Neurocranium

The neurocranium is most conveniently divided into two portions: (a) the membranous part, consisting of flat bones, which surround the brain as a vault; and (b) the cartilaginous part, or chondrocranium, which forms bones of the base of the skull.

Membranous Neurocranium

The membranous portion of the skull is derived from neural crest cells and paraxial mesoderm as indicated in Figure 8.3. Mesenchyme from these two sources invests the brain and undergoes membranous ossification. The result is formation of a number of flat, membranous bones that are characterized by the presence of needle-like bone spicules. These spicules progressively radiate

Figure 8.3 Skeletal structures of the head and face. Mesenchyme for these structures is derived from neural crest (blue), lateral plate mesoderm (yellow), and paraxial mesoderm (somites and somitomeres) (red).

Grow Neurocranium Bones

Figure 8.4 Skull of a newborn, seen from above (A) and the right side (B). Note the anterior and posterior fontanelles and sutures. The posterior fontanelle closes about 3 months after birth; the anterior fontanelle, about the middle of the second year. Many of the sutures disappear during adult life.

Figure 8.4 Skull of a newborn, seen from above (A) and the right side (B). Note the anterior and posterior fontanelles and sutures. The posterior fontanelle closes about 3 months after birth; the anterior fontanelle, about the middle of the second year. Many of the sutures disappear during adult life.

from primary ossification centers toward the periphery (Fig. 8.2). 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.

Newborn Skull

At birth the flat bones of the skull are separated from each other by narrow seams of connective tissue, the sutures, which are also derived from two sources: neural crest cells (sagittal suture) and paraxial mesoderm (coronal suture). At points where more than two bones meet, sutures are wide and are called fontanelles (Fig. 8.4). 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 (Fig. 8.4B).

Several sutures and fontanelles remain membranous for a considerable time after birth. The bones of the vault continue to grow after birth, mainly because the brain grows. Although a 5- to 7-year-old child has nearly all of its cranial capacity, some sutures remain open until adulthood. In the first few years after birth palpation of the anterior fontanelle may give valuable information

Mastoid Fontanelles

Figure 8.5 Dorsal view of the chondrocranium, or base of the skull, in the adult. On the right side are the various embryonic components participating in formation of the median part of the chondrocranium (blue) and components of the lateral part (red). On the left are the names of the adult structures. 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. Those forming posterior to this landmark arise from paraxial mesoderm (chordal chondrocranium).

Figure 8.5 Dorsal view of the chondrocranium, or base of the skull, in the adult. On the right side are the various embryonic components participating in formation of the median part of the chondrocranium (blue) and components of the lateral part (red). On the left are the names of the adult structures. 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. Those forming posterior to this landmark arise from paraxial mesoderm (chordal chondrocranium).

as to whether ossification of the skull is proceeding normally and whether intracranial pressure is normal.

Cartilaginous Neurocranium or Chondrocranium

The cartilaginous neurocranium or chondrocranium of the skull initially consists of a number of separate cartilages (Fig. 8.5). 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 paraxial mesoderm and form the chordal chondrocranium. The base of the skull is formed when these cartilages fuse and ossify by endochondral ossification.

The base of the occipital bone is formed by the parachordal cartilage and the bodies of three occipital sclerotomes (Fig. 8.5). Rostral to the occipital base plate are the hypophyseal cartilages and trabeculae cranii. These cartilages soon fuse to form the body of the sphenoid and ethmoid, respectively In this manner an elongated median plate of cartilage extending from the nasal region to the anterior border of the foramen magnum forms.

A number of other mesenchymal condensations arise on either side of the median plate. The most rostral, the ala orbitalis, forms the lesser wing of the sphenoid bone. Caudally it is followed by the ala temporalis, which gives rise to the greater wing of the sphenoid. A third component, the periotic capsule, gives rise to the petrous and mastoid parts of the temporal bone. These components later fuse with the median plate and with each other, except for openings through which cranial nerves leave the skull (Fig. 8.5).

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Responses

  • gordon
    What bones are separated by the sagittal suture?
    8 years ago
  • lotta suomalainen
    When s the bone developed embrologically?
    9 months ago

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