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The axial skeleton consists of the:
  • skull
  • vertebral column
  • hyoid bone
  • ribs
  • sternum

It is derived from both
paraxial (somitic) mesoderm and neural crest sources.

The
neural crest is by way of the branchial arches and it contributes to the development of the anterior parts of the skull and hyoid bone while the paraxial mesoderm forms masses of tissue called somites that contribute to the posterior parts of the skull and the vertebral column.

The Skull

  • Overview

    The skull can be divided into two parts:
    Viscerocranium - skeleton of the face
    Neurocranium - portion of the skull forming the cranial vault

  • Viscerocranium
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    The viscerocranium consists of the bones that form the skeleton of the face and lower jaw and hyoid. They are derived from the mesenchyme of the first three branchial arches. The first branchial arch divides into a maxillary process and a mandibular process. The second branchial arch is called the hyoid arch and it, together with the third branchial arch form the hyoid bone. The mesenchyme of first three branchial arches is of neural crest origin. In addition to formation of the bones of the face, the first two arches also give rise to the three pairs of ossicles of the middle ear cavity, the malleus, incus and stapes.

  • Chondrocranium

    he neurocranium is the part of the skull that surrounds and protects the brain. The base of the neurocranium consists of bones that form by endochondral ossification and which are derived from mesenchyme of both neural crest and somitic origin. This part of the neurocranium is called the chondrocranium.

    Those of
    neural crest origin include the ethmoid bones, greater and lesser wings of the sphenoid bone and the anterior part of the sella turcica.

    Those derived from
    somatic origin, originate in the sclerotomes of occipital somites and include the posterior part of the sella turcica, body of the sphenoid, base of the occipital bone and petrous part of the temporal bone.

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  • Membranous Neurocranium

    The membranous neurocranium forms by intramembranous ossification of several flat bones. Those located more anteriorly in the skull such as the frontal bone and squamous part of the temporal bones are from neural crest origin, while those more posterior such as the parietal bones and upper part of the occipital bone are from somitic mesoderm.

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  • Fontanelles
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    At birth, the flat bones of the skull are separated by connective tissue, which will eventually form the sutures of the skull. The fontanelles are located where more than two bones meet and are generally wide. The fact that the sutures are not completely ossified at birth, allows the bones of the skull to overlap during birth. After birth, the bones move back to their original position. The cranium of the neonate is about 4/5 of the skull and the face about 1/5.
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    The anterior fontanelle is the largest and is often referred to in lay terms as the “soft spot” on the baby’s head. Palpation of the anterior fontanelle can provide information about the normal progress of ossification of the skull and whether intracranial pressure is normal. The posterior fontanelle generally closes around the second month after birth while the larger anterior fontanelle closes around the 18th month post partum.

    At around 5 to 7 years old, the child has nearly all of the cranial volume and subsequent growth of the skull is due to increase in the size of the facial skeleton. In adults, the facial skeleton and the cranial vault are each about 50% of the skull volume.

The Vertebral Column

  • Overview

    The vertebral column develops from the sclerotome portion of masses of paraxial mesoderm called somites. The sclerotome cells surround the developing neural tube and notochord to begin endochordral ossification to form the specific components of the vertebrae; the vertebral body, vertebral arch (pedicles & laminae), surrounding the vertebral foramen, spinous process and transverse process.

    In addition to the vertebrae, the sclerotome gives rise to the annulus fibrosis portions of the intervertebral discs.

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  • Somites
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    Somites begin to appear in the midline of the embryo around the 20th day of development. They are on both sides of the midline, on either side of the neural tube and consisting of condensations of paraxial mesoderm. By day 30 of development, 44 pairs of somites can be identified. These are named according to their location.

    • 4 occipital
    • 8 cervical
    • 12 thoracic
    • 5 lumbar
    • 5 sacral
    • 10 coccygeal
    Many of the coccygeal somites disappear while the occipital somites contribute to skull and head structures. The others differentiate into components of the vertebral column, muscles and dermis of the skin.

  • Subdivisions of the Somite
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    The ventromedial part of each somite is induced to become a sclerotome by factors secreted by the ventral neural tube and notochord. The sclerotomes will form the vertebrae and intervertebral discs.
    Factors secreted by the dorsal neural tube induce the
    dorsomedial part of the somite to become the epimere which will develop into the epaxial musculature, ie. deep muscles of the back (erector spinae).
    Factors produced by the
    surface ectoderm and the somatic layer of the lateral plate mesoderm induce the formation of the hypomere from the lateral ventral part of the myotome.
    The dorsal part of the neural tube induces the formation of the
    dermatome to form the dermis of the skin in each specific body segment.

  • Vertebrae

    Developing spinal nerves that grow from the neural tube pass through the cranial part of the sclerotome to reach their developing muscle masses. The sclerotomes divide into cranial and caudal parts at the place where the nerves are passing through to create intervertebral foramina.

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    Vertebral bodies form by the fusion of the caudal end of one sclerotome with the cranial end of the sclerotome caudal to it.