Axial Skeleton: Morphogenesis
The vertebrae, ribs, and sternum all arise by different signaling mechanisms.
The vertebrae begin to develop with a sonic hedgehog (shh) mediated induction by the notochord. This continued signaling causes the ventromedial portion of the somite to form the centrum. The ventral portion of the vertebrae does not receive the shh signaling from the notochord, however, the roof plate of the developing neural tube induces the adjacent scleratome to express Pax-9 and Msx-2, which cause formation of the neural arch. [A] In the absence of this signalling, the neural arches are incomplete, resulting in local exposure of the spinal cord, which is usually covered with skin and hair and can go unnoticed for many years in mild cases [A]
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The vertebrae in each region of the spinal column are designed a little differently. This specialization is controlled by combining expression of thirteen differentHox genes, which organize the embryo along the anterior posterior axis. Retanoic acid plays a role in the definition of this craniocaudal organization: if administered too early, it can cause a cranial level shift (the last cervical vertebrae become thoracic) and if administered too late, causes a caudal shift (thoracic vertebrae become lumbar, etc.). This follows with the general anterior to posterior development that is observed in other parts of the embryo. |
The ribs arise from condensations of mesenchymal cells lateral to the forming vertebrae and are also under the control of Hox genes. They separate from the vertebrae at the point of vertebral ossification. [A]
The sternum, which appears in land
dwelling tetrapods and allows the negative pressure in the pleural cavity
necessary for the expansion of the lungs, arises first as cartilage in bands
across the ventral midline of the forming body. These segments fuse during
ossification to form the sternum. [A]
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