Occipital somites guide motor axons of the accessory nerve in the avian embryo

• We examined the influence of occipital somites on the axonal pathfinding of the accessory nerve.
• The first occipital somite acts as a “permissive gate”, allowing accessory axons to pass through it but does not attract them.
• The more caudal occipital somites repel the accessory axons.

The accessory nerve (nervus accessorius) displays a unique organization in that its axons ascend along the rostrocaudal axis after exiting the cervical spinal cord and medulla oblongata and thereafter project ventrally into the periphery at the first somite level. Little is known about how this organization is achieved. We have investigated the role of somites in the guidance of motor axons of the accessory nerve using heterotopic transplantations of somites in avian embryos. The formation of not only accessory nerve but also the vagal nerve was affected, when a more caudal occipital somite (somites 2–4) was grafted to the position of the first occipital somite. Our study reveals that only the first occipital somite permits the development of ventral projection of accessory axons, a process that is inhibited by more caudal occipital somites.