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Axial Skeleton

The neurocranium is a critical part of the axial skeleton, protecting the brain and sensory structures. It reflects the evolutionary phylogeny common to all vertebrates. Its development involves various components, including chondrocranium replacement bone, prechordal cartilage forming under the brain, and parachordal cartilage forming alongside the notochord. Key structures like the olfactory and optic capsules develop concurrently. Ossification centers arise in the occipital, otic, sphenoid, and ethmoid regions, leading to the formation of critical structures such as the foramen magnum and occipital condyles.

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Axial Skeleton

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  1. Axial Skeleton • Skull • Cephalization • Neurocranium • Protects brain & sensory structures • Phylogeny – common to all vertebrates • Replacement bone

  2. Chondrocranium development • Prechordal cartilage (trabeculae cranii) form • Underneath brain • Anterior to notochord

  3. Prechordal cartilage • Form under forebrain region • Form nasal = olfactory capsules • Form optic capsules in orbit • Form rostrum • Finally expands to become ethmoid plate

  4. Parachordal cartilage • Forms beside and posterior to notochord • Forms floor of hindbrain • Expands to form basal plate • Forms occipital condyles (1-2) and foramen magnum

  5. Sensory capsules • As floor of cranium forms, so do sensory capsules • Olfactory capsule • Otic capsule for inner ear structures • Optic capsule (not attached – outer covering of eyeball)

  6. Plate expansion • Ethmoid plate expands and unites with olfactory capsule • Basal plate expands and unites with otic capsule • Formina remain for nerves & blood vessels • Hypophyseal fenestra remains for pituitary gland

  7. Question • Singular of foramina? • Fenestra?

  8. Ossification Centers • Ethmoid plate, basal plate & sensory capsules ossify • Four ossification centers • Occipital (back), Otic (ear), Sphenoid (floor), Ethmoid (floor)

  9. Occipital Ossification Center • Foramen magnum • Basioccipital • Exoccipital • Supraoccipital • Occipital Condyles

  10. Otic ossification center

  11. Sphenoid ossification center • Basisphenoid • Presphenoid/Sphenethmoid

  12. Ethmoid ossification center • Ethmoid plate • Olfactory capsules • Turbinates in mammals

  13. Neurocranium • Look at Skull & Lower Jaw Components page

  14. Key Points • What is one important trend of the NEUROCRANIUM from the Skull & Lower Jaw Components page?

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