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DEVELOPMENT OF THE SKULL

So today lecture will be the last lecture

we will talk about the development of the skull, so it is more embrogically lecture

than gross anatomy.

When we speak about the development of the skull, we have to know that the skull

bones develop from loosly organized embryonic C.T called mesenchyme; it is usually

surround the brain tissue, and it going to form the skull bone.

There is 2 ways for this mesenchyme to be transform into bone ( YOU HAVE

ALREADY STUDIED THAT IN GENERAL HISTOLOGY) :

1- intramembranous ossification

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formation of the bone, it will remain there it will NOT disappear so( bone

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Here, the mesenchymal cells start to change into chondrocytes (cartilage cells),

then after they become mature, the start to calcify, after they calcify, they

become bone tissue (osteocyte).

(THE DR SAID: I dont wanna go in too many details, because you have alreadystudied that in histology )

SO, WHAT WE CONCERN HERE???

IS THE DEVELOPMENT OF THE SKULL?

**the prenatal skull(also postnatal ) consists of 2 parts:

1-neuorocranium 2- viscerocranium

-the neuorocranium part is the brain case or brain box that gonna surround the

whole brain .the viscerocranium part is the bone of the facial skeleton.

both of these part of the skull are developed by either intrmembranous or

endochondral ossification (depend on the shape of the bone).

In the neuorocranium :

-the membranous part will be form by intramembranous ossification and it will

involve just the vault (frontal bone, parietal bone , squamous part of temporal

bone,squamous part of occipital bone) .the cartilaginous part will be formed by endochondral ossification, and it will

involve the cranial base (basilar part of occipital, sphenoid bone, ethmoid bone,

peterous part of temporal bone).

In the viscerocranium :

-the membranous part usually involve the jaw bone (maxilla &mandible) also the

zygomatic bone , all of them formed by intramembranous ossification.

the cartilaginous part will involve more delicate and complicated bone as ear

ossicles (malleus, incus, and stapes), also mandibular condyle.

So, NOT the whole mandible is formed by intramembranous ossification, there is

exception =the condyle.

because the condyle is very delicate and important structure (because it gonna

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form TMJ) ,that is why we have to form cartilage model of condyle at first to

make sure it is fitting in mandibular fossa properly ,then it will be calcify.

Again, THE WHOLE MANDIBLE IS FORMED BY INTRAMEMBRANOUS

OSSIFICATION EXCEPET ???THE CONDYLAR PART.

* membranous neurocranium

In the membranous neurocranium it forms by intramemmbranous ossification .

It`s formed by frontal bone , parietal bone , squamous part of temporal bone and

sqaumous part of occipital bone

* Cartilagenous neurocranium

The part that formed by endochondral ossification

It`s formed by cranial base : basilar part of occipital bone , petrous part of

temporal bone , sphenoid and ethmoid

** for the cranial base around the age of 6 week several cartilages start to form,

you start to see centers of cartilaginous formation

So what is happening there in the cranial base the mesenchyme start to form a

cartilaginous base which we refer to it chondrocranuim ( chondro : cartilage ,

cranium : skull and to be specific the base of the skull )

SO,,,

**membranous neorocranium

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chondrocranium usually formed at the age of 12 weeks intra uterine ( 3 months)

you will have a complete base ..

but how it is formed ??

_ first you have centers of cartilages for chondroformation these centers we

refer to them chondrocranium cartilages , those centers start to grow up and fuse

together , ones they fuse together they form a complete base of the skull but it is

made of cartilage

NOTE : in the same time the vault formed by woven bone

Later on the chondrocranium start to calssified to become the bones of the cranial

base

Now the most imp chondrocraniam cartilages are :

1- parachordal cartilage : they are 2 ones around the neural cord ( the cordthat gonna form the spinal cord and the brain ) and when they fuse they

form the basilar part of occipital bone

2- hypopheseal cartilages ( infront the neural cord cartilage ) : they surroundedthe hypophesis at this time ( pituitary gland ) they end to form the body of

sphenoid

3- trabeculae carnii : are two colomns located most anteriorly those twocolomns start to fuse together and grow up in the middle to form the body

ethmoid bone4- ala orbitalis : its closed to the orbital bone and it will form the the lesser

wing of sphenoid

5- otic capsule : its a capsule surrounding the ear i will form the petrous partof temporal bone

