1 Development of Skull

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Developmentof

Skull



Development of Skull

The development of the skull is divided into two parts:

1. Neurocranium: It forms the brain box.

2. Viscerocranium: It forms the facial skeleton.

The neurocranium is subdivided into:

A. Membranous neurocranium. It forms the vault of the

skull. This part is ossified in membrane.

B. Cartilaginous neurocranium. It forms the skull base. Thispart ossifies in cartilage.



Membranous neurocranium(vault of the skull)

It starts as a membranous

mesodermal condensation

overlying the brain.

The centers of ossification

are developed directly in thismesodermal condensation to

form:

2 parietal bones

2 halves of the frontal bone

squamous part of occipital

bone

squamous part of both

temporal bones.




Ossification spreads until the

bones become separated by

more or less wide sutures.

At the points of meeting of

sutures there are wider gapsof non-ossified membranes

which are called fontanelles:

Anterior fontanelle.

Posterior fontanelle.

Sphenoid fontanelle.

Mastoid fontanelle.




The frontal bone is ossified

from two centers; the bone is

early formed of two halves

separated by interfrontal

(metopic) suture. The 2 halves of the frontal

bone begin fusion in the 2nd

year after birth and the

metopic suture is

completely obliterated by

the age of 6-8 years.



Cartilaginous neurocranium(base of the skull)

Close to the midline the 4 paired

cartilages are developed according

to their anteroposterior

arrangement:

1) Interorbital cartilage (tuberculacranii): It lies between the orbit and

nasal cavity.

2) Hypophyseal cartilage: in

relation to the developing

hypophysis cerebri (pituitary gland).3) Parachordal cartilage: in relation to

the upper part of the notochord.

4) Occipital sclerotomes: 4 in number

and lie close to the notochord.




More laterally 3 paired

cartilages are developing

from mesodermal

condensation:

1) Ala orbitalis: primordiumof lesser wing of sphenoid.

2) Ala temporalis:

primordium of the greater

wing of sphenoid.

3) Periotic capsule: form the

petrous and mastoid parts

of the temporal bone.




The cartilages fuse with each

other across the middle line

and with the neighboring

ones anteroposteriorly and

mediolaterally to form thecartilaginous models of the

bone of the base of the skull

which are later ossified.

However gaps (foramen or

sutures) are left for passage

of cranial nerves and vessels

from or to brain.



Viscerocranium (facial Skeleton)

The bones of the face are

developed in membrane

from the mesoderm of the

first arch.

The mesoderm of the maxillaryprocess forms mesodermal

condensation which ossifies in

membrane to form:

The maxilla.

Premaxilla.

Palate.

Zygomatic bone.

Part of temporal bone.



Viscerocranium (facial Skeleton)

The mandibular processcontains the Meckel's cartilage.

The mesoderm of the

mandibular processes forms a


around the Meckel's cartilage. The mandible is formed by

membranous ossification in the


around Meckel's cartilage.

At the same time the Meckel'scartilage regress and

disappears.

The 2 halves of the mandible is

united by a median fibrous

connection called symphysismenti.



Abnormalities of skull

development These result from:

1. Failure of cranial sutures

to form.

2. Premature closure ofsutures

(craniosynostosis).

Acrocephaly

It is a tower - like skullcaused by premature

closure of the coronal

suture.




development

Scaphocephaly It is a long skull caused by

premature closure of the

sagittal suture.




developmentPlagiocephaly

It is an asymmetric skull caused by premature closure ofthe lambdoid and coronal sutures on one side of the skull.

Microcephaly It results from failure of the brain to grow and the skull

fails to expand.

It is usually associated with mental retardation.

Anencephaly The skull never forms and the brain degenerates.

1 Development of Skull

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