Nervous system development of the sea cucumber Stichopus japonicus

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.     

Ciliary Bands in Echinoderm Larvae: Evidence for Structural Homologies and a Common Plan

Abstract A series of laterally projecting ridges develop along the ciliary band of late stage auricularia larvae. These are similar in position to the larval arms of bipinnaria larvae and the epaulettes and vibratile lobes of echinoid pluteus larvae, all of which structures are potentially homologous. When the auricularia is converted to a doliolaria with a series of circumferential ciliary bands, the ridges of the former are retained as basic elements from which the circumferential bands of the latter then develop. There is a simple repeating pattern in the arrangement of these elements in which bands composed of two elements alternate with bands composed of four. The available evidence does not resolve the question of which of the above four larval types, whether feeding or non-feeding, is more primitive. The common plan apparent among them suggests, however, that this plan, whatever its origin, predates the diversification of larval types among eleutherozoan echinoderms.     

Group B sox genes that contribute to specification of the vertebrate brain are expressed in the apical organ and ciliary bands of hemichordate larvae

We have identified and characterized the sequence and expression of two Group B Sox genes in the acorn worm, Ptychodera flava. One sequence represents a Group B1 Sox gene and is designated Pf-SoxB1; the other is a Group B2 Sox gene and is designated Pf-SoxB2. Both genes encode polypeptides with an HMG domain in the N-terminal half. Whole-mount in situ hybridization to embryonic and larval stages of P. flava shows that the two genes are expressed in rather similar patterns at these stages. Expression is first detected in the cells of the blastula and subsequently localizes to the ectoderm during gastrulation. As the mouth forms, expression becomes concentrated in the stomodeum region. During morphogenesis of the tornaria larva, expression in the stomodeal ectoderm remains prominent around the mouth and under the oral hood. Later the genes are prominently upregulated in the ciliary bands and the apical organ. These results provide additional evidence that genes playing essential roles in the formation of the chordate dorsal central nervous system function in the development of the ciliary bands and apical organ, neural structures of this non-chordate deuterostome larva.     

Development of the nervous system in the acorn worm Balanoglossus simodensis: insights into nervous system evolution

SUMMARY To examine the evolutionary origin of the chordate nervous system, an outgroup comparison with hemichordates is needed. When the nervous systems of chordates and hemichordates are compared, two possibilities have been proposed, one of which is that the chordate nervous system has evolved from the nervous system of hemichordate‐like larva and the other that it is comparable to the adult nervous system of hemichordates. To address this issue, we investigated the entire developmental process of the nervous system in the acorn worm Balanoglossus simodensis. In tornaria larvae, the nervous system developed along the longitudinal ciliary band and the telotroch, but no neurons were observed in the ventral band or the perianal ciliary ring throughout the developmental stages. The adult nervous system began to develop at the dorsal midline at the Krohn stage, considerably earlier than metamorphosis. During metamorphosis, the larval nervous system was not incorporated into the adult nervous system. These observations strongly suggest that the hemichordate larval nervous system contributes little to the newly formed adult nervous system.     

Ascidians as excellent chordate models for studying the development of the nervous system during embryogenesis and metamorphosis

The swimming larvae of the chordate ascidians possess a dorsal hollowed central nervous system (CNS), which is homologous to that of vertebrates. Despite the homology, the ascidian CNS consists of a countable number of cells. The simple nervous system of ascidians provides an excellent experimental system to study the developmental mechanisms of the chordate nervous system. The neural fate of the cells consisting of the ascidian CNS is determined in both autonomous and non-autonomous fashion during the cleavage stage. The ascidian neural plate performs the morphogenetic movement of neural tube closure that resembles that in vertebrate neural tube formation. Following neurulation, the CNS is separated into five distinct regions, whose homology with the regions of vertebrate CNS has been discussed. Following their larval stage, ascidians undergo a metamorphosis and become sessile adults. The metamorphosis is completed quickly, and therefore the metamorphosis of ascidians is a good experimental system to observe the reorganization of the CNS during metamorphosis. A recent study has shown that the major parts of the larval CNS remain after the metamorphosis to form the adult CNS. In contrast to such a conserved manner of CNS reorganization, most larval neurons disappear during metamorphosis. The larval glial cells in the CNS are the major source for the formation of the adult CNS, and some of the glial cells produce adult neurons.     

