Nervous network in larvae of the ascidian Ciona intestinalis

 With the use of the monoclonal antibody UA301, which specifically recognizes the nervous system in ascidian larvae, the neuronal connections of the peripheral and central nervous systems in the ascidian Ciona intestinalis were observed. Three types of peripheral nervous system neurons were found: two located in the larval trunk and the other in the larval tail. These neurons were epidermal and their axons extended to the central nervous system and connected with the visceral ganglion directly or indirectly. The most rostral system (rostral trunk epidermal neurons, RTEN) was distributed bilateral-symmetrically. In addition, presumptive papillar neurons in palps were found which might be related to the RTEN. Another neuron group (apical trunk epidermal neurons, ATEN) was located in the apical part of the trunk. The caudal peripheral nervous system (caudal epidermal neurons, CEN) was located at the dorsal and ventral midline of the caudal epidermis. In the larval central nervous system, two major axon bundles were observed: one was of a photoreceptor complex and the other was connected with RTEN. These axon bundles joined in the posterior sensory vesicle, ran posteriorly through the visceral ganglion and branched into two caudal nerves which ran along the lateral walls of the caudal nerve tube. In addition, some immunopositive cells existed in the most proximal part of the caudal nerve tube and may be motoneurons. Received: 8 September 1997 / Accepted: 14 December 1997     

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.     

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.     

<Emphasis Type="Italic">Engrailed</Emphasis> is expressed in larval development and in the radial nervous system of <Emphasis Type="Italic">Patiriella</Emphasis> sea stars

We documented expression of the pan-metazoan neurogenic gene engrailed in larval and juvenile Patiriella sea stars to determine if this gene patterns bilateral and radial echinoderm nervous systems. Engrailed homologues, containing conserved En protein domains, were cloned from the radial nerve cord. During development, engrailed was expressed in ectodermal (nervous system) and mesodermal (coeloms) derivatives. In larvae, engrailed was expressed in cells lining the larval and future adult coeloms. Engrailed was not expressed in the larval nervous system. As adult-specific developmental programs were switched on during metamorphosis, engrailed was expressed in the central nervous system and peripheral nervous system (PNS), paralleling the pattern of neuropeptide immunolocalisation. Engrailed was first seen in the developing nerve ring and appeared to be up-regulated as the nervous system developed. Expression of engrailed in the nerve plexus of the tube feet, the lobes of the hydrocoel along the adult arm axis, is similar to the reiterated pattern of expression seen in other animals. Engrailed expression in developing nervous tissue reflects its conserved role in neurogenesis, but its broad expression in the adult nervous system of Patiriella differs from the localised expression seen in other bilaterians. The role of engrailed in patterning repeated PNS structures indicates that it may be important in patterning the fivefold organisation of the ambulacrae, a defining feature of the Echinodermata.     

A developmental study of serotonin-immunoreactive neurons in the larval central nervous system of the spider crabHyas araneus (Decapoda,Brachyura)

Larval development in crabs is characterized by a striking double metamorphosis in the course of which the animals change from a pelagic to a benthic life style. The larval central nervous system has to provide an adequate behavioural repertoire during this transition. Thus, processes of neuronal reorganization and refinement of the early larval nervous system could be expected to occur in the metamorphosing animal. In order to follow identified sets of neurons throughout metamorphosis, whole mount preparations of the brain and ventral nerve cord of laboratory reared spider crab larvae (Hyas araneus) were labelled with an antibody against the neurotransmitter serotonin. The system of serotonin-immunoreactive cell bodies, fibres and neuropils is well-developed in newly hatched larvae. Most immunoreative structures are located in the protocerebrum, with fewer in the suboesophaegeal ganglia, while the thoracic and abdominal ganglia initially comprise only a small number of serotonergic neurons and fibres. However, there are significant alterations in the staining pattern through larval development, some of which are correlated to metamorphic events. Accordingly, new serotonin-immunoreactive cells are added to the early larval set and the system of immunoreactive fibres is refined. These results are compared to the serotonergic innervation in other decapod crustaceans.     

