EARLY DEVELOPMENT OF THE PERIPHERAL NERVOUS-SYSTEM IN A LANCELET SPECIES

The developmental pattern of the lancelet (amphioxus) peripheral nervo us system from embryos to larvae has been studied by using wholemount immunostaining and transmission electron microscopy. The peripheral ne rves first appeared on the anterior dorsal surface of the medulla at t he middle neurula stage, when the anterior nerve cord was just closing . A single axon with a large growth cone was the progenitor of each ne rve. 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 topograph ically to the preoptic nerve (the posterior branch of the terminal ner ve) 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 Quatrefag es, appeared in larvae with the mouth and two anterior gill pores. The ir axons were seemingly fasciculated with the efferent axon of the fir st nerve. The second nerve, the most complex one to appear during embr yonic and early larval development, innervated the preoral pit and the buccal region. The third and fourth nerves on the left side also inne rvated the buccal region. The larval innervation patterns in the anter ior region differed from the adult organization, suggesting a segmenta l rearrangement of the nerve supply during development. There was no e vidence to dichotomize the peripheral nerves into cranial and spinal n erves, as exist in vertebrates. These characteristics of the periphera l nervous system in the lancelet indicate that this animal has a rathe r derived or primitive developmental system of peripheral nerves, maki ng the analysis of homology with vertebrates difficult. (C) 1998 Wiley -Liss, Inc.