The nonfeeding auricularia of Holothuria mexicana (Echinodermata,Holothuroidea)

Among echinoderms, nonfeeding larvae usually are simplified in body shape, have uniform ciliation, and have lost the larval gut. A few species have nonfeeding larvae that express some remnant features of feeding larvae like ciliated bands and larval skeleton or larval arms, but typically their larval mouth never opens and their gut does not function. Still other species have retained the feeding larval form, a functional gut, and can feed, but they do not require food to metamorphose. The present note describes the development of a tropical holothurian, Holothuria mexicana, which hatches as a gastrula that is already generating coelomic structures. A translucent auricularia forms with a mouth that opens but becomes reduced soon thereafter. In form and ciliation this auricularia resembles a feeding larva, but it does not respond to food. A doliolaria forms on day 4 and the pentactula on day 6 post‐fertilization. Further study of this larva and that of its closely related congener, Holothuria floridana, is warranted.     

Ontogeny of the holothurian larval nervous system: evolution of larval forms

Echinoderm larvae share numerous features of neuroanatomy. However, there are substantial differences in specific aspects of neural structure and ontogeny between the dipleurula-like larvae of asteroids and the pluteus larvae of echinoids. To help identify apomorphic features, we have examined the ontogeny of the dipleurula-like auricularia larva of the sea cucumber, Holothuria atra. Neural precursors arise in the apical ectoderm of gastrulae and appear to originate in bilateral clusters of cells. The cells differentiate without extensive migration, and they align with the developing ciliary bands and begin neurogenesis. Neurites project along the ciliary bands and do not appear to extend beneath either the oral or aboral epidermis. Apical serotonergic cells are associated with the preoral loops of the ciliary bands and do not form a substantial commissure. Paired, tripartite connectives form on either side of the larval mouth that connect the pre-oral, post-oral, and lateral ciliary bands. Holothurian larvae share with hemichordates and bipinnariae a similar organization of the apical organ, suggesting that the more highly structured apical organ of the pluteus is a derived feature. However, the auricularia larva shares with the pluteus larva of echinoids several features of neural ontogeny. Both have a bilateral origin of neural precursors in ectoderm adjacent to presumptive ciliary bands, and the presumptive neurons move only a few cell diameters before undergoing neurogenesis. The development of the holothurian nervous systems suggests that the extensive migration of neural precursors in asteroids is a derived feature. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Novel and conserved roles for orthodenticle/ otx and orthopedia/ otp orthologs in the gastropod mollusc Patella vulgata

The orthodenticle/ otx and orthopedia/ otp classes of homeobox gene families have been identified in all three major classes of bilaterians: deuterostomes, lophotrochozoans, and ecdysozoans. Otx genes have been studied extensively and play a role in the development of anterior neural structures. Otp genes have been found to be involved in nervous system development in mouse and Drosophila. To date, no members of these genes are known in molluscs. We cloned orthologs of orthodenticle/ otx and orthopedia/ otpfrom the gastropod Patella vulgata, and designated them Pv-otx and Pv-otprespectively. Our analysis of the spatio-temporal expression pattern of otx and otp orthologs during P. vulgata embryogenesis leads to the following conclusions. First, Pv-otx is expressed in and around the stomodaeum and our analysis thus supports the previously suggested conservation of the protostome and deuterostome larval mouth regions. Second, we find that Pv-otp is involved in the development of the larval apical sensory organ, suggesting a conserved role for this gene family in nervous system development. A similar conserved role in nervous system development has been proposed for orthodenticle/otx genes and we suggest that part of the cells expressing Pv-otx are involved in the development of the anterior nervous system. Last, we postulate that otx genes were ancestrally involved in the development of ciliary bands in bilaterians.     

Nervous system development of two crinoid species, the sea lily Metacrinus rotundus and the feather star Oxycomanthus japonicus

Nervous system development in echinoderms has been well documented, especially for sea urchins and starfish. However, that of crinoids, the most basal group of extant echinoderms, has been poorly studied due to difficulties in obtaining their larvae. In this paper, we report nervous system development from two species of crinoids, from hatching to late doliolaria larvae in the sea lily Metacrinus rotundus and from hatching to cystidean stages after settlement in the feather star Oxycomanthus japonicus. The two species showed a similar larval nervous system pattern with an extensive anterior larval ganglion. The ganglion was similar to that in sea urchins which is generally regarded as derived. In contrast with other echinoderm and hemichordate larvae, synaptotagmin antibody 1E11 failed to reveal ciliary band nerve tracts. Basiepithelial nerve cells formed a net-like structure in the M. rotundus doliolaria larvae. In O. japonicus, the larval ganglion was still present 1 day after settlement when the adult nervous system began to appear inside the crown. Stalk nerves originated from the crown and extended down the stalk, but had no connections with the remaining larval ganglion at the base of the stalk. The larval nervous system was not incorporated into the adult nervous system, and the larval ganglion later disappeared. The aboral nerve center, the dominant nervous system in adult crinoids, was formed at the early cystidean stage, considerably earlier than previously suggested. Through comparisons with nervous system development in other ambulacraria, we suggest the possible nervous system development pattern of the echinoderm ancestor and provide new implications on the evolutionary history of echinoderm life cycles.     