The fate of specific motoneurons and sensory neurons of the pregenital abdominal segments inTenebrio molitor (Insecta : Coleoptera) during metamorphosis

The anatomy and innervation of the lateral external muscle and sensory cells located in the ventral region of pregenital abdominal segments were examined at the larval and adult stages ofTenebrio molitor (Coleoptera). All seven muscles located in this region degenerate during the pupal stage, whilst only the lateral external median (lem) appears in the adult. Backfillings of the motor nerve innervating this muscle reveal that, at both larval and adult stages, it is innervated by ten neurons. Intracellular records from the muscle fibres show that two neurons are inhibitory, and at least five are excitatory. There are also two unpaired neurons. A variety of sensory organs are located in the ventral region of the larvae, whilst only campaniform sensilla are found in the adult. At both stages, the innervation pattern of the sensory nerve branches is very similar. Also, the central projections of the sensory cells occupy similar neuropilar areas. Finally, prolonged intracellular records from the lem muscle revealed that, at the larval stage, it participates only in segmental or intersegmental reflexes, whilst in the adult it has a primary expiratory role in ventilation. The results show that extensive changes occur in the number of muscles located in the ventral region of the pregenital abdominal segments, as well as in the arrangement and number of sensory neurons, in the structure of the exoskeleton, and even in the central nervous system. In contrast, only minor changes are observed in the sensory and motor nerve branches, in the sensory projections, and in the number and the location of the motoneurons innervating the lateral external median muscle. Correspondence to: G. Theophilidis     

Evolutionary and structural diversification of the larval nervous system among marine bryozoans

Regardless of the morphological divergence among larval forms of marine bryozoans, the larval nervous system and its major effector organs (musculature and ciliary fields) are largely molded on the basis of functional demands of feeding, ciliary propulsion, phototactic behaviors, and substrate exploration. Previously published ultrastructural information and immunohistochemical reconstructions presented here indicate that neuronal pathways are largely ipsilateral, with more complex synaptic connections localized within the nerve nodule. Multiciliated sensory-motor neurons diversify structurally and functionally on the basis of their position along the axis of swimming largely due to the functional demands of photoklinotaxis and substrate exploration. Vesiculariform, buguliform, and ascophoran coronate larvae all have patches of sensory neurons bordering the pyriform organ's ciliated groove (juxtapapillary cells and border cells) that are active during substrate selection. Despite their simplified form, cyclostome larvae maintain swimming and probing behaviors with sensory-motor systems functionally similar to those of some parenchymella and planula larval types. Considering the evolutionary relationships among the morphological grades of marine bryozoans, particular lineages within the gymnolaemates have independently evolved larval traits that convey a greater range of sensory abilities and increased propulsive capacity. The larval nervous system of bryozoans may be evolutionarily derived from the pretrochal region of a trochophore-like larval form.     

The structure of the larval nervous system of Pisaster ochraceus (Echinodermata: Asteroidea)

Ultrastructural observations and glyoxilic acid-induced fluorescence of catecholamines indicate that tracts of axons lie at the base of the ciliary bands and run throughout their length in bipinnaria and brachiolaria larvae of Pisaster ochraceus. Two types of nerve cells occur at regular intervals within the ciliary bands. Type I nerve cells are associated with the axonal tracts, and type II nerve cells, which are ciliated, occur along the edge of the ciliary bands. Two prominent ganglia, which appear as accumulations of nerve cells and neuropile, occur on the lower lip of the larval mouth. Smaller ganglia occur irregularly throughout the ciliary band. Synapses were never clearly identified and were assumed to be unspecialized. Nervous tissues were also found associated with the esophageal muscles, the attachment organ, and the larval arms. Organization of the nervous system and its association with effectors suggest it controls swimming and feeding. Several similarities exist between the nervous systems of larval asteroids, larval echinoids, and adult echinoderms.     