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* membranous veserocranium ( facial part )

the maxillary bone

zygomatic bone

the nasal bone ( 2 small plates )

the mandible except the chondylar part

* cartilaginous veserocranium

The middle ear ossicles

Mandibular chondyle

* * After the age of 12 weeks the woven bone continue to form a complete

bone and the cartilaginous base ( chondrocranium ) start to ossify to form the

cranial base bone until bearth , at the age of nine month the complete skull is

bony but not the whole bones is bony part we still can see some spaces (

mesenchyme ) so the bone not fuse together

For example the parietal bone formed but it does not fuse with the anothor

parietal bone

*The sutures still opens and have mesenchymal tissue that will allow the

growth in size so when you look to a new born skull it is small round in shape ( in

adult it is oval ) and the bones are very thin because of that they not commonly

fracture( ones there is a hit a curvatures will form but the bone will not

fracture ) because they are thin

* they are large in proportion to the whole body because the body is not yet

growing much as the skull

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* the neurocranium part is much much larger than the facial part (

veserocranium ) because the jaws are not completely form you don`t have the

alveolar process which contain the eruption of the teeth , so facial part will

later start to grow in size and increase in height by the formation of alveolar

process and the eruption of the teeth

*The calvarial bone are separated by connective tissue ( membranous area )

those connective tissue is losse which are the mesenchyme those areas we

refer to them as the fontanelles that allow the skull to growth , in the first 2

years after birth the sutures are still open and you can see some mesenchyme

tissue btw them .

Eg .

sagittal suture : allow the skull to grow in width

coronal suture : allow the skull to grow in length

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There is NO air sinuses in the new born baby because the bone is still very small

but with continuous growth and after 2 years the fontanelles and the sutures

close and no mesenchyme tissue remain the skull now grow by different way

by reabsorption from inside and deposition from out side ( the way of skull

growth from 2years until 18 years )

HOW dose the sinuses form ?!

reabsorption by the bone itself :)

CLINCAL Advantage of the fontanelles :

Measure the Intracranial pressure :

JUST touch the fontanelle and if there any depresstion then up back to the

bone that indicate dehydrationtreatment : IV

But if it up that indicate an increase of the intracranial pressure and the fluid

increase

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We have 6 fontanelles the most important one the ANT and POST cus they

accommodate the growth of the brain The increase in calvaria size is greatest

during first 2 yaers of lifemost head growth is accomplished by expansion along suture lines & fontanelles.

Thereafter, the skull grows by internal resorption and external replacement.

Skull continues to growth in capacity until adulthood (16-18y)

Increase In facial height in the new bone there is no facial height coincides withthe growth of alveolar processes & eruption of deciduous teeth

6 unossified membranous areas

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Anterior fontanelle:

Located btw the 2 frontals and 2 parietal bones

Posterior fontanelle:

2 parietal & occipital

(2) Anterolateral (Sphenoid):

F, P, T, & S

This area after the closer of the sutures will become the area of pterion

(2) Posterolateral (Mastoid) very close to mastoid:

P, T, & Occip.

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Developmental abnormalities of the skull

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Human body contain Thousands of genes a mistake of the expression one gene atleast can lead to different abnormality in the skull of the infant :

the first one which we refer it ACRANIA A: without , crania : cranium

we dont have a nurocranium falier of the cranial vault and part of the cranial

base to develop absence of the Calvaria because of the brain or partial lose of the

brain this is due of some molecules and genes in the formation of the brain will

influence the growth of the skull bone if you have no development of the brain that

mean also no development of the skull bones .

Happen in one /1000 birth , Incompatible with life he will born dead cus he dont

have a brain .

this condition most of the time usually join with abnormal on the spinal cord and

vertebral column .

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The Second condition we have a very small cranium so we call it Microcephaly

Small size calvaria with normal size facial skeleton so in this case the infant will

has a small underdevelopment brain the brain not totally develop not growing insize so the skull bone around it still samall in size

So the baby will live but will have mental retardation .

Causes : cus of the closer of fontanelles and sutures

the brain is not developing and the skull bone Because there is no signaling to them

to induce the continuation of the growth because the brain is under developed.

There is a genetic factors as I told you , mutation in the genes or some

environmental factors ( as maternal infection with some viruses like chickenpox) ,

or most commonly there is radiation as what happening in atomic pumps in(

Hiroshima and Nagasaki) , several cases where suffering from microcephaly.

Or there is another cause which is the maternal alcohol abuse, because they found

that sever abuse from continuation of alcohol drinking from the mother (pregnantmother ) will affect the development , so the brain will not develop, and you have a

microcepaly

Craniosynostosis:

cranio: cranium.

syno: sutures.

stosis : premature closure of one of the sutures.

