Somitic Mesoderm
Skeletal muscles of the neck, torso and limbs come from two major subdivisions of the somitic mesoderm:
- epimere - dorsomedial part of somite
- hypomere - dorsolateral part of somite
The epimere forms the erector spinae group of muscles and the deep muscles of the back, innervated by dorsal primary rami of spinal nerves.
The hypomere forms the remaining muscles of the torso; infrahyoid muscles, intercostal muscles, abdominal wall muscles, thoracoappendicular muscles and muscles of the upper and lower limbs. These muscles are all supplied by ventral primary rami of spinal nerves.
The somatic layer of lateral plate mesoderm forms the skeletal elements of the limbs, tendons.as well as connective tissue septa in the body wall and limbs that divide muscles into functional groups.
A diagram of a typical spinal nerve shows that the muscles of the epimere, i.e. the erector spinae and deep muscles of the back are innervated by the dorsal primary rami of spinal nerves.
The muscles derived from the hypomere are innervated by ventral primary rami of the spinal nerves
- The primaxial domain refers to muscles of the body axis proper (primaxial) derived exclusively from the somite mesoderm, the epimere and part of the hypomere. Muscles derived from the primaxial domain include the erector spinae and related deep back muscles innervated by the dorsal primary rami of spinal nerves but also some of the muscles of the shoulder girdle and intercostal muscles, innervated by ventral primary rami of spinal nerves.
- The abaxial domain refers to muscles of the appendages, not part of the body axis (abaxial) derived from mesoderm of the hypomere, infrahyoid muscles, abdominal wall muscles and the limb muscles. These muscles are all innervated by the ventral primary rami of spinal nerves.
To be honest, I don't see the value of this distinction in terms of anatomical understanding of the muscles or its clinical significance and prefer to use the classical definitions. I include it here because several standard texts have introduced the concept, though without universal agreement on what's what.
Molecular Regulation
The dorsomedial portion of the myotome, the epimere, develops the deep muscles of the back, the erector spinae, by the inductive influence of Wnt proteins from the dorsal part of the neural tube,
Wnt proteins from the surface ectoderm and BMP4 from the lateral plate mesoderm induces the ventrolateral portion of the myotome, the hypomere, to develop muscles of the torso, such as the intercostals and abdominal musculature and musculature of the upper and lower limbs.
Mesoderm that migrates into the limb bud to form the muscles aggregates into two masses.
One mass is located posterior (dorsally) in the limb and the other is located (anterior) ventrally.
The posterior muscle mass will give rise to the posterior muscles of the limb, which in turn are innervated by nerves containing fibers from the posterior divisions of their respective nerve plexuses.
The anterior muscle mass will give rise to the anterior muscles of the limb, that are innervated by nerves containing fibers from anterior divisions of the nerve plexuses that innervate them.
Gradients of Wnt signaling from the posterior surface ectoderm and sonic hedge hog from ZPA guide the development of the posterior and anterior muscle masses.
In the upper limb the posterior muscle mass generally produces the extensors supinators and abductors. They are mostly supplied by nerve branches of the posterior divisions of the brachial plexus, that form the posterior cord and include the axillary and radial nerves.
In the upper limb the anterior muscle mass generally produces the flexors, pronators and adductors. These muscles are supplied by nerve branches of the anterior divisions of the brachial plexus that form the medial and lateral cords and include the musculocutaneous, ulnar and median nerves.
Gross Anatomy Correlates
Myotomes
Myotome refers to specific spinal nerves innervating any given muscle. Every muscle is innervated by two or more consecutive spinal nerves.
In both upper and lower limbs, more cranial spinal nerves in a plexus innervate proximal muscles and caudal spinal nerves in a plexus the distal muscles (myotomes).
Dermatomes
The dermatome of each somite contributes to the CT of each particular body segment, and the sensory innervation of the associated skin segment is also called a dermatome. Dermatome charts indicate the segmental distribution of spinal nerves.
Dermatome segments overlap slightly with the assigned spinal nerve making the major contribution. For example, the belly button or umbilicus is in the dermatome innervated by T10.
In the upper limb, dermatomes of the hand are C6 for the thumb, C7 for the middle finger and C8 for the little finger.
In the lower limb, dermatomes of the foot are L4 for the great toe, L5 for the middle of the foot and S1 for the lateral side of the foot.
Dermatomes, are aligned in a cranial to caudal sequence the entire length of the limb, with the preaxial side of the limb being innervated by more cranial spinal nerves and the postaxial side innervated by the more caudal spinal nerves.
Knowledge of dermatome & myotome distribution is useful in evaluating sensory and motor deficits that may accompany specific nerve injuries.
Rotation of the LImbs
Final positioning of the limbs is the result of rotation of each limb, in different directions. The upper limb undergoes a slight degree of lateral rotation (yellow arrows) so that the thumb points laterally and the original ventral surface (i.e. flexor surface) of the limb still faces anteriorly.
The lower limb undergoes a great degree of medial rotation (yellow arrows). As a result, the great toe moves medially and the original dorsal surface (i.e. extensor surface) faces anteriorly.
Clinical Correlates
Deformations vs Amputations
Anomalies of the limbs are relatively common and are grouped into two major classes: amputations and disruptions.
Amputations may occur due to mechanical trauma from entanglement in amniotic bands or umbilical cord, resulting in loss of a part of the limb.
Disruptions are interruptions in the developmental process related to genetic or environmental (teratogenic) causes.
Limb Deformities
Partial absence of a limb (meromelia) or complete absence of a limb (amelia) are seen.
In some cases, all segments of the limb may be present, but the limb is abnormally short (micromelia).
Phocomelia is a form of meromelia in which the long bones are missing and rudimentary hands or feet are connected to the trunk by irregular and misshapen versions of girdle bones.
Though naturally occurring instances of phocomelia due to genetic causes are rare, it is also known to be a teratogenic consequence of thalidomide, a drug that was prescribed as a sleep aid and anti nausea drug for pregnant women in the UK in the late 1950's.
Case courtesy of Dr Jayanth Keshavamurthy, Radiopaedia.org. From the case rID: 33175
Ulnar Dimelia
The role of ZPA in determining the post axial development of the limb has been studied in chick embryos. Transplantation of cells from the ZPA from the post axial side of the limb bud to the preaxial side of the limb, results in a limb that is a mirror image of post axial limb structures.
Ulnar dimelia is a naturally occurring example of this in humans in which duplication of the ZPA results in a forearm with two ulnae and a hand that is a mirror image of itself.
Case courtesy of Dr Jeremy Jones, Radiopaedia.org. From the case rID: 24352