Development of the nervous system in the brittle star Amphipholis kochii

There are several studies of neural development in various echinoderms, but few on ophiuroids, which develop indirectly via the production of pluteus larvae, as do echinoids. To determine the extent of similarity of neuroanatomy and neural development in the ophiuroids with other echinoderm larvae, we investigated the development of the nervous system in the brittle star Amphipholis kochii (Echinodermata: Ophiuroidea) by immunohistochemistry. Immunoreactive cells first appeared bilaterally in the animal pole at the late gastrula stage, and there was little migration of the neural precursors during A. kochii ontogeny, as is also the case in echinoids and holothuroids. On the other hand, neural specification in the presumptive ciliary band near the base of the arms does occur in ophiuroid larvae and is a feature they share with echinoids and ophiuroids. The ophiopluteus larval nervous system is similar to that of auricularia larvae on the whole, including the lack of a fine network of neurites in the epidermis and the presence of neural connections across the oral epidermis. Ophioplutei possess a pair of bilateral apical organs that differ from those of echinoid echinoplutei in terms of relative position. They also possess coiled cilia, which may possess a sensory function, but in the same location as the serotonergic apical ganglia. These coiled cilia are thought to be a derived structure in pluteus-like larvae. Our results suggest that the neural specification in the animal plate in ophiuroids, holothuroids, and echinoids is a plesiomorphic feature of the Ambulacraria, whereas neural specification at the base of the larval arms may be a more derived state restricted to pluteus-like larvae.     

Peripheral neurosecretory cells on the thoracic median nerves of the locust,Schistocerca gregaria

The distribution of neutral red staining peripheral cell bodies along the nerve trunks of the thoracic median nervous system of the locust, Schistocerca gregaria, is described. Backfilling of the cells with cobalt chloride solution reveals that they are neurones with characteristic axonal processes that terminate in the neurohaemal areas of the median nerve. The neurones react with the dye acridine orange, indicating their neurosecretory nature. This is confirmed by their ultrastructural appearance at the electron microscope level. The distribution and staining properties of the cells are compared with those of peripheral neurones from other insects and the nature of their neurosecretory product is discussed.     

The peripheral nervous system of mutants of early neurogenesis inDrosophila melanogaster

Summary Mutations previously known to affect early neurogenesis inDrosophila melanogaster have been found also to affect the development of the peripheral nervous system. Anti-HRP antibody staining has shown that larval epidermal sensilla of homozygous mutant embryos occur in increased numbers, which depend on the allele considered. This increase is apparently due to the development into sensory organs of cells which in the wild-type would have developed as non-sensory epidermis. Thus, neurogenic genes act whenever developing cells have to decide between neurogenic and epidermogenic fates, both in central and peripheral nervous systems. Different regions of the ectodermal germ layer are distinguished with respect to their neurogenic abilities.     

On the morphology of the central nervous system in larval stages ofCarcinus maenas L. (Decapoda,Brachyura)

We investigated the morphology of the central nervous system throughout the larval development ofCarcinus maenas. For that purpose single larvae were reared in the laboratory from hatching through metamorphosis. Complete series of whole mout semithin sections were obtained from individuals of all successive larval stages and analysed with a light microscope. Morphological feature and spatial arrangement of discernable neural cell clusters, fibre tracts and neuropile are described and compared with the adult pattern. We found that most of the morphological features characterizing the adult nervous system are already present in the zoea-1. Nevertheless, there are marked differences with respect to the arrangement of nerve cell bodies, organization of cerebral neuropile, and disposition of ganglia in the ventral nerve cord. It appears that complexity of the central nervous neuropile is selectively altered during postmetamorphotic development, probably reflecting adaptive changes of sensory-motor integration in response to behavioural maturation. In contrast, during larval development there was little change in the overall structural organization of the central nervous system despite some considerable growth. However, the transition from zoea-4 to megalopa brings about multiple fundamental changes in larval morphology and behavioural pattern. Since central nervous integration should properly adapt to the altered behavioural repertoire of the megalopa, it seems necessary to ask in which respect synaptic rearrangement might characterize development of the central nervous system.     