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So it is several kinds of deformities in the skull , depends on premature closure of

one suture , as you know you have :

1- Coronal suture : which allows the skull to grow in length.

2- Sagittal suture: which allow the skull to grow in width.

3- Lambdoid suture: for roundation of the skull posterirly.

So those are important suture , a closure in one of those sutures will prevent the

growth in one side , so the growth will continue in the other side , this will lead to

deformable skull.

They found that it is happened in males more than females, but there is no specific

reason why .

There are 3 different types and each one depend on which suture is closing:

1- Scaphcephaly: (~50%)

premature closure of sagittal suture.

2- Oxycephaly (Turricephaly):

premature closure of coronal Suture.

3- Plagiocephaly:

premature closure of coronal or lambdoid

suture on one side.

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1- the most common one is the scaphcephaly :

if you remember a scaphoid bone which means ( boat like shape ) >> so scaphcephaly

( ) it is going more in length along the coronal suture , and it is due to

closure of sagittal suture, so more growth in width.

2- Oxycaphaly (Turricephaly):

Premature closure of coronal suture.

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3- Plagiocephaly:

Premature closure of lambdoid suture or coronal suture on one side only.

Plagio : mean asymmetry .

Because it is close one of the coronal from right or left.

Or one of the lambdoid from right or left .

So the growth in one side is stopped , and the other is continue.

There is another type which is (trianglocephaly ) in the infants , there is a metopic

sutures ( frontal sutures) if this suture close early >> the skull will be like a

triangle.

So the scaphcephaly is the most common kind of Craniosynostosis, 50% of kids

suffering from scaphcephaly.

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the second one is the closure of the coronal suture >> once the coronal is closed >>

that means no growth in length , in this case the brain start to compensate by

growing in height ( Turricephaly ) , the head will go up ( tower like skull ) , like a

tower shape skull >> this is 30% of Craniosynostosis.

The remaining 20% will be trianglocephaly and plagiocephaly ( asymmetrical twisted

skull because of the closure of lambdoid suture or coronal suture on one side only.).

The growth continue in one side only, so there is asymmetrical shape of the baby

skull.

However plagiocephaly causes arent only genetic causes, like scaphcephaly and

Turricephaly, in plagiocephaly there is :

1-genetic causes leading to premature closure in the sutures.

2-Environmental causes :

Sleeping position : the baby always sleeps on one side , so the suture is open and

the bone is very thin, and you have depressed the side of the skull, so there is no

continuation of the growth here, the growth will continue there >> so there is what

we refer to it ( as plagiocephaly positional ) to distinguish it from the genetic one.

The environmental causes >>> east to be treated, and it will involve complication as

in the genetic one .

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To treat plagiocephaly positional you have to make sure of 2 things :

1-the baby each night sleeps on a different side , one night on the right side, the

other on the left.

2-But the most important one is to have a very soft pillow ( will resorb the skull, as

I told you the size of the head in the baby is very big compared to the body).

It is a cosmetic issue >> it has no problems , the kids will not have any mental

retardation or problems like in a scaphcephaly and Turricephaly m but however

they found that there is somewhat in 10-15% of kids are suffering from

plagiocephaly , they will increased in intra-cranial pressure>> so the fluid will not

distributed equally .

The sooner discovered >> the better the outcome.

When you discovered the problem within first 6 months >> it is still very easy, just

start to alternate the skull of the baby m thats will solve the problem.

however after 1st year or after the first 6 months in this case it is more difficult

to treat, but it is just a cosmetic issue , but you can do some orthoatic management

( the baby wears head caps ) , this cap will distributed the pressure , it is a

rounded cap so as the baby sleeps on cap on one side , the space will remain there

>> the pressure will be on the cap , and the cap will not pressure the head . so it is

will allow it to grow.

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Cloverleaf Skull

Very severe condition m in this case you dont have just closure of one side suture

>> you have the closure of all sutures , so premature closure of all sutures together>> so there is no growing in the skull at all >> so the cranial vault growth stopped m

and the cranial base growth continues to grow in size .

Most common characteristic of Thanatophoric Dwarfism type II and it is kind of

associated with syndrome ( more than one problem ).

And the most common reason they found it is the mutation in fibroblast growth

factor receptor # 3 (FGFR3).

Done by : Heba Radaideh , Weam Rahamnh , Sondos Harbieh , Haya Momani

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