Post on 30-Dec-2015
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Skeletal SystemSkeletal System It develops from mesodermal and neural crest It develops from mesodermal and neural crest
cellscells As the notochord and neural tube formsAs the notochord and neural tube forms
Embryonic mesoderm on each side of them Embryonic mesoderm on each side of them proliferatesproliferates
Form a thick longitudinal columns of paraxial Form a thick longitudinal columns of paraxial mesodermmesoderm
Each column is continuous with intermediate Each column is continuous with intermediate mesodermmesoderm
SomitesSomites
Paraxial mesoderm differentiates and Paraxial mesoderm differentiates and begins to divide into cuboidal bodies called begins to divide into cuboidal bodies called somites by the end of 3somites by the end of 3rdrd week week
These blocks of mesoderm are located on These blocks of mesoderm are located on each side of developing neural tubeeach side of developing neural tube
About 38 pairs of somites form during the About 38 pairs of somites form during the somite period of human development (20-somite period of human development (20-30 days)30 days)
SomitesSomites
Each somite differentiates into two parts:Each somite differentiates into two parts:
The ventromedial part is sclerotomeThe ventromedial part is sclerotome
Its cells form the vertebrae and ribsIts cells form the vertebrae and ribs
The dorsolateral part is the dermomyotomeThe dorsolateral part is the dermomyotome
Cells from myotome form myoblastsCells from myotome form myoblasts
Cells from dermatome form the dermisCells from dermatome form the dermis
Axial SkeletonAxial Skeleton
The axial skeleton is composed of:The axial skeleton is composed of:
Cranium (skull)Cranium (skull) Vertebral columnVertebral column RibsRibs SternumSternum
FormationFormation
During formation of this part of the During formation of this part of the skeleton, the cells in the sclerotomes of skeleton, the cells in the sclerotomes of the somites change their positionthe somites change their position
During the fourth week they surround the During the fourth week they surround the neural tube and the notochordneural tube and the notochord
Vertebral ColumnVertebral Column
During the precartilaginous or During the precartilaginous or mesenchymal stage, mesenchymal cells mesenchymal stage, mesenchymal cells are found in three main areas:are found in three main areas:
Around the notochordAround the notochord Surrounding the neural tubeSurrounding the neural tube In the body wallIn the body wall
Vertebral ColumnVertebral Column
In a frontal section of a 4 week embryo, In a frontal section of a 4 week embryo, the sclerotomes appear as paired the sclerotomes appear as paired condensations of mesenchymal cells condensations of mesenchymal cells around the notochordaround the notochord
Each sclerotome consists of loosely Each sclerotome consists of loosely arranged cells cranially and densely arranged cells cranially and densely packed cells caudallypacked cells caudally
Intervertebral DiscIntervertebral Disc
Some densely packed cells move cranially, Some densely packed cells move cranially, opposite the centre of the myotome, where they opposite the centre of the myotome, where they form the intervertebral discform the intervertebral disc
The remaining densely packed cells fuse with The remaining densely packed cells fuse with the loosely arranged cells of the immediately the loosely arranged cells of the immediately caudal sclerotome to form the mesenchymal caudal sclerotome to form the mesenchymal centrumcentrum
This is primordium of the body of a vertebraThis is primordium of the body of a vertebra
Intervertebral DiscIntervertebral Disc
Thus each centrum develops from two adjacent Thus each centrum develops from two adjacent sclerotomes and becomes an intersegmental sclerotomes and becomes an intersegmental structurestructure
The nerves lie in close relationship to the IV The nerves lie in close relationship to the IV discsdiscs
The intersegmental arteries lie on each side of The intersegmental arteries lie on each side of the vertebral bodiesthe vertebral bodies
In the thorax the dorsal intersegmental arteries In the thorax the dorsal intersegmental arteries become the intercostal arteriesbecome the intercostal arteries
Nucleus PulposusNucleus Pulposus
The notochord degenerates and disappears The notochord degenerates and disappears where it is surrounded by the developing where it is surrounded by the developing vertebral bodiesvertebral bodies
Between the vertebrae, the notochord expands Between the vertebrae, the notochord expands to form the gelatinous center of the intervertebral to form the gelatinous center of the intervertebral disc called nucleus pulposusdisc called nucleus pulposus
The nucleus later surrounded by circularly The nucleus later surrounded by circularly arranged fibers that form the anulus fibrosusarranged fibers that form the anulus fibrosus
Vertebral ColumnVertebral Column
