[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Embryology](https://durgical.com/en/embryology/) › [General embryology: weeks 1 to 8](https://durgical.com/en/embryology/#a-general) › Paraxial mesoderm and somites: sclerotome, myotome, dermatome

[Embryology](https://durgical.com/en/embryology/) · [General embryology: weeks 1 to 8](https://durgical.com/en/embryology/#a-general)

# 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)](https://durgical.com/en/anatomy/atlas/) · [C7 vertebra](https://durgical.com/en/anatomy/vertebra-prominens/) · [T12 vertebra](https://durgical.com/en/anatomy/t12/) · [L4 vertebra](https://durgical.com/en/anatomy/lumbar-vertebra-spine-and-pelvis-l4/) · [Ribs (back part)](https://durgical.com/en/anatomy/ribs/) · [Multifidus](https://durgical.com/en/anatomy/multifidus-muscles/) · [Rectus abdominis](https://durgical.com/en/anatomy/rectus-abdominis-muscle/) · [Skin](https://durgical.com/en/anatomy/skin/)

## Key points

- Somite → sclerotome (bone), myotome (muscle), dermatome (dermis).
- Epaxial = back muscles, dorsal rami; hypaxial = body wall and limbs, ventral rami.
- Resegmentation: the spinal nerve passes between vertebrae; 8 cervical nerves, 7 vertebrae.
- Hemivertebra → congenital scoliosis; Klippel–Feil → cervical fusion.
- SHH (notochord) → sclerotome; Wnt (dorsal ectoderm) → dermomyotome.

## 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.

## Memory aid

Each somite is a three-piece 'kit': skeleton at the bottom (sclero), muscle in the middle (myo) and skin on top (dermo); to build the spine, each vertebra takes half a piece from two neighboring kits.

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/embryology/paraxial-mesoderm-and-somites-sclerotome-myotome-dermatome/)

## See it in 3D

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

- [Atlas (C1)](https://durgical.com/en/anatomy/atlas/)
- [C7 vertebra](https://durgical.com/en/anatomy/vertebra-prominens/)
- [T12 vertebra](https://durgical.com/en/anatomy/t12/)
- [L4 vertebra](https://durgical.com/en/anatomy/lumbar-vertebra-spine-and-pelvis-l4/)
- [Ribs (back part)](https://durgical.com/en/anatomy/ribs/)
- [Multifidus](https://durgical.com/en/anatomy/multifidus-muscles/)
- [Rectus abdominis](https://durgical.com/en/anatomy/rectus-abdominis-muscle/)
- [Skin](https://durgical.com/en/anatomy/skin/)

## Related

- [Week 3: gastrulation and the trilaminar disc](https://durgical.com/en/embryology/week-3-gastrulation-and-the-trilaminar-disc/)
- [Primitive streak and notochord (nucleus pulposus, sacrococcygeal teratoma)](https://durgical.com/en/embryology/primitive-streak-and-notochord/)
- [Vertebrae: resegmentation of the sclerotomes](https://durgical.com/en/embryology/vertebrae-resegmentation-of-the-sclerotomes/)
- [Muscles: somites, epaxial and hypaxial](https://durgical.com/en/embryology/muscles-somites-epaxial-and-hypaxial/)
- [Skin, hair, nails and the mammary glands](https://durgical.com/en/embryology/skin-hair-nails-and-the-mammary-glands/)
- [Hemivertebra, congenital scoliosis and Klippel–Feil](https://durgical.com/en/embryology/hemivertebra-congenital-scoliosis-and-klippel-feil/)
- [Germ-layer derivatives: ectoderm, mesoderm, endoderm](https://durgical.com/en/embryology/germ-layer-derivatives-ectoderm-mesoderm-endoderm/)
- [Molecular regulation: Shh, Wnt, FGF, BMP, Hox genes and retinoic acid](https://durgical.com/en/embryology/molecular-regulation-shh-wnt-fgf-bmp-hox-genes-and-retinoic-acid/)

## Related conditions

- [Scoliosis](https://durgical.com/en/conditions/scoliosis/)
- [Torticollis](https://durgical.com/en/conditions/torticollis/)

[← All of embryology](https://durgical.com/en/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](https://medlineplus.gov/genetics/condition/klippel-feil-syndrome/) (MedlinePlus (U.S. National Library of Medicine), 2026); [SHH gene: MedlinePlus Genetics](https://medlineplus.gov/genetics/gene/shh/) (MedlinePlus (U.S. National Library of Medicine), 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/embryology/paraxial-mesoderm-and-somites-sclerotome-myotome-dermatome/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/embryology/paraxial-mesoderm-and-somites-sclerotome-myotome-dermatome/