The central nervous system, its cellular organisation and development, in the tadpole larva of the ascidian Ciona intestinalis

From its numerical composition, the central nervous system (CNS) of the ascidian larva is one of the simplest known nervous systems having a chordate plan. Fewer than 350 cells together constitute a caudal nerve cord, an interposed visceral ganglion containing motor circuits for swimming and, rostrally, an expanded sensory vesicle containing major sensory and interneuron regions of the CNS. Some cells are ependymal, with ciliated surfaces lining the neural canal, while others are clearly either sensory receptors or motoneurons, but most are distinguishable only on cytological grounds. Although reassignments between categories are still being made, there is evidence for determinancy of total cell number. We have made three-dimensional cell maps either from serial semithin sections, or from confocal image stacks of whole-mounted embryos and larvae stained with nuclear markers. Comparisons between the maps of neural tubes in embryos of successive ages, that is, between cells in one map and their progeny in older maps, enable us to follow the line of mitotic descent through successive maps, at least for the caudal neural tube. Details are clear for the lateral cell rows in the neural tube, at least until the latter contains approximately 320 cells, and somewhat for the dorsal cell row, but the ventral row is more complex. In the hatched larva, serial-EM reconstructions of the visceral ganglion reveal two ventrolateral fibre bundles at the caudalmost end, each of 10-12 axons. These tracts include at least five pairs of presumed motor axons running into the caudal nerve cord. Two pairs of axons decussate. Complementing this vertebrate feature in the CNS of the larval form of Ciona, we confirm that synapses form upon the somata and dendrites of its neurons, and that its motor tracts are ventral.     

Suppression of neural fate and control of inner ear morphogenesis by Tbx1

Inner ear sensory organs and VIIIth cranial ganglion neurons of the auditory/vestibular pathway derive from an ectodermal placode that invaginates to form an otocyst. We show that in the mouse otocyst epithelium, Tbx1 suppresses neurogenin 1-mediated neural fate determination and is required for induction or proper patterning of gene expression related to sensory organ morphogenesis (Otx1 and Bmp4, respectively). Tbx1 loss-of-function causes dysregulation of neural competence in otocyst regions linked to the formation of either mechanosensory or structural sensory organ epithelia. Subsequently, VIIIth ganglion rudiment form is duplicated posteriorly, while the inner ear is hypoplastic and shows neither a vestibular apparatus nor a coiled cochlear duct. We propose that Tbx1 acts in the manner of a selector gene to control neural and sensory organ fate specification in the otocyst.     

Nervous System of the Larva of the Ascidian Molgula citrina(Alder and Hancock, 1848)

Features of the nervous system, especially those of the peripheral nervous system, are described in the larva of Molgula citrina . In the peripheral nervous system, antibodies raised against acetylated α–tubulins mark a pair of rostral nerves arising from 8 to 10 sensory cells in the trunk epithelium, and a pair of tail nerves. In the sensory vesicle of the trunk, a pair of antennal cells is associated with the statocyte, and a tuft of ca. 150 cilia is labelled inside the hypophysial duct. A dorsal bundle of fibres forms a plexus over the surface of the sensory vesicle which extends caudally over the visceral ganglion. The latter contains somala that were back-filled by Co²⁺–lysine through the cut tip of the tail. Antibodies directed against the transmitter candidates: peptides substance P, FMRFamide, somatostatin, neuropeptide Y, CGRP, VIP, and the amines: 5-HT, dopamine, noradrenaline and GABA, all failed to demonstrate immunoreactivity anywhere in the nervous system. The trunk epithelium is ciliated uniformly