The nucleus pulposus and anulus fibrosus The nucleus pulposus and anulus fibrosus together constitute the IV disctogether constitute the IV disc
The mesenchymal cells, surrounding the The mesenchymal cells, surrounding the neural tube, form the vertebral archneural tube, form the vertebral arch
The mesenchymal cells in the body wall The mesenchymal cells in the body wall form the costal processes that form ribs in form the costal processes that form ribs in the thoracic regionthe thoracic region
Cartilaginous StageCartilaginous Stage
During the sixth week chondrification During the sixth week chondrification centers appear in each mesenchymal centers appear in each mesenchymal vertebravertebra
The two centers in each centrum fuse at The two centers in each centrum fuse at the end of the embryonic period to form a the end of the embryonic period to form a cartilaginous centrumcartilaginous centrum
The centers in the vertebral arches fuse The centers in the vertebral arches fuse with each other and the centrumwith each other and the centrum
Cartilaginous StageCartilaginous Stage
The spinous and transverse processes The spinous and transverse processes develop from extensions of chondrification develop from extensions of chondrification centers in the vertebral archcenters in the vertebral arch
Chondrification spreads until a Chondrification spreads until a cartilaginous vertebral column is formedcartilaginous vertebral column is formed
Bony StageBony Stage
Ossification of typical vertebrae begins Ossification of typical vertebrae begins during the embryonic periodduring the embryonic period
It usually ends by the twenty-fifth yearIt usually ends by the twenty-fifth year
There are two primary ossification centers, There are two primary ossification centers, ventral and dorsal for the centrumventral and dorsal for the centrum
These primary ossification centers soon These primary ossification centers soon fuse to form one centerfuse to form one center
Bony StageBony Stage
Three primary centers are present by the Three primary centers are present by the end of the embryonic period:end of the embryonic period:
One in the centrumOne in the centrum One in each half of the vertebral archOne in each half of the vertebral arch
Ossification becomes evident in the Ossification becomes evident in the vertebral arches during the eighth weekvertebral arches during the eighth week
Bony StageBony Stage
At birth each vertebra consists of three bony At birth each vertebra consists of three bony parts connected by cartilageparts connected by cartilage
The bony halves of the vertebral arch usually The bony halves of the vertebral arch usually fuse during the first 3 to 5 yearsfuse during the first 3 to 5 years
The arches first unite in the lumber regionThe arches first unite in the lumber region
This union progresses craniallyThis union progresses cranially
The vertebral arch articulates with the centrum The vertebral arch articulates with the centrum at cartilaginous neurocentral jointsat cartilaginous neurocentral joints
Bony StageBony Stage
These articulations permit the vertebral These articulations permit the vertebral arches to grow as the spinal cord enlargesarches to grow as the spinal cord enlarges
These joints disappear when the vertebral These joints disappear when the vertebral arch fuses with the centrum during the arch fuses with the centrum during the third to sixth yearsthird to sixth years
The vertebral body is a composite of the The vertebral body is a composite of the anular epiphyses and the mass of bone anular epiphyses and the mass of bone between thembetween them
Bony StageBony Stage
Five secondary ossification centers appear in the Five secondary ossification centers appear in the vertebrae after puberty:vertebrae after puberty:
One for the tip of the spinous processOne for the tip of the spinous process
One for the tip of each transverse processOne for the tip of each transverse process
Two anular epiphysis, one on the superior and Two anular epiphysis, one on the superior and one on the inferior rim of the vertebral bodyone on the inferior rim of the vertebral body
Bony StageBony Stage
The vertebral body includes the centrum, The vertebral body includes the centrum, parts of the vertebral arch, and the facets parts of the vertebral arch, and the facets for the heads of the ribsfor the heads of the ribs
All secondary centers unite with the rest of All secondary centers unite with the rest of the vertebra around 25 years of agethe vertebra around 25 years of age
Exceptions to the typical ossification of Exceptions to the typical ossification of vertebrae occur in the atlas, axis, C7, vertebrae occur in the atlas, axis, C7, lumbar vertebrae, sacrum and coccyxlumbar vertebrae, sacrum and coccyx
Development of RibsDevelopment of Ribs
The ribs develop from the mesenchymal costal The ribs develop from the mesenchymal costal processes of the thoracic vertebraeprocesses of the thoracic vertebrae
