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Skeletal system
Dr: Eman Khammas Al-sadi
Embryology lecturer 1
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Skeletal System
The skeletal system develops
from1. paraxial mesoderm.
2. lateral plate (parietal layer)mesoderm.
3.from neural crest 2
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.
At the end of the fourth week,Paraxial mesoderm(sclerotome cells)
become mesenchymal, may become
fibroblasts,
chondroblasts,
orosteoblasts (bone-forming cells).
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form ing capacity o f the lateral plate mesoderm-The bone
will forms bones of the
1. pelvic girdle,
2. shoulder girdle,
3. limbs,4. and sternum.
Neural crest c ells in th e head region
also participate in formation of bones of1. the face
2. and skull.
somi tomeresandsomi tesOccipi ta l
also contribute to formation of the
1. cranial vault
2. and base of the skull.
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intramembranous
ossification
in some bones, such
as the f lat bones o f
the skul l,
mesenchyme in the
dermis differentiates
directly into bone, a
process known as(intramembranous
ossification)
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endochondral
ossification
In most bones,
mesenchymal cells
first give rise to
hyaline cartilage
models, which in
turn become
ossified byendochondral
ossification
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SKULL
The skull can be dividedinto two parts: the 1-
neurocranium, which
forms a protective casearound the brain,
2-viscerocranium,
which forms theskeleton of the face.
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NeurocraniumThe neurocranium divided
into two portions:
(1) the membranous part,
consisting offlat bones,
which surround the brain
as a vault, and
(2) the cartilaginous part,
orchondrocranium,
which forms bones of the
base of the skull.
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Membranous
Neurocranium
derived from
1. neural crest cells
2. and paraxial mesoderm
Mesenchyme undergoes membranousossification. -----number of flat, membranous
bones that are characterized by bone spicules.Which progressively radiate from primaryossification centers toward the periphery
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With further growth during fetal and
postnatal life, membranous bones
enlarge by
1. apposition of new layers on the
outer surface2. and by simultaneous
osteoclastic resorption from the
inside.
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Newbo rn Skul l
At birth,
the flat bones of the skull areseparated from each other by narrowsutures of connective tissue, whichare also derived from two sources:
1.neural crest cells (sagittal suture)2. and paraxial mesoderm (coronal
suture).
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At points wheremore than twobones meet,sutures are wide
and are calledfontanelles
anterior fontanelle,which is found
where the twoparietal and twofrontal bones
meet. Sutures andfontanelles allowthe bones of theskull to overlap(molding) during
birth. 13
Sk l l f
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3Skeletal structures ofthe head and face.
Mesenchyme for these
structures isderived from
1. neural crest (blue),
2. paraxial mesoderm(somites andsomitomeres) (red),
3. and lateral platemesoderm (yel low).
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In the first few years after birth, palpation of the
anterior fontanelle may give valuable
information as to
1. whether ossification of the skull is proceeding
normally
2. and whether intracranial pressure is normal.
In most cases, the anterior fontanelle closes by 18
months of age,and the posterior fontanelle closes by 1 to 2
months of age
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Cart ilaginous Neurocranium or
Chondrocranium
1-prechordal chondrocraniumcartilages lie in front of the notochord, which ends at
the level of the pituitary gland in the center of the
sella turcica, are derived from neural crest cells.2-chordal chondrocraniumcartilages posterior to this limit arise from occipital
sclerotomes formed by paraxial mesoderm
The base of the skullis formed when these cartilages fuse and ossify by
endochondral ossification
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Viscerocranium
ViscerocraniumThe viscerocranium, which
consists of the bones of
the face, is formed mainlyfrom the first two pharyngealarches
The first arch gives rise to
A-dorsal portion,the maxillary process, gives
rise to
1. the maxilla,
2. the zygomatic bone,3. and part of the temporal
bone
B-The ventral portion,
the mandibular process, 17
The dorsal tip of the mandibular process along with
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The dorsal tip of the mandibular process, along withthat of the second pharyngeal arch, later givesrise to
1. theincus,
2. the malleus,3. and the stapes
Ossification of the three ossicles begins in thefourth month, making these the first bones tobecome fully ossified.