Larval and adult brains

Apical organs are a well-known structure in almost all ciliated eumetazoan larvae, although their function is poorly known. A review of the literature indicates that this small ganglion is the "brain" of the early larva, and it seems probable that it represents the brain of the ancestral, holopelagic ancestor of all eumetazoans, the gastraea. This early brain is lost before or at metamorphosis in all groups. Protostomes (excluding phoronids and brachiopods) appear to have brains of dual origin. Their larvae develop a pair of cephalic ganglia at the episphere lateral to the apical organ, and these two ganglia become an important part of the adult brain. The episphere and the cerebral ganglia show Otx expression, whereas Hox gene expression has not been seen in this part of the brain. A ventral nervous system develops around the blastopore, which becomes divided into mouth and anus by fusion of the lateral blastopore lips. The circumblastoporal nerve ring becomes differentiated into a nerve ring around the mouth, becoming part of the adult brain, a pair of ventral nerve cords, in some cases differentiated into a chain of ganglia, and a ring around the anus. This part of the nervous system appears to be homologous with the oral nerve ring of cnidarians. This interpretation is supported by the expression of Hox genes around the cnidarian mouth and in the ventral nervous system of the protostomes. The development of phoronids, brachiopods, echinoderms, and enteropneusts does not lead to the formation of an episphere or to differentiation of cerebral ganglia. In general, a well-defined brain is lacking, and Hox genes are generally not expressed in the larval organs, although this has not been well studied.     

The auricularia-to-doliolaria transformation in two aspidochirote holothurians, Holothuria mexicana and Stichopus californicus

Abstract. The fragmentation and rearrangement of the ciliary bands that occurs during the auricularia-to-doliolaria transformation is described for the non-feeding auricularia larva of Holothuria mexicana and the more typical planktotrophic auricularia of Stichopus californicus. The ciliary band of the auricularia larva runs along a series of ridges that project from the sides of the body. Fragmentation results from a loss of ciliary band cells from the zones between the ridges. The remaining fragments then reorient, elongate, and fuse to form the 5 circumferential bands of the doliolaria. The fate of the band cells lost during this process could not be determined with certainty, but they disappear after being sequestered beneath the epithelium for a time, probably through histolysis. Cell counts indicate that significant numbers of cells are also lost from the ridges. Normal swimming ceases just before transformation begins, probably because the nerve supply to all or parts of the band is disrupted, and this may play a role in initiating morphogenesis.     

Development of the central nervous system in the larvacean Oikopleura dioica and the evolution of the chordate brain

In non-vertebrate chordates, central nervous system (CNS) development has been studied in only two taxa, the Cephalochordata and a single Class (Ascidiacea) of the morphologically diverse Urochordata. To understand development and molecular regionalization of the brain in a different deeply diverging chordate clade, we isolated and determined the expression patterns of orthologs of vertebrate CNS markers (otxa, otxb, otxc, pax6, pax2/5/8a, pax2/5/8b, engrailed, and hox1) in Oikopleura dioica (Subphylum Urochordata, Class Larvacea). The three Oikopleura otx genes are expressed similarly to vertebrate Otx paralogs, demonstrating that trans-homologs converged on similar evolutionary outcomes by independent neo- or subfunctionalization processes during the evolution of the two taxa. This work revealed that the Oikopleura CNS possesses homologs of the vertebrate forebrain, hindbrain, and spinal cord, but not the midbrain. Comparing larvacean gene expression patterns to published results in ascidians disclosed important developmental differences and similarities that suggest mechanisms of development likely present in their last common ancestor. In contrast to ascidians, the lack of a radical reorganization of the CNS as larvaceans become adults allows us to relate embryonic gene expression patterns to three subdivisions of the adult anterior brain. Our study of the Oikopleura brain provides new insights into chordate CNS evolution: first, the absence of midbrain is a urochordate synapomorphy and not a peculiarity of ascidians, perhaps resulting from their drastic CNS metamorphosis; second, there is no convincing evidence for a homolog of a midbrain-hindbrain boundary (MHB) organizer in urochordates; and third, the expression pattern of "MHB-genes" in the urochordate hindbrain suggests that they function in the development of specific neurons rather than in an MHB organizer.     