They become cartilaginous during the embryonic They become cartilaginous during the embryonic period period
They ossify during the fetal periodThey ossify during the fetal period
The original site of union of the costal processes The original site of union of the costal processes with the vertebra is replaced by costovertebral with the vertebra is replaced by costovertebral jointsjoints
Development of RibsDevelopment of Ribs
These are the plane type of synovial jointThese are the plane type of synovial joint
Seven pairs of ribs (1 to 7) are true ribs Seven pairs of ribs (1 to 7) are true ribs
They attach through their own cartilages to the sternumThey attach through their own cartilages to the sternum
Five pairs of ribs (8 to 12) are false ribsFive pairs of ribs (8 to 12) are false ribs
They attach to the sternum through the cartilage of They attach to the sternum through the cartilage of another rib or ribsanother rib or ribs
The last two pairs (11 - 12) are floating ribsThe last two pairs (11 - 12) are floating ribs
Development of SternumDevelopment of Sternum
A pair of vertical mesenchymal bands, A pair of vertical mesenchymal bands, sternal bars develop ventrolaterally in the sternal bars develop ventrolaterally in the body wallbody wall
Chondrification occurs in these bars as Chondrification occurs in these bars as they move mediallythey move medially
They fuse craniocaudally in the median They fuse craniocaudally in the median plane to form the cartilaginous models of plane to form the cartilaginous models of the manubrium, sternebrae and xiphoid the manubrium, sternebrae and xiphoid processprocess
Development of SternumDevelopment of Sternum
Fusion at the inferior end of the sternum is Fusion at the inferior end of the sternum is sometimes incompletesometimes incomplete
As a result the xiphoid process in these infants As a result the xiphoid process in these infants is bifid or perforatedis bifid or perforated
Centers of ossification appear craniocaudally in Centers of ossification appear craniocaudally in the sternum before birththe sternum before birth
But xiphoid process appears during childhoodBut xiphoid process appears during childhood
Development of CraniumDevelopment of Cranium
The cranium develops from mesenchyme The cranium develops from mesenchyme around the developing brainaround the developing brain
The cranium consists of:The cranium consists of:
The neurocranium, a protective case for The neurocranium, a protective case for the brainthe brain
The viscerocranium, the skeleton of the The viscerocranium, the skeleton of the faceface
Cartilaginous NeurocraniumCartilaginous Neurocranium
Initially the cartilaginous neurocranium or Initially the cartilaginous neurocranium or chondrocranium consists of the chondrocranium consists of the cartilaginous base of the developing cartilaginous base of the developing craniumcranium
It forms by the fusion of several cartilagesIt forms by the fusion of several cartilages
Later, endochondral ossification of the Later, endochondral ossification of the chondrocranium forms the bones in the chondrocranium forms the bones in the base of the craniumbase of the cranium
Cartilaginous NeurocraniumCartilaginous Neurocranium
The ossification pattern of these bones The ossification pattern of these bones beginning with occipital bone, body of sphenoid, beginning with occipital bone, body of sphenoid, and ethmoid boneand ethmoid bone
The parachordal cartilage or basal plate forms The parachordal cartilage or basal plate forms around the cranial end of the notochordaround the cranial end of the notochord
It fuses with the cartilages derived from the It fuses with the cartilages derived from the sclerotome regions of the occipital somitessclerotome regions of the occipital somites
Cartilaginous NeurocraniumCartilaginous Neurocranium
This cartilaginous mass contributes to the This cartilaginous mass contributes to the base of the occipital bonebase of the occipital bone
Later extensions grow around the cranial Later extensions grow around the cranial end of the spinal cordend of the spinal cord
These extensions form the boundaries of These extensions form the boundaries of the foramen magnumthe foramen magnum
Cartilaginous NeurocraniumCartilaginous Neurocranium
Hypophysial cartilage forms around the Hypophysial cartilage forms around the developing pituitary glanddeveloping pituitary gland
It fused to form the body of the sphenoid boneIt fused to form the body of the sphenoid bone
The trabeculae cranii fuse to form the body of The trabeculae cranii fuse to form the body of the ethmoid bonethe ethmoid bone
The ala orbitalis forms the lesser wing of the The ala orbitalis forms the lesser wing of the sphenoid bonesphenoid bone
Cartilaginous NeurocraniumCartilaginous Neurocranium
Otic capsules develop around the otic vesicles, Otic capsules develop around the otic vesicles, the primordia of the internal earsthe primordia of the internal ears
They form the petrous and mastoid parts of the They form the petrous and mastoid parts of the temporal bonetemporal bone
