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Embryology · General embryology: weeks 1 to 8

Paraxial mesoderm and somites: sclerotome, myotome, dermatome !

  • Embryonic structure

Blocks of mesoderm on each side of the neural tube that build the vertebrae, trunk muscles and the dermis of the back.

Updated 2026-11-01

See it in 3D: Atlas (C1) · C7 vertebra · T12 vertebra · L4 vertebra · Ribs (back part) · Multifidus · Rectus abdominis · Skin

What it is

Paraxial mesoderm, beside the neural tube, segments into paired blocks called somites from day 20, cranial to caudal, about three pairs a day. Each somite splits into sclerotome (vertebrae and ribs), myotome (skeletal muscle) and dermatome (dermis of the back). This segmentation explains the level-by-level layout of the spine, the spinal nerves and skin innervation.

Key facts

  • Day 20: the first pair of somites appears in the occipital region.
  • 42–44 pairs form in all; the last coccygeal ones and the first occipital pair disappear.
  • Somite count is used to estimate embryonic age in weeks 4–5.
  • A segmentation clock (oscillating Notch, Wnt and FGF) sets each somite boundary.
  • The sclerotome forms under SHH from the notochord and floor plate (PAX1).
  • Resegmentation: each vertebra forms from the caudal half of one sclerotome and the cranial half of the next; hence 8 cervical nerves and 7 vertebrae.

Structure

  • Somitomeres: whorls of paraxial mesoderm that precede somites; in the head they never fully segment.
  • Each somite is an epithelial ball that then opens: a ventromedial part (sclerotome) and a dorsolateral part (dermomyotome).

What it becomes

  • Sclerotome: vertebral bodies and arches, ribs, the disc's annulus fibrosus and part of the skull base.
  • Epaxial myotome: deep back muscles (supplied by dorsal rami).
  • Hypaxial myotome: trunk wall and limb muscles (ventral rami).
  • Dermatome: dermis and subcutaneous tissue of the neck and back.

Markers

  • PAX1
  • PAX3
  • MYF5
  • Notch
  • SHH
  • HOX

Clinical notes

  • Hemivertebra: half of a sclerotome is missing; causes congenital scoliosis.
  • Klippel–Feil syndrome: fused cervical vertebrae; short neck and limited motion.
  • Spondylocostal dysostosis: segmentation-clock (Notch pathway) errors with disordered vertebrae and ribs.
  • The adult dermatome map (skin supplied by one spinal nerve) reflects this embryonic segmentation.
  • Spina bifida occulta: the sclerotome-derived vertebral arches fail to fuse.

Test yourself

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

Into which three parts does a somite divide, and what does each form?

Sclerotome (vertebrae and ribs), myotome (skeletal muscle) and dermatome (dermis of the back).

Why are there 8 cervical nerves but only 7 cervical vertebrae?

Resegmentation: each vertebra forms from halves of two neighboring sclerotomes, leaving the first nerve above the first vertebra.

Which vertebral malformation causes congenital scoliosis?

Hemivertebra, from failure to form half of a sclerotome.

Which muscles come from the epaxial myotome, and which nerves supply them?

The deep back muscles (erector spinae), supplied by the dorsal rami of the spinal nerves.

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The structures of Durgical's 3D anatomy where it is found.

← All of embryology

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: Langman's Medical Embryology, 13th ed., ch. 6: Third to Eighth Weeks: The Embryonic Period (Wolters Kluwer, 2015); Klippel-Feil syndrome: MedlinePlus Genetics (MedlinePlus (U.S. National Library of Medicine), 2026); SHH gene: MedlinePlus Genetics (MedlinePlus (U.S. National Library of Medicine), 2026).

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