Larval development of the featherstar Aporometra wilsoni (Echinodermata: Crinoidea)

Abstract. Larval development of a small ovoviviparous comatulid crinoid, Aporometra wilsoni , was investigated using a population from South Australia. The genital pinnules of reproductive females each contain an ovary, within which are oocytes of various stages. Generally, one or more developing larvae lie outside the ovary, but within each pinnule. Larvae pass through "uniformly ciliated" and doliolaria stages before they exit the pinnule via the genital pore. The doliolariae lack the usual ciliary bands and are unable to swim. Doliolariae dissected from pinnules were followed through metamorphosis to the cystidean stage until the pentacrinoid larval stage. While previous reports on Aporometra have noted pentacrinoids attached to the female, virtually no pentacrinoids were found attached to any of the hundreds of adult females observed during this study. Females sampled from the mid-reproductive season (September and October) were found to bear >2500 developing eggs and larvae at a time. It appears that emerging doliolariae fall from the female and attach to other substrates to complete development. Aporometra is classified in Notocrinida with the Antarctic crinoid Notocrinus. This classification is based in part on their supposedly homologous larval brooding. However, the reproductive mechanism in A. wilsoni is quite different from Notocrinus , calling into question their current status as sister taxa.     

Apical organs in echinoderm larvae: insights into larval evolution in the Ambulacraria

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.     

Region specific gene expressions in the central nervous system of the ascidian embryo

The vertebrate brain is regionalized during development into forebrain, midbrain and hindbrain. Fibroblast growth factor 8 (FGF8) is expressed in the midbrain/hindbrain boundary (MHB) and functions as an organizer molecule. Previous studies demonstrated that the brain of basal chordates or ascidians is also regionalized at least into fore/midbrain and hindbrain. To better understand the ascidian brain regionalization, the expression of the Ciona Fgf8/17/18 gene was compared with the expression of Otx, En and Pax2/5/8 genes. The expression pattern of these genes resembled that of the genes in the vertebrate forebrain, midbrain, MHB and hindbrain, each of those domains being characterized by sole or combined expression of Otx, Pax2/5/8, En and Fgf8/17/18. In addition, the putative forebrain and midbrain expressed Ci-FgfL and Ci-Fgf9/16/20, respectively. Therefore, the regionalization of the ascidian larval central nervous system was also marked by the expression of Fgf genes.     

A Distinctive Nerve Cell Type Common to Diverse Deuterostome Larvae: Comparative Data from Echinoderms,Hemichordates and Amphioxus

Abstract The larval ciliary bands of echinoderm bipinnaria and pluteus larvae and the hemichordate tornaria contain similar multipolar or bipolar nerve cells with unusual apical processes that run across the surface of the band between the bases of its cilia. We report on some distinctive ultrastructural features of these cells. Among these are specialized junctions that occur between the cells' apical processes and adjacent ciliary band cells near the base of each cilium. Such structures are best developed in pluteus larvae. Many nerve cells in the larval spinal cord of amphioxus also have large apical processes that cross the central lumen of the cord. We interpret our observations on these cells in terms of Garstang's hypothesis, which derives the chordate neural tube from a larval ciliary band, and suggest that multipolar cells like those in echinoderm and tornaria bands may be the antecedents of some categories of neurons in the chordate spinal cord.     

Region specific gene expressions in the central nervous system of the ascidian embryo

The vertebrate brain is regionalized during development into forebrain, midbrain and hindbrain. Fibroblast growth factor 8 (FGF8) is expressed in the midbrain/hindbrain boundary (MHB) and functions as an organizer molecule. Previous studies demonstrated that the brain of basal chordates or ascidians is also regionalized at least into fore/midbrain and hindbrain. To better understand the ascidian brain regionalization, the expression of the Ciona Fgf8/17/18 gene was compared with the expression of Otx, En and Pax2/5/8 genes. The expression pattern of these genes resembled that of the genes in the vertebrate forebrain, midbrain, MHB and hindbrain, each of those domains being characterized by sole or combined expression of Otx, Pax2/5/8, En and Fgf8/17/18. In addition, the putative forebrain and midbrain expressed Ci-FgfL and Ci-Fgf9/16/20, respectively. Therefore, the regionalization of the ascidian larval central nervous system was also marked by the expression of Fgf genes.