Nasal capsules develop around the nasal sacsNasal capsules develop around the nasal sacs
They contribute to the formation of the ethmoid They contribute to the formation of the ethmoid bonebone
Membranous NeurocraniumMembranous Neurocranium
Intramembranous ossification occurs in the Intramembranous ossification occurs in the mesenchyme at the sides and top of the brain mesenchyme at the sides and top of the brain forming calvaria (cranial vault)forming calvaria (cranial vault)
During fetal life the flat bones of the calvaria are During fetal life the flat bones of the calvaria are separated by dense connective tissue separated by dense connective tissue membranes, that form the suturesmembranes, that form the sutures
Six large fibrous areas fontanelles are present Six large fibrous areas fontanelles are present where several sutures meetwhere several sutures meet
Membranous NeurocraniumMembranous Neurocranium
The softness of bones and their loose connections at the The softness of bones and their loose connections at the sutures enable the calvaria to change shape during birthsutures enable the calvaria to change shape during birth
During molding of the fetal cranium, the frontal bones During molding of the fetal cranium, the frontal bones become flat become flat
The occipital bone is drawn outThe occipital bone is drawn out
Parietal bone overrides the other oneParietal bone overrides the other one
Shape of the calvaria returns to normal in few days after Shape of the calvaria returns to normal in few days after birthbirth
Cartilaginous ViscerocraniumCartilaginous Viscerocranium These parts of the fetal cranium are derived from These parts of the fetal cranium are derived from
the cartilaginous skeleton of the first two pairs of the cartilaginous skeleton of the first two pairs of pharyngeal archespharyngeal arches
11stst arch: malleus and incus arch: malleus and incus
22ndnd arch: stapes, styloid process, lesser cornu arch: stapes, styloid process, lesser cornu and body of hyoid boneand body of hyoid bone
33rdrd arch: greater horn and lower part of hyoid arch: greater horn and lower part of hyoid bonebone
44thth to 6 to 6thth arches: laryngeal cartilages arches: laryngeal cartilages
Membranous ViscerocraniumMembranous Viscerocranium
Intramembranous ossification occurs in the Intramembranous ossification occurs in the maxillary prominence of the first pharyngeal archmaxillary prominence of the first pharyngeal arch
Subsequently forms the squamous temporal, Subsequently forms the squamous temporal, maxillary, and zygomatic bonesmaxillary, and zygomatic bones
The squamous temporal bones become part of The squamous temporal bones become part of the neurocraniumthe neurocranium
Mandibular prominence undergoes Mandibular prominence undergoes intramembranous ossification to form mandibleintramembranous ossification to form mandible
Newborn CraniumNewborn Cranium
Newborn’s cranium is round and thinNewborn’s cranium is round and thin
It is large in proportion to the rest of the skeletonIt is large in proportion to the rest of the skeleton
Face is relatively small compared with the Face is relatively small compared with the calvariacalvaria
The small facial region of cranium results from:The small facial region of cranium results from:
Small size of the jawSmall size of the jaw Absence of paranasal air sinusesAbsence of paranasal air sinuses Underdeveloped facial bones at birthUnderdeveloped facial bones at birth
Postnatal Growth of CraniumPostnatal Growth of Cranium
The fibrous sutures of the newborn’s calvaria The fibrous sutures of the newborn’s calvaria permit the brain to enlarge during infancy and permit the brain to enlarge during infancy and childhoodchildhood
The increase in the size of the calvaria is The increase in the size of the calvaria is greatest during the first 2 yearsgreatest during the first 2 years
This is the period of rapid postnatal growth of This is the period of rapid postnatal growth of the brainthe brain
Calvaria normally increases in capacity until Calvaria normally increases in capacity until about 16 years of ageabout 16 years of age
Postnatal Growth of CraniumPostnatal Growth of Cranium
There is a rapid growth of the face and jaws coinciding There is a rapid growth of the face and jaws coinciding with eruption of teethwith eruption of teeth
These facial changes are more marked after the These facial changes are more marked after the secondary teeth eruptsecondary teeth erupt
Enlargement of frontal and facial regions also increase Enlargement of frontal and facial regions also increase with increase in size of paranasal sinuseswith increase in size of paranasal sinuses
Most paranasal sinuses are rudimentary or absent at Most paranasal sinuses are rudimentary or absent at birthbirth
Growth of these sinuses alter the shape of the face and Growth of these sinuses alter the shape of the face and adding resonance to the voiceadding resonance to the voice