Fine structure of the doliolaria larva of the feather star Florometra serratissima (Echinodermata: Crinoidea), with special emphasis on the nervous system

The epidermis of the doliolaria larva of the Florometra serratissima is differentiated into distinct structures including an apical organ, adhesive pit, ganglion, ciliary bands, nerve plexus, and vestibular invagination. All these structures possess unique cell-types, suggesting that they are functionally specialized in the larva, except the vestibular invagination that becomes the postmetamorphic stomodeum. The epidermis also contains yellow cells, amoeboid-like cells, and secretory cells. The enteric sac, hydrocoel, axocoel, and somatocoels have differentiated but are probably not functional in the doliolaria stage. Mesenchymal cells, around the enteric sac and coeloms, appear to be actively secreting the endoskeleton and connective tissue fibers. The nervous system is composed of a nerve plexus, ganglion, and sensory receptor cells in the apical organ. The apical organ is a larval specialization of the anterior end; the ganglion is located in the base of the epidermis at the anterior dorsal end of the larva. The nerve plexus underlies most of the epidermis, although it is more prominent in the anterior region. Here, processes from sensory receptor cells of the apical organ, as well as those from nerve cells, contribute to the plexus. These processes contain one or a combination of organelles including vesicles, vacuoles, microtubules, and mitochondria. The configuration of glyoxylic acid-induced fluorescence, revealing catecholamine activity, correlates to the apical organ, nerve cells, and nerve plexus. Morphological evidence suggests that the nervous system may function in initiation and control of settlement, attachment, and metamorphosis. The crinoid larval nervous system is discussed and compared to that found in other larval echinoderms.     

The development of the serotonergic and FMRF-amidergic nervous system in<Emphasis Type="Italic"> Antalis entalis</Emphasis> (Mollusca,Scaphopoda)

The morphogenesis of serotonin- and FMRF-amide-bearing neuronal elements in the scaphopod Antalis entalis was investigated by means of antibody staining and confocal laser scanning microscopy. Nervous system development starts with the establishment of two initial, flask-like, serotonergic central cells of the larval apical organ. Slightly later, the apical organ contains four serotonergic central cells which are interconnected with two lateral serotonergic cells via lateral nerve projections. At the same time the anlage of the adult FMRF-amide-positive cerebral nervous system starts at the base of the apical organ. Subsequently, the entire neuronal complex migrates behind the prototroch and the six larval serotonergic cells lose transmitter expression prior to metamorphic competence. There are no strictly larval FMRF-amide-positive neuronal structures. The development of major adult FMRF-amide-containing components such as the cerebral system, the visceral loop, and the buccal nerve cords, however, starts before the onset of metamorphosis. The anlage of the putative cerebral system is the only site of adult serotonin expression in Antalis larvae. Establishment of the adult FMRF-amidergic and serotonergic neuronal bauplan proceeds rapidly after metamorphosis. Neurogenesis reflects the general observation that the larval phase and the expression of distinct larval morphological features are less pronounced in Scaphopoda than in Gastropoda or Bivalvia. The degeneration of the entire larval apical organ before metamorphic competence argues against an involvement of this sensory system in scaphopod metamorphosis. The lack of data on the neurogenesis in the aplacophoran taxa prevent a final conclusion regarding the plesiomorphic condition in the Mollusca. Nevertheless, the results presented herein shed doubts on general theories regarding possible functions of larval "apical organs" of Lophotrochozoa or even Metazoa.     

