12 Skeletal System 1

7/27/2019 12 Skeletal System 1

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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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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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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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12 Skeletal System 1