AXIAL SKELETON By Dr Mujahid Khan Skeletal System
AXIAL SKELETON By: Dr. Mujahid Khan
Skeletal System Ø It develops from mesodermal and neural crest cells Ø As the notochord and neural tube forms Ø Embryonic mesoderm on each side of them proliferates Ø Form a thick longitudinal columns of paraxial mesoderm Ø Each column is continuous with intermediate mesoderm
Somites Ø Paraxial mesoderm differentiates and begins to divide into cuboidal bodies called somites by the end of 3 rd week Ø These blocks of mesoderm are located on each side of developing neural tube Ø About 38 pairs of somites form during the somite period of human development (2030 days)
Somites Each somite differentiates into two parts: Ø The ventromedial part is sclerotome Ø Its cells form the vertebrae and ribs Ø The dorsolateral part is the dermomyotome Ø Cells from myotome form myoblasts Ø Cells from dermatome form the dermis
Axial Skeleton The axial skeleton is composed of: Ø Cranium (skull) Ø Vertebral column Ø Ribs Ø Sternum
Formation Ø During formation of this part of the skeleton, the cells in the sclerotomes of the somites change their position Ø During the fourth week they surround the neural tube and the notochord
Vertebral Column Ø During the precartilaginous or mesenchymal stage, mesenchymal cells are found in three main areas: Ø Around the notochord Ø Surrounding the neural tube Ø In the body wall
Vertebral Column Ø In a frontal section of a 4 week embryo, the sclerotomes appear as paired condensations of mesenchymal cells around the notochord Ø Each sclerotome consists of loosely arranged cells cranially and densely packed cells caudally
Intervertebral Disc Ø Some densely packed cells move cranially, opposite the centre of the myotome, where they form the intervertebral disc Ø The remaining densely packed cells fuse with the loosely arranged cells of the immediately caudal sclerotome to form the mesenchymal centrum Ø This is primordium of the body of a vertebra
Intervertebral Disc Ø Thus each centrum develops from two adjacent sclerotomes and becomes an intersegmental structure Ø The nerves lie in close relationship to the IV discs Ø The intersegmental arteries lie on each side of the vertebral bodies Ø In the thorax the dorsal intersegmental arteries become the intercostal arteries
Nucleus Pulposus Ø The notochord degenerates and disappears where it is surrounded by the developing vertebral bodies Ø Between the vertebrae, the notochord expands to form the gelatinous center of the intervertebral disc called nucleus pulposus Ø The nucleus later surrounded by circularly arranged fibers that form the anulus fibrosus
Vertebral Column Ø The nucleus pulposus and anulus fibrosus together constitute the IV disc Ø The mesenchymal cells, surrounding the neural tube, form the vertebral arch Ø The mesenchymal cells in the body wall form the costal processes that form ribs in the thoracic region
Cartilaginous Stage Ø During the sixth week chondrification centers appear in each mesenchymal vertebra Ø The two centers in each centrum fuse at the end of the embryonic period to form a cartilaginous centrum Ø The centers in the vertebral arches fuse with each other and the centrum
Cartilaginous Stage Ø The spinous and transverse processes develop from extensions of chondrification centers in the vertebral arch Ø Chondrification spreads until a cartilaginous vertebral column is formed
Bony Stage Ø Ossification of typical vertebrae begins during the embryonic period Ø It usually ends by the twenty-fifth year Ø There are two primary ossification centers, ventral and dorsal for the centrum Ø These primary ossification centers soon fuse to form one center
Bony Stage Three primary centers are present by the end of the embryonic period: Ø One in the centrum Ø One in each half of the vertebral arch Ø Ossification becomes evident in the vertebral arches during the eighth week
Bony Stage Ø At birth each vertebra consists of three bony parts connected by cartilage Ø The bony halves of the vertebral arch usually fuse during the first 3 to 5 years Ø The arches first unite in the lumber region Ø This union progresses cranially Ø The vertebral arch articulates with the centrum at cartilaginous neurocentral joints
Bony Stage Ø These articulations permit the vertebral arches to grow as the spinal cord enlarges Ø These joints disappear when the vertebral arch fuses with the centrum during the third to sixth years Ø The vertebral body is a composite of the anular epiphyses and the mass of bone between them
Bony Stage Five secondary ossification centers appear in the vertebrae after puberty: Ø One for the tip of the spinous process Ø One for the tip of each transverse process Ø Two anular epiphysis, one on the superior and one on the inferior rim of the vertebral body
Bony Stage Ø The vertebral body includes the centrum, parts of the vertebral arch, and the facets for the heads of the ribs Ø All secondary centers unite with the rest of the vertebra around 25 years of age Ø Exceptions to the typical ossification of vertebrae occur in the atlas, axis, C 7, lumbar vertebrae, sacrum and coccyx
