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Embryology · Skeleton, muscles, limbs and skin

Vertebrae: resegmentation of the sclerotomes !

  • Process

Each vertebra is built from half of one embryonic block (somite) and half of the next, which is why muscles and nerves end up where they do.

Updated 2026-11-01

See it in 3D: Atlas (C1) · Axis (C2) · L4–L5 disc · L4–L5 nucleus · L4 vertebra · L5 vertebra · T6 vertebra · Sacrum · Coccyx

What it is

Vertebrae come from the sclerotome, the part of each somite that migrates around the neural tube and notochord in week 4. Each sclerotome splits into a cranial and a caudal half, and the caudal half of one fuses with the cranial half of the next: this is resegmentation. It explains why segmental muscles cross the discs and can move the spine, and why there are eight cervical nerves but only seven cervical vertebrae.

Key facts

  • Week 4: sclerotome cells surround the notochord (future body) and the neural tube (future arch).
  • Resegmentation: definitive vertebra = caudal half of one sclerotome + cranial half of the next.
  • Cells left between the two halves form the annulus fibrosus of the intervertebral disc.
  • The notochord disappears inside the vertebral bodies and persists within the disc as the nucleus pulposus.
  • Intersegmental arteries end up beside the vertebral bodies, and spinal nerves exit at the level of the discs.
  • HOX genes give each vertebra its regional identity (cervical, thoracic, lumbar); the occipital somites contribute to the skull base.

Structure

  • Vertebral body: sclerotome around the notochord.
  • Vertebral arch: sclerotome around the neural tube; if its two halves fail to close, spina bifida results.
  • Costal processes: in the thoracic region they grow out to form the bony part of the ribs.
  • Intervertebral disc: nucleus pulposus (notochord) inside the annulus fibrosus (sclerotome).

Step by step

  1. Days 20 to 30: paraxial mesoderm segments into paired somites, cranial to caudal, paced by a Notch-driven clock.
  2. The notochord and neural tube floor plate release SHH and noggin: the ventromedial somite becomes sclerotome (PAX1).
  3. Week 4: the sclerotome migrates and wraps around the notochord and neural tube.
  4. Each sclerotome splits into a loose cranial half and a dense caudal half.
  5. The caudal half of one segment fuses with the cranial half of the next to form the precartilaginous vertebral body.
  6. The myotome now bridges the disc and links two neighbouring vertebrae; the spinal nerve runs between them.
  7. Chondrification centres appear, then ossification; the arches fuse with the body after birth.

Markers

  • SHH
  • Noggin
  • PAX1
  • PAX9
  • Notch
  • DLL3
  • HOX

Clinical notes

  • If half a vertebra is missing (hemivertebra) or two segments fail to separate (block vertebra), congenital scoliosis follows.
  • Klippel–Feil: congenital fusion of cervical vertebrae from failed segmentation.
  • Spina bifida occulta: the vertebral arches fail to fuse in the midline; usually an incidental, symptom-free X-ray finding.
  • Segmentation-clock genes (Notch pathway, e.g. DLL3) cause spondylocostal dysostosis: fused vertebrae and ribs.

Test yourself

Think of the answer, then tap the question to see it.

Which embryonic structure gives rise to the nucleus pulposus?

The notochord, which disappears in the vertebral bodies but persists inside the intervertebral disc.

What is sclerotome resegmentation?

The fusion of the caudal half of one sclerotome with the cranial half of the next to form each definitive vertebra.

Why are there eight cervical nerves but only seven cervical vertebrae?

After resegmentation the first cervical nerve exits between the occipital bone and the atlas, and the eighth exits between C7 and T1.

Which developmental failure produces a hemivertebra?

Failure of formation of one lateral half of the vertebral body; a classic cause of congenital scoliosis.

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Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.

Sources: Spondylocostal dysostosis: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Klippel-Feil syndrome: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Langman's Medical Embryology, 13th ed., ch. 10: The Axial Skeleton (Wolters Kluwer, 2015).

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