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.

The corticospinal (CS) tract is essential for voluntary movement, but what we know about the organization and development of the CS tract remains limited. To determine the total cortical area innervating the seventh cervical spinal cord segment (C7), which controls forelimb movement, we injected a retrograde tracer (fluorescent microspheres) into C7 such that it would spread widely within the unilateral gray matter (to Ͼ80%), but not to the CS tract. Subsequent detection of the tracer showed that, in both juvenile and adult mice, neurons distributed over an unexpectedly broad portion of the rostral two-thirds of the cerebral cortex converge to C7. This even included cortical areas controlling the hindlimbs (the fourth lumbar segment, L4). With aging, cell densities greatly declined, mainly due to axon branch elimination. Whole-cell recordings from spinal cord cells upon selective optogenetic stimulation of CS axons, and labeling of axons (DsRed) and presynaptic structures (synaptophysin) through cotransfection using exo utero electroporation, showed that overgrowing CS axons make synaptic connections with spinal cells in juveniles. This suggests that neuronal circuits involved in the CS tract to C7 are largely reorganized during development. By contrast, the cortical areas innervating L4 are limited to the conventional hindlimb area, and the cell distribution and density do not change during development. These findings call for an update of the traditional notion of somatotopic CS projection and imply that there are substantial developmental differences in the cortical control of forelimb and hindlimb movements, at least in rodents.

The nervous system of the brachiolaria larva of the starfish, Asterina pectinifera, was characterized using immunohistochemistry with the neuron-specific monoclonal antibodies 1E11 and 1F9 and an anti-serotonin antibody. The antigen recognized by 1F9 was determined by immunoprecipitation, peptide identification by mass spectrometry, and cDNA cloning as a novel START (steroidogenic acute regulatory protein [StAR]-related lipid transfer) domain-containing protein. Nerve cells are prominent in the brachiolar arms, ciliary bands, and adult rudiment. The brachiolar arms contain sensory-like nerve cells in the adhesive papillae, flask-shaped nerve cells in the adhesive disk, and bundles of fibers with branches interconnecting them. In the ciliary bands, nerve cells are interconnected with axon bundles along the ciliary bands and some neurons send fibers toward the oral and aboral epidermis. These neural components of the ciliary bands are regionally modified to form masses such as lateral and oral ganglia. The future aboral epidermis of the adult rudiment forms a nerve plexus with cell bodies enriched over spicules. Serotonergic nerve cell bodies are found throughout the nervous system except in the adhesive disk, the bipinnaria arms, and the adult rudiment. In addition, there are neural components in the esophagus and in the coelom where nerve fibers or bundles have distinct orientations with respect to the muscle fibers. The neuroanatomy of the brachiolaria suggests how it may function in controlling larval physiology and identifies intriguing problems on the origin of larval and adult nerves.