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Skull Development

Skull

• Divided into two parts:• Neurocranium: forms a protective case

around the brain. • Viscerocranium: forms the skeleton of

the face.

Neurocranium

• Divided into two portions:

(1) membranous part, consisting of flat bones, which surround the brain as a vault.

(2) cartilaginous part, or chondrocranium, which forms bones of the base of the skull.

Membranous Neurocranium

• Derived from neural crest cells and paraxial mesoderm.

• Mesenchyme of those two sources undergoes membranous ossification.

• Resulting in a number of flat, membranous bones that are characterized by the presence of needlelike bone spicules.

• Spicules progressively radiate from primary ossification centers toward the periphery.

• With further growth during fetal and postnatal life, membranous bones enlarge by apposition of new layers on the outer surface and by simultaneous osteoclastic resorption from the inside.

Skull bones of a 3-month-old fetus show the spread of bone spicules from primary ossification centers in the flat bones of the skull.

Skeletal structures of the head and face. Mesenchyme for these structures is derived from neural crest (blue), paraxial mesoderm (somites and somitomeres) (red), lateral plate mesoderm (yellow).

New Born Skull• At birth, the flat bones of the skull are separated from each other

by narrow seams of connective tissue, the sutures, derived from two sources:

• neural crest cells (sagittal suture) • paraxial mesoderm (coronal suture).• At points where more than two bones meet, sutures are wide

and are called fontanelles • The most prominent of these is the anterior fontanelle, which is

found where the two parietal and two frontal bones meet. • Sutures and fontanelles allow the bones of the skull to overlap

(molding) during birth.• Soon after birth, membranous bones move back to their original

positions, and the skull appears large and round.• In fact, the size of the vault is large compared with the small

facial region

Skull of a newborn,

Note the anterior and posterior fontanelles and sutures. The posterior fontanelle closes about 3 months after birth; the anterior fontanelle closes around the middle of the second year. Many of the sutures disappear during adult life.

• Several sutures and fontanelles remain membranous for a considerable time after birth.

• Vault continue to grow after birth, because the brain grows.

• Although a 5-to 7-year-old child has nearly all of his or her cranial capacity.

• Some sutures remain open until adulthood. • In the first few years after birth, palpation of the anterior

fontanelle may give valuable information as to whether ossification of the skull is proceeding normally and whether intracranial pressure is normal.

• Anterior fontanelle closes by 18 months of age,• Posterior fontanelle closes by 1 to 2 months of age.

After Birth

A fetal cranium showing the bones, fontanelles, and sutures. A, Lateral view.

B, Superior view. The posterior and

C.,3D-dimensional ultrasound rendering of the fetal head at 22 weeks (gestational age). Note the anterior fontanelle (*) and the frontal suture (arrow). The coronal and sagittal sutures are also shown.

Chondrocranium

• The cartilaginous neurocranium (chondrocranium) of the skull initially consists of a number of separate cartilages.

• Those that lie in front of the rostral limit 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.

• They form the prechordal chondrocranium.

• Those that lie posterior to this limit arise from occipital sclerotomes formed by paraxial mesoderm and form the chordal chondrocranium.

• The base of the skull is formed when these cartilages fuse and ossify by endochondral ossification

Dorsal view of the chondrocranium, or base of the skull, in the adult showing bones formed by endochondral ossification.

Bones that form rostral to the rostral half of the sella turcica arise from neural crest and constitute the prechordal (in front of the notochord) chondrocranium (blue).

Those forming posterior to this landmark arise from paraxial mesoderm (chordal chondrocranium) (red).

Viscerocranium• consists of the bones of the face, is formed mainly from the first two

pharyngeal arches.• The first arch gives rise to a dorsal portion, the maxillary process, which

extends forward beneath the region of the eye and gives rise to the maxilla, the zygomatic bone, and part of the temporal bone

• The ventral portion, the mandibular process, contains the Meckel cartilage.• Mesenchyme around the Meckel cartilage condenses and ossifies by

membranous ossification to give rise to the mandible. • The Meckel cartilage disappears except in the sphenomandibular

ligament. • The dorsal tip of the mandibular process, along with that of the second

pharyngeal arch, later gives rise to the incus, the malleus, and the stapes.• Ossification of the three ossicles begins in the fourth month, making these

the first bones to become fully ossified.• Mesenchyme for formation of the bones of the face is derived from neural

crest cells, including the nasal and lacrimal bones .

Lateral view of the head and neck region of an older fetus, showing derivatives of the arch cartilages participating in formation of bones of the face.

THE END