Embryology · Skeleton, muscles, limbs and skin
Muscles: somites, epaxial and hypaxial !
- Embryonic structure
Almost all skeletal muscle comes from the somites: the back part makes the back muscles, the front part makes the body wall and the limbs.
Updated 2026-11-01
See it in 3D: Iliocostalis · Longissimus · Spinalis · Multifidus · Rectus abdominis · External oblique · Internal oblique · Transversus abdominis · External intercostals · Pectoralis major · Tongue · Diaphragm
What it is
Skeletal muscle of the trunk and limbs comes from the myotome of each somite; head muscle comes from the somitomeres and the mesoderm of the pharyngeal arches. Each myotome divides into a dorsal (epaxial) part that forms the true back muscles and a ventral (hypaxial) part that forms the trunk wall and the limbs. This split explains their nerve supply: dorsal rami for the back, ventral rami for everything else.
Key facts
- Skeletal muscle: paraxial mesoderm (somites and somitomeres).
- Visceral smooth muscle: splanchnic mesoderm; cardiac muscle: splanchnic mesoderm of the cardiogenic field.
- Ectodermal exception: the sphincter and dilator pupillae, and myoepithelial cells of glands.
- Week 7: the first muscle condensation appears in the limb buds.
- End of the third month: the typical cross-striations of skeletal muscle appear.
- Each muscle's shape is set by the connective tissue the myoblasts migrate into (neural crest in the head, lateral plate mesoderm in the limbs).
Structure
- Epaxial (dorsal): erector spinae, multifidus and the other deep back muscles; dorsal ramus of the spinal nerve.
- Hypaxial (ventral): intercostals, obliques, transversus and rectus abdominis, scalenes, infrahyoids and limb muscles; ventral ramus.
- Tongue: myoblasts from the occipital somites, hence the hypoglossal nerve.
- Pharyngeal arch muscles (mastication, facial expression, pharynx, larynx): somitomeres, with the nerve of their arch.
What it becomes
- Myoblasts fuse into multinucleated fibres; some remain as satellite cells to repair muscle.
- A sequence of myogenic factors (MYF5, MYOD, myogenin) turns PAX3/PAX7 progenitors into fibres.
- The dorsal and ventral masses of each limb give the extensors and flexors.
- The myotome drags its nerve along: a muscle keeps the nerve of the segment where it was born, even after migrating far (e.g. the diaphragm with C3–C5).
Markers
- PAX3
- PAX7
- MYF5
- MYOD
- Myogenin
- WNT
- BMP4
Clinical notes
- Partial absence of a muscle is common and usually symptom-free (palmaris longus, for example).
- Poland sequence: the sternal head of pectoralis major is missing, with a small hand and joined fingers on the same side.
- Prune belly syndrome: deficient abdominal wall muscles (hypaxial).
- Congenital torticollis: fibrosis or shortening of sternocleidomastoid; associated with hip dysplasia.
Test yourself
Think of the answer, then tap the question to see it.
Which branch of the spinal nerve supplies the epaxial muscles?
The dorsal (posterior) primary ramus; hypaxial muscles are supplied by the ventral ramus.
Where do the muscles of the tongue come from?
From occipital somite myoblasts that migrate into the tongue, which is why the hypoglossal nerve supplies them.
Which muscles are of ectodermal origin?
The sphincter and dilator pupillae, derived from the optic cup, and the myoepithelial cells of glands.
Are limb muscles epaxial or hypaxial?
Hypaxial: they come from the ventral part of the myotome and are supplied by ventral rami.
See it in 3D
The structures of Durgical's 3D anatomy where it is found.
- Iliocostalis
- Longissimus
- Spinalis
- Multifidus
- Rectus abdominis
- External oblique
- Internal oblique
- Transversus abdominis
- External intercostals
- Pectoralis major
- Tongue
- Diaphragm
Related
- Paraxial mesoderm and somites: sclerotome, myotome, dermatome
- Limb development: the apical ectodermal ridge, ZPA and limb rotation
- Vertebrae: resegmentation of the sclerotomes
- Poland sequence and prune belly syndrome
- Pharyngeal arches: nerve, muscles, skeleton and artery (the table)
- Diaphragm: its four components and its innervation
- Tongue: development and innervation
Related conditions
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: Poland syndrome: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Prune belly syndrome: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Langman's Medical Embryology, 13th ed., ch. 11: Muscular System (Wolters Kluwer, 2015).
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Representative model, for educational purposes only.