Myotomes

Origin of Myotomes

Each myotome originates from an early embryological structure known as a somite. These are paired blocks of mesoderm that give rise to skeletal muscle, bone, and dermis.

The development of skeletal muscle can be traced to the formation of these somites. By approximately day 20, the trilaminar disc has formed, and the mesoderm has differentiated into several regions. The area immediately adjacent to the neural tube is known as the paraxial mesoderm.

From around day 20, the paraxial mesoderm begins to segment into paired blocks called somites. 44 pairs of somites initially form, but several regress to leave 31 pairs (these correspond to the 31 pairs of spinal nerves in the adult).

From day 20 onwards the paraxial mesoderm begins to differentiate further into segments known as somites. 44 pairs of somites are formed, however some of these regress until 31 pairs remain, corresponding to 31 pairs of spinal nerves in the adult.

Each somite differentiates into two main components:

• Ventromedial sclerotome –  gives rise to the vertebrae and ribs.
• Dorsolateral dermomyotome – forms the dermis and skeletal muscle.

As development continues, the myotome portion of each somite proliferates and differentiates into skeletal muscle fibres.

By TeachMeSeries Ltd (2025)

Fig 1
Somites adjacent to the neural tube.

Distribution of Myotomes

Most muscles in the upper and lower limbs receive innervation from more than one spinal nerve root. They are therefore comprised of multiple myotomes.

For example, the biceps brachii muscle performs flexion at the elbow. It is innervated by the musculocutaneous nerve, which is derived from C5-7 nerve roots. All three of these spinal nerve roots can be said to be associated with elbow flexion.

The table below details which movement is most associated with each myotome: