1 Development of Skull
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Developmentof
Skull
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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.
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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.
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Membranous neurocranium(vault of the skull)
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.
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Membranous neurocranium(vault of the skull)
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.
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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.
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Cartilaginous neurocranium(base of the skull)
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.
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Cartilaginous neurocranium(base of the skull)
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.
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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.
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Viscerocranium (facial Skeleton)
The mandibular processcontains the Meckel's cartilage.
The mesoderm of the
mandibular processes forms a
mesodermal condensation
around the Meckel's cartilage. The mandible is formed by
membranous ossification in the
mesodermal condensation
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.
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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.
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Abnormalities of skull
development
Scaphocephaly It is a long skull caused by
premature closure of the
sagittal suture.
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Abnormalities of skull
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.