Mesenchyme for formation of the bones of the faceis derived from neural crest cells, including thenasal and lacrimal bones
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At first, the face is small in comparison with the
neurocranium?.
This appearance is caused by(1)virtual absence of the paranasal air sinuses and
(2)the small size of the bones, particularly the jaws.
With the appearance of teeth and development of the
air sinuses, the face loses its babyish
characteristics.
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Clinical Correlates
Cranio facial Defects and Skeletal Dysp lasias
Neural Crest Cells
Neural crest cells originating in the neuroectoderm
form the facial skeleton and most of the skull.
they are often a target for teratogens. Therefore,it is not surprising that craniofacial abnormalities
are common birth defects.
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Cranioschis isWhen the cranial vault fails to form because of failure of the
cranial neuropore to close is called (cranioschisis),
anencephaly:whenthe cranial vault fails to form and brain tissue exposed to
amniotic fluid& degenerates,Children with such severe skulland brain defects cannot survive.
cranial meningocele and meningoencephalocele,Children with relatively small defects in the skull through which
meninges and/or brain tissue herniated may be treatedsuccessfully. In such cases, the extent of neurological deficitsdepends on the amount of damage to brain tissue
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View
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Craniosynos tosis and Dwarf ism
craniosynostosis :1. premature closure of one or more
sutures.
2. occurs in 1\2,500 births
3. . is a feature of more than 100 geneticsyndromes.
4. The shape of the skull depends onwhich of the sutures closed
prematurely.
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Scaphocephaly:
Early closure of the sagittal suture (57%of cases) results in frontal and occipitalexpansion, and the skull becomes longand narrow (scaphocephaly)
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brachycephaly
Premature closure of the coronal suture
results in a short skull calledbrachycephaly
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l i h l
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plagiocephalyIf the coronal and lambdoid sutures close
prematurely on one side only, then theresult is an asymmetric flattening of theskull called plagiocephaly
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Child ith h h l d b l l f th
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A. Child with scaphocephaly causedby early closure of the
sagittal suture. Note the frontal and occipital bossing.
B. Child with brachycephaly caused by early closure of the
coronal and lambdoidal sutures
C. Child with plagiocephaly resulting from premature closure of
the coronal and lambdoid sutures on one side of the skull.
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Achondroplasia (ACH),the most common form of dwarfism (one per
26,000 live births),primarily affects the long bones.
Other skeletal defects include a large skull
(megalocephaly)
with a small midface,
short fingers,
and accentuated spinal curvature
ACH is inherited as an autosomal dominant,
and 80% of cases appear sporadically.
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Thanatopho r ic dysp lasiais the most common neonatal lethal form ofdwarfism (one per 20,000 live births). There aretwo types; both are autosomal dominant.
Hypochondroplas ia,
another autosomal dominant form ofdwarfism, appears to be a milder type ofACH.
The defect in all these conditions is abnormalendochondral bone formation, so that growthof the long bones and the base of the skull isadversely affected.
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.Three-month-old infant with achondroplasia. Note the large head, shortextremities, and protruding abdomen.
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Acromegaly
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Acromegaly
is caused by congenital hyperpituitarism and
excessive production of growth hormone.
It is characterized by disproportional
enlargement of the face, hands, and feet.
Sometimes, it causes more symmetrical
excessive growth and gigantism.
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Patient with cloverleaf skullcharacteristic ofthanatophoric dwarfismtype II.
The shape of the skullresults from abnormal
growth of the cranial base,caused by a mutation inFGFR3,
followed by
craniosynostosis.The sagittal, coronal, andlambdoid sutures arecommonly involved.
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Microcephalyis usually an abnormality in which the brain
fails to grow and the skull fails to expand.Many children with microcephaly are
severely retarded
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Thank you
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