The Skull

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The Skull

• Its bones can be divided into
• 1. NEUROCRANIUM
• It forms a protective case around the brain.
• 2. VISCEROCRANIUM
• It forms the skeleton of the face.

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NEUROCRANIUM

• Is divided into
• (A) Membranous
• It forms Flat bones of (Calvaria) or Cranial
  Vault, which surround the brain.

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NEUROCRANIUM

• (B) Cartilaginous part (Chondrocranium)
• It forms the bones of the base of the skull.

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MEMBRANOUS NEUROCRANIUM

• Mesenchymal cells derived mostly from the Neural
  Crest and partly from the Para Axial mesoderm.
• They ossify directly into bone (flat bones)
• at the top and most of the sides of the brain.

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SUTURES

• At birth, the neural crest cells form condensed
  Fibrous joints between the flat bones of the
  skull.

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FONTANELLS

• They are at the meeting of more than two sutures
  which becomes wide.
• The fontanells are (6) in number.
• The Anterior Fontanell is the most prominent.
• Its palpation gives an idea about the
  intracranial pressure and the process of
  ossification of the skull.

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MOLDING

• It is adaptation of the shape of the fetal skull
  to the pelvic cavity during birth.
• It depends on the softness of the bones and their
  loose connections at the sutures to allow the
  overlapping of the bones of the skull.

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MOLDING

• 1. The Frontal bone becomes flat.
• 2. The Occipital bone is drawn out.
• 3. Slight overriding of one Parietal bone on the
  other.
• Restoration of the normal shape is within few
  days after birth.

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CHONDROCRANIUM

• It is formed by fusion of several separate
  cartilages.

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(1) PARACHORDAL CARTILAGE (BASAL PLATE)

• It is the cartilage in around the rostral limit
  of the notochord.
• It is derived from the paraxial mesoderm.

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PARACHORDAL (BASAL) PLATE

• It fuses with the cartilages derived from the
  Sclerotomes of the occipital somites
  (somitomers).

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PARACHORDAL PLATE

• It forms the Base of the Occipital bone.
• It extends around the cranial end of the spinal
  cord to form the boundaries of the foramen
  magnum.

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(2) HYPOPHYSEAL CARTILAGE

• It is formed around the developing pituitary
  gland,rostral to the occipital basal plate.
• It is derived from the Neural Crest cells.
• Its cartilage fuses to form the Body of the
  sphenoid bone.

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CHONDROCRANIUM

• (3) TRABECULAE CRANII
• They fuse to form the body of the ethmoid.
• (4) ALA ORBITALIS
• It forms the lesser wing of the sphenoid.
• (5) ALA TEMPORALIS
• It forms the greater wing of the sphenoid.

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CHONDROCRANIUM

• (6) OTIC CAPSULES
• They surround the otic vesicles (primordia of the
  internal ear).
• They form the petrous and mastoid parts of the
  temporal bone.

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CHONDROCRANIUM

• (7) NASAL CAPSULES
• They surround the nasal sacs and contribute to
  the formation of the ethmoid bone.

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CARTILAGENOUS NEUROCRANIUM

• An elongated median plate of cartilage is formed
  between the nasal region and the anterior border
  of the foramen magnum.

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OSSIFICATION

• It begins in the
• 1. Occipital bone
• 2. Basisphenoid (body of sphenoid) bone.
• 3. Ethmoid bone. (In that order).

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VISCEROCRANIUM

• The mesenchyme forming the bones of the face
  including the nasal and lacrimal bones is derived
  from the Neural Crest cells that migrate into the
  first two pharyngeal arches.

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1ST ARCH

• The Dorsal end of the first arch (Meckel
  cartilage) forms
• The bony ossicles of the middle ear
• (Malleus and Incus).
• Its remnant is the Sphenomandibular ligament.
• .

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2ND ARCH

• The (Dorsal) end of the Second arch (Reichert
  cartilage) forms
• 1. Stapes of the middle ear.
• 2. Styloid process of the temporal bone.

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2ND ARCH

• The (Ventral) end of the 2nd arch forms
• 1.Lesser horn.
• 2. Superior part of the (body of hyoid bone).

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3RD ARCH

• The (Ventral) part of the 3rd arch forms
• 1. Greater horn. 2.Inferior part of the (body of
  hyoid).

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4TH 6TH ARCHES

• They fuse to form
• the Laryngeal cartilages Except the Epiglottis.

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MEMBRANOUS VISCEROCRANIUM

• Intramembranous ossification of the Maxillary
  prominence (dorsal portion of the first arch )
  gives the following bones
• 1. Squamotemporal (becomes part of the
  neurocranium) .
• 2. Maxilla.
• 3. Zygomatic.

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MANDIBLE

• It is formed from
• Intramembranous ossification of the Mandibular
  prominence (ventral portion of the first arch).
• Endochondral ossification
• The mandibular condyles and the median plane

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SKULL OF NEW BORN

• It is large in proportion to the rest of the
  skeleton.
• Small face is due to
• 1. Small size of the jaws.
• 2. Absence of the Para nasal sinuses.
• 3. Underdeveloped facial bones at birth.

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POST NATAL GROWTH OF THE SKULL

• The greatest increase of size of the calvaria is
  during the first two years. This is the period of
  most rapid increase of size of the brain.
• The calvaria normally increases in capacity until
  about 16 years of age.
• A slight increase of its size for 3 to 4 years
  because of increase thickening of the bone.

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CRANIOSYNOSTOSIS

• It is premature closure of the skull sutures.
• Its cause is unknown but genetic factors are
  important it is more common in males.

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(1) SCAPHOCEPHALY

• Premature closure of the Sagittal suture.
• The skull becomes long, narrow and wedge- shaped.

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(2) OXYCEPHALY

• Premature closure of the Coronal suture.
• The skull is high and tower like.

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(3) PLAGIOCEPHALY

• Premature closure of the Coronal or Lambdoid
  sutures.
• The skull is twisted and asymmetric.