Early development of the peripheral nervous system in a lancelet species.

The developmental pattern of the lancelet (amphioxus) peripheral nervous system from embryos to larvae has been studied by using wholemount immunostaining and transmission electron microscopy. The peripheral nerves first appeared on the anterior dorsal surface of the medulla at the middle neurula stage, when the anterior nerve cord was just closing. A single axon with a large growth cone was the progenitor of each nerve. The nerve roots adopted an asymmetric arrangement soon after. The first nerve, likely a pair of pure sensory nerves, sprouted from the anterior tip of the nerve cord. This nerve may be comparable topographically to the preoptic nerve (the posterior branch of the terminal nerve) in lungfishes. However, the neuron that first extends its axon was located in the medulla, as in the other posterior nerves. One of the extramedullary primary sensory neurons, the corpuscles of de Quatrefages, appeared in larvae with the mouth and two anterior gill pores. Their axons were seemingly fasciculated with the efferent axon of the first nerve. The second nerve, the most complex one to appear during embryonic and early larval development, innervated the preoral pit and the buccal region. The third and fourth nerves on the left side also innervated the buccal region. The larval innervation patterns in the anterior region differed from the adult organization, suggesting a segmental rearrangement of the nerve supply during development. There was no evidence to dichotomize the peripheral nerves into cranial and spinal nerves, as exist in vertebrates. These characteristics of the peripheral nervous system in the lancelet indicate that this animal has a rather derived or primitive developmental system of peripheral nerves, making the analysis of homology with vertebrates difficult.