[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Embryology](https://durgical.com/en/embryology/) › [Skeleton, muscles, limbs and skin](https://durgical.com/en/embryology/#a-musculoskeletal) › Muscles: somites, epaxial and hypaxial [Embryology](https://durgical.com/en/embryology/) · [Skeleton, muscles, limbs and skin](https://durgical.com/en/embryology/#a-musculoskeletal) # 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](https://durgical.com/en/anatomy/iliocostalis-muscles/) · [Longissimus](https://durgical.com/en/anatomy/longissimus-muscles/) · [Spinalis](https://durgical.com/en/anatomy/spinalis-muscles/) · [Multifidus](https://durgical.com/en/anatomy/multifidus-muscles/) · [Rectus abdominis](https://durgical.com/en/anatomy/rectus-abdominis-muscle/) · [External oblique](https://durgical.com/en/anatomy/external-abdominal-oblique-muscle/) · [Internal oblique](https://durgical.com/en/anatomy/internal-abdominal-oblique-muscle/) · [Transversus abdominis](https://durgical.com/en/anatomy/transversus-abdominis-muscle/) · [External intercostals](https://durgical.com/en/anatomy/external-intercostal-muscles/) · [Pectoralis major](https://durgical.com/en/anatomy/pectoralis-major-muscle/) · [Tongue](https://durgical.com/en/anatomy/tongue/) · [Diaphragm](https://durgical.com/en/anatomy/diaphragm/) ## Key points - Epaxial = deep back muscles = dorsal ramus. - Hypaxial = trunk wall and limbs = ventral ramus. - Tongue muscle = occipital somites (hypoglossal). - Iris muscles = ectoderm (optic cup), not mesoderm. - Connective tissue, not the myoblast, sets the shape and pattern of the muscle. ## 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. ## Memory aid 'Behind the spine, behind ramus': whatever sits behind the vertebral column is epaxial and gets the dorsal ramus; everything else rides with the ventral ramus. ## 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. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/embryology/muscles-somites-epaxial-and-hypaxial/) ## See it in 3D The structures of Durgical's 3D anatomy where it is found. - [Iliocostalis](https://durgical.com/en/anatomy/iliocostalis-muscles/) - [Longissimus](https://durgical.com/en/anatomy/longissimus-muscles/) - [Spinalis](https://durgical.com/en/anatomy/spinalis-muscles/) - [Multifidus](https://durgical.com/en/anatomy/multifidus-muscles/) - [Rectus abdominis](https://durgical.com/en/anatomy/rectus-abdominis-muscle/) - [External oblique](https://durgical.com/en/anatomy/external-abdominal-oblique-muscle/) - [Internal oblique](https://durgical.com/en/anatomy/internal-abdominal-oblique-muscle/) - [Transversus abdominis](https://durgical.com/en/anatomy/transversus-abdominis-muscle/) - [External intercostals](https://durgical.com/en/anatomy/external-intercostal-muscles/) - [Pectoralis major](https://durgical.com/en/anatomy/pectoralis-major-muscle/) - [Tongue](https://durgical.com/en/anatomy/tongue/) - [Diaphragm](https://durgical.com/en/anatomy/diaphragm/) ## Related - [Paraxial mesoderm and somites: sclerotome, myotome, dermatome](https://durgical.com/en/embryology/paraxial-mesoderm-and-somites-sclerotome-myotome-dermatome/) - [Limb development: the apical ectodermal ridge, ZPA and limb rotation](https://durgical.com/en/embryology/limb-development-the-apical-ectodermal-ridge-zpa-and-limb-rotation/) - [Vertebrae: resegmentation of the sclerotomes](https://durgical.com/en/embryology/vertebrae-resegmentation-of-the-sclerotomes/) - [Poland sequence and prune belly syndrome](https://durgical.com/en/embryology/poland-sequence-and-prune-belly-syndrome/) - [Pharyngeal arches: nerve, muscles, skeleton and artery (the table)](https://durgical.com/en/embryology/pharyngeal-arches-nerve-muscles-skeleton-and-artery/) - [Diaphragm: its four components and its innervation](https://durgical.com/en/embryology/diaphragm-its-four-components-and-its-innervation/) - [Tongue: development and innervation](https://durgical.com/en/embryology/tongue-development-and-innervation/) ## Related conditions - [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: [Poland syndrome: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/poland-syndrome/) (MedlinePlus, US National Library of Medicine, 2026); [Prune belly syndrome: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/001269.htm) (MedlinePlus, US National Library of Medicine, 2026); Langman's Medical Embryology, 13th ed., ch. 11: Muscular System (Wolters Kluwer, 2015). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/embryology/muscles-somites-epaxial-and-hypaxial/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/embryology/muscles-somites-epaxial-and-hypaxial/