Development of the Back
- Vertebral bodies and arches: Vertebral bodies and arches develop from sclerotomes of somites. Sclerotome cells surround the notochord and neural tube. The developmental relationship explains important adult anatomic connections.
- Somites are segmented embryonic tissue blocks, sclerotomes are the parts that form vertebrae and ribs, the notochord helps pattern the axial skeleton, and the neural tube forms the central nervous system.
- Each vertebral body: Each vertebral body forms from the caudal half of one sclerotome and cranial half of the next, allowing spinal nerves to pass between vertebrae. This relationship helps localize neurologic deficits after injury.
- This resegmentation shifts vertebral bodies relative to spinal nerves, so spinal nerves exit between adjacent vertebrae rather than through the middle of a vertebral body.
- Development of the Back: The notochord persists in the nucleus pulposus. The developmental relationship explains important adult anatomic connections.
- Development of the Back: The spinal cord develops from neural tube. Neural-crest cells form dorsal root ganglia and autonomic ganglia. The developmental relationship explains important adult anatomic connections.
- Development of the Back: The spinal cord initially extends to the sacrum but grows more slowly than the vertebral column, producing the conus, cauda equina, and filum terminale. This relationship is an important landmark for clinical anatomy.
- Because the vertebral column outgrows the spinal cord, lower spinal nerve roots must descend inside the canal before exiting, forming the cauda equina below the conus.
- Meninges: Meninges develop around the neural tube. Failure of neural-tube or vertebral-arch closure can cause spina bifida. The developmental relationship explains important adult anatomic connections.
- Development of the Back: Intrinsic back muscles derive from epimere portions of somites and retain dorsal-ramus innervation. Superficial extrinsic muscles derive mainly from migratory muscle masses and receive ventral- or cranial-nerve supply. This relationship helps localize neurologic deficits after injury.
- The developmental rule is: intrinsic back muscles stay in the back and keep dorsal-ramus innervation, while many extrinsic muscles migrate into the back and keep the nerves of their original functional group.
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