FlyPNS,a database of the <Emphasis Type="Italic">Drosophila</Emphasis> embryonic and larval peripheral nervous system

Background  

The embryonic and larval peripheral nervous system of Drosophila melanogaster is extensively studied as a very powerful model of developmental biology. One main advantage of this system is the ability to study the origin and development of individual sensory cells. However, there remain several discrepancies regarding the organization of sensory organs in each abdominal segment A1-A7.     

Development of the sensory system in larvae and pupae of Chaoborus crystallinus (DeGeer, 1776; Diptera, Chaoboridae): sensory cells, nerves and ganglia of the tail region

Using various microscopical techniques, we have studied changes in the sensory equipment and architecture of the peripheral nervous system (PNS) around the first metamorphic molt from larva to pupa in the phantom midge Chaoborus. The transparent larvae and pupae of this dipteran with ancestral features allow us to investigate sensilla and their central projections from whole-mount preparations of complete groups of segments. Each sensillum on the posterior larval and pupal segments was identified using its external shape and position, and the morphology of the abdominal ganglia and segmental nerves was investigated. In addition, retrograde fills with the carbocyanine dye DiI were used to trace the axonal paths of most of the extero- and proprioreceptors. These findings were combined to produce maps of the sensory elements of larval and pupal abdomens that were analyzed at three levels: seriality (homonomy), ontogenetic changes of individual sensilla, and homology of the PNS between different species. Comparison of different segments shows for both stages that primarily there is a homonomous basic design of the PNS, but segment-specific modifications are evident in segments 8-10. Comparison of corresponding larval and pupal segments shows that many sensilla retain their internal structure and axonal projections. However, their external cuticular parts are changed in relation to the different life habits of larvae and pupae. Furthermore, some sensilla are completely reduced during the pupal molt, especially those of the tenth segment which appears as a distinct larval structure (caenogenesis). Comparison between species indicates that despite the varying types of sensilla their basic segmental arrangement and their axonal trajectories are conserved.     

Peculiarities of ultrastructural organization of the metathoracic ganglion in the locustLocusta migratoria larva

In electron microscopic study of structural organization of the thoracic ganglion of the locust larva of the 1st age (1–2 days after hatching), the data on the structure of motoneurons of the 1st nerve, basal and motor neuropil of the larva were obtained. The effector elements of the larval locust CNS are formed rather early and have the structural plan similar to that in adult insects. However, in the larval motoneurons innervating the flight muscles (longitudinal dorsal muscles, wing depressors) the clearly seen features of immaturity of these nervous elements are revealed. Study of the larval ganglion neuropil has shown that the basal neuropil is morphologically formed sufficiently completely as early as in larvae of the first days after hatching. There are shown longitudinal contacts between axons of the ventral neuropil zone, the presence of axons forming theen-passant contacts as well as the synapses with a heterogeneous set of vesicles in the presynaptic area. The presence of the great number of granular vesicles in the basal neuropil of the locust larva may indicate an important role of catecholamines in the early development of the nervous system in the locust larva.     

Alterations of the eyes of Carinaria lamarcki (Gastropoda, Heteropoda) during the long pelagic cycle

 The eyes of different larval stages of Carinaria lamarcki were examined ultrastructurally. In all larval stages the eyes consist of a cornea, a lens and an everse retina. The photoreceptors in young larvae are exclusively of the ciliary type. In old larvae, however, two types of photoreceptors are present and the retina is composed of two segments: a posterior segment with altered ciliary photoreceptors (=type I sensory cells) and an anterior segment with what are presumably rhabdomeric photoreceptors (=type II sensory cells). The anterior retina is interpreted here as an accelerted character. Furthermore, the arrangement of the pigment granules changes during the long larval development being cup shaped in young larvae versus ribbon shaped in old larvae. The findings allow for the conclusions that: (a) the ciliary photoreceptors are correlated with the long larval period of Heteropoda and that (b) the eyes are altered continuously during the larval cycle. Accepted: 6 July 1998