Development of Ribs Ø The ribs develop from the mesenchymal costal processes of the thoracic vertebrae Ø They become cartilaginous during the embryonic period Ø They ossify during the fetal period Ø The original site of union of the costal processes with the vertebra is replaced by costovertebral joints
Development of Ribs Ø These are the plane type of synovial joint Ø Seven pairs of ribs (1 to 7) are true ribs Ø They attach through their own cartilages to the sternum Ø Five pairs of ribs (8 to 12) are false ribs Ø They attach to the sternum through the cartilage of another rib or ribs Ø The last two pairs (11 - 12) are floating ribs
Development of Sternum Ø A pair of vertical mesenchymal bands, sternal bars develop ventrolaterally in the body wall Ø Chondrification occurs in these bars as they move medially Ø They fuse craniocaudally in the median plane to form the cartilaginous models of the manubrium, sternebrae and xiphoid process
Development of Sternum Ø Fusion at the inferior end of the sternum is sometimes incomplete Ø As a result the xiphoid process in these infants is bifid or perforated Ø Centers of ossification appear craniocaudally in the sternum before birth Ø But xiphoid process appears during childhood
Development of Cranium Ø The cranium develops from mesenchyme around the developing brain Ø The cranium consists of: Ø The neurocranium, a protective case for the brain Ø The viscerocranium, the skeleton of the face
Cartilaginous Neurocranium Ø Initially the cartilaginous neurocranium or chondrocranium consists of the cartilaginous base of the developing cranium Ø It forms by the fusion of several cartilages Ø Later, endochondral ossification of the chondrocranium forms the bones in the base of the cranium
Cartilaginous Neurocranium Ø The ossification pattern of these bones beginning with occipital bone, body of sphenoid, and ethmoid bone Ø The parachordal cartilage or basal plate forms around the cranial end of the notochord Ø It fuses with the cartilages derived from the sclerotome regions of the occipital somites
Cartilaginous Neurocranium Ø This cartilaginous mass contributes to the base of the occipital bone Ø Later extensions grow around the cranial end of the spinal cord Ø These extensions form the boundaries of the foramen magnum
Cartilaginous Neurocranium Ø Hypophysial cartilage forms around the developing pituitary gland Ø It fused to form the body of the sphenoid bone Ø The trabeculae cranii fuse to form the body of the ethmoid bone Ø The ala orbitalis forms the lesser wing of the sphenoid bone
Cartilaginous Neurocranium Ø Otic capsules develop around the otic vesicles, the primordia of the internal ears Ø They form the petrous and mastoid parts of the temporal bone Ø Nasal capsules develop around the nasal sacs Ø They contribute to the formation of the ethmoid bone
Membranous Neurocranium Ø Intramembranous ossification occurs in the mesenchyme at the sides and top of the brain forming calvaria (cranial vault) Ø During fetal life the flat bones of the calvaria are separated by dense connective tissue membranes, that form the sutures Ø Six large fibrous areas fontanelles are present where several sutures meet
Membranous Neurocranium Ø The softness of bones and their loose connections at the sutures enable the calvaria to change shape during birth Ø During molding of the fetal cranium, the frontal bones become flat Ø The occipital bone is drawn out Ø Parietal bone overrides the other one Ø Shape of the calvaria returns to normal in few days after birth
Cartilaginous Viscerocranium Ø These parts of the fetal cranium are derived from the cartilaginous skeleton of the first two pairs of pharyngeal arches Ø 1 st arch: malleus and incus Ø 2 nd arch: stapes, styloid process, lesser cornu and body of hyoid bone Ø 3 rd arch: greater horn and lower part of hyoid bone Ø 4 th to 6 th arches: laryngeal cartilages
Membranous Viscerocranium Ø Intramembranous ossification occurs in the maxillary prominence of the first pharyngeal arch Ø Subsequently forms the squamous temporal, maxillary, and zygomatic bones Ø The squamous temporal bones become part of the neurocranium Ø Mandibular prominence undergoes intramembranous ossification to form mandible
Newborn Cranium Ø Newborn’s cranium is round and thin Ø It is large in proportion to the rest of the skeleton Ø Face is relatively small compared with the calvaria Ø The small facial region of cranium results from: Ø Ø Ø Small size of the jaw Absence of paranasal air sinuses Underdeveloped facial bones at birth
Postnatal Growth of Cranium Ø The fibrous sutures of the newborn’s calvaria permit the brain to enlarge during infancy and childhood Ø The increase in the size of the calvaria is greatest during the first 2 years Ø This is the period of rapid postnatal growth of the brain Ø Calvaria normally increases in capacity until about 16 years of age
Postnatal Growth of Cranium Ø There is a rapid growth of the face and jaws coinciding with eruption of teeth Ø These facial changes are more marked after the secondary teeth erupt Ø Enlargement of frontal and facial regions also increase with increase in size of paranasal sinuses Ø Most paranasal sinuses are rudimentary or absent at birth Ø Growth of these sinuses alter the shape of the face and adding resonance to the voice
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