Early emergence of axons and formation of accessory nerve (XI) in avian embryos

Development of the nervous system involves specification of distinct classes of neurons at defined locations within the central nervous system (CNS). In the head–trunk-transitory region accessory nerve (XI) display a unique axonal trajectory in that they ascend along the lateral margin of the spinal cord and the medulla oblongata. At present, we lack a detailed description of early emergence and development of the accessory nerve (XI) in avian embryos. To know the early projection of accessory axons from the central nervous system (CNS) and formation of accessory nerve (XI), whole-mount immunostaining of chick embryos was performed. Our results showed that axons start to project from the CNS as early as Hamilton and Hamburger (HH) stage 18 at the level of occipital somites (1 to 5). In the succeeding developmental stages, the axons developed, united with each other and ran dorsolaterally along the longitudinal axis of the embryo. Finally, it formed a bent near the first occipital somite and passed along with vagus nerve. This study will give us an idea on the topographic anatomy of accessory nerve (XI) during development of chick embryos.