bone 2 - parietal bone • Bones are separated by fibrous joints (sutures) • Six fontanelles (site of meeting of several sutures) are present
cartilaginous viscerocranium • it’s derived from pharyngeal arches: • Dorsal end of 1 st arch gives malleus & incus • Dorsal end of 2 nd arch gives stapes & styloid process of temporal bone • Ventral end of 2 nd arch gives lesser horn & superior part of body of hyoid bone • Ventral end of 3 rd arch gives greater horn & inferior part of body of hyoid bone
membranous viscerocranium • consist of : 1 - Squamous part of temporal bone 2 - Maxillary bone 3 - Zygomatic bone 4 - Mandible
Postnatal Growth of Cranium • At birth, calvaria is larger than the face • Increase in size of calvaria is greatest during first 2 years • this is due to the rapid postnatal growth of brain at that period The rapid growth of face & jaws occur with : 1 - Eruption of teeth 2 - 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 • Sex : more frequent in females • Incidence : 0. 04 – 0. 15% • Types : 1 - Spina bifida occulta (20%) 2 - Spina bifida cystica (80%)
SPINA BIFIDA OCCULTA • The closed and mildest type • Only one vertebra is affected • No clinical symptoms • Skin over it is intact • Sometimes covered by a tuft of hair
SPINA BIFIDA CYSTICA • The open type • Neurological symptoms are present • Subdivided into: 1 - Spina bifida with meningocele: protrusion of the sac containing meninges & cerebrospinal fluid 2 - Spina bifida with meningomyelocele: protrusion of sac containing meninges with spinal cord and / or nerve roots 3 - Spina bifida with myeloschisis: spinal cord is open ( due to failure of fusion of neural folds )
CRANIOSYNOSTOSIS • Cause : Premature closure of cranial sutures (more severe if occurs prenatally) • Sex : more frequent in males • Types: depend upon which suture closes prematurely 1 - Scaphocephaly )50%) : sagittal suture 2 - Oxycephaly )30%) : coronal suture 3 – Plagiocephaly : both coronal and lambdoid sutures on one side 4 – Trigonocephaly : frontal suture
Pure spirit
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