The development and organization of the nervous systems of echinoderm larvae are incompletely described. We describe the development and organization of the larval nervous systems of Strongylocentrotus purpuratus and Asterina pectinifera using a novel antibody, 1E11, that appears to be neuron specific. In the early pluteus, the antibody reveals all known neural structures: apical ganglion, oral ganglia, lateral ganglia, and an array of neurons and neurites in the ciliary band, the esophagus, and the intestine. The antibody also reveals several novel features, such as neurites that extend to the posterior end of the larva and additional neurons in the apical ganglion. Similarly, in asteroid larvae the antibody binds to all known neural structures and identifies novel features, including large numbers of neurons in the ciliary bands, a network of neurites under the oral epidermis, cell bodies in the esophagus, and a network of neurites in the intestine. The 1E11 antigen is expressed during gastrulation and can be used to trace the ontogenies of the nervous systems. In S. purpuratus, a small number of neuroblasts arise in the oral ectoderm in late gastrulae. The cells are adjacent to the presumptive ciliary bands, where they project neurites with growth cone-like endings that interconnect the neurons. In A. pectinifera, a large number of neuroblasts appear scattered throughout the ectoderm of gastrulae. The cells aggregate in the developing ciliary bands and then project neurites under the oral epidermis. Although there are several shared features of the larval nervous systems of echinoids and asteroids, the patterns of development reveal fundamental differences in neural ontogeny.

The anatomy and cellular organization of serotonergic neurons in the echinoderm apical organ exhibits class-specific features in dipleurula-type (auricularia, bipinnaria) and pluteus-type (ophiopluteus, echinopluteus) larvae. The apical organ forms in association with anterior ciliary structures. Apical organs in dipleurula-type larvae are more similar to each other than to those in either of the pluteus forms. In asteroid bipinnaria and holothuroid auricularia the apical organ spans ciliary band sectors that traverse the anterior-most end of the larvae. The asteroid apical organ also has prominent bilateral ganglia that connect with an apical network of neurites. The simple apical organ of the auricularia is similar to that in the hemichordate tornaria larva. Apical organs in pluteus forms differ markedly. The echinopluteus apical organ is a single structure on the oral hood between the larval arms comprised of two groups of cells joined by a commissure and its cell bodies do not reside in the ciliary band. Ophioplutei have a pair of lateral ganglia associated with the ciliary band of larval arms that may be the ophiuroid apical organ. Comparative anatomy of the serotonergic nervous systems in the dipleurula-type larvae of the Ambulacraria (Echinodermata+Hemichordata) suggests that the apical organ of this deuterostome clade originated as a simple bilaterally symmetric nerve plexus spanning ciliary band sectors at the anterior end of the larva. From this structure, the apical organ has been independently modified in association with the evolution of class-specific larval forms.

Interest in chordate evolution has emphasized a need for a better understanding of the comparative neuroanatomy of invertebrate deuterostomes. However, molecular and genetic approaches to neurobiological studies in these groups are hampered by a lack of neuron-specific molecular markers. A monoclonal antibody, 1E11, is neuron specific and is useful in identification of neural structures in larvae and adults of echinoderms, hemichordates, and urochordates. To identify a neuron-specific gene product, we have characterized the antigen recognized by 1E11. In immunoblots and immunoprecipitations of neural tissue from adult Strongylocentrotus purpuratus, 1E11 recognizes a 57-kDa band. Tandem mass spectrometry of trypsin digests of the 57-kDa band permitted peptide mass mapping and sequencing of five peptides. All of the sequenced peptides, and 12 additional mass-mapped peptides, are found within the open reading frame of a cDNA encoding synaptotagmin B (Sp-SynB). In situ RNA hybridizations with synaptotagmin B probes with S. purpuratus larvae reveal a pattern of expression that is similar to that revealed by the antibody 1E11. Antibodies produced against a bacterially expressed Sp-SynB protein recognize a 57-kDa protein and colocalize with 1E11. When a full-length Sp-SynB cDNA is expressed in chicken embryonic cells, the cells become immunoreactive to 1E11. We conclude that synaptotagmin B is a gene expressed in neurons that has conserved epitopes in other invertebrate deuterostomes.

The nervous system development of the sea cucumber Stichopus japonicus was investigated to explore the development of the bilateral larval nervous system into the pentaradial adult form typical of echinoderms. The first nerve cells were detected in the apical region of epidermis in the late gastrula. In the auricularia larvae, nerve tracts were seen along the ciliary band. There was a pair of bilateral apical ganglia consisted of serotonergic nerve cells lined along the ciliary bands. During the transition to the doliolaria larvae, the nerve tracts rearranged together with the ciliary bands, but they were not segmented and remained continuous. The doliolaria larvae possessed nerves along the ciliary rings but strongly retained the features of auricularia larvae nerve pattern. The adult nervous system began to develop inside the doliolaria larvae before the larval nervous system disappears. None of the larval nervous system was observed to be incorporated into the adult nervous system with immunohistochemistry. Since S. japonicus are known to possess an ancestral mode of development for echinoderms, these results suggest that the larval nervous system and the adult nervous system were probably formed independently in the last common ancestor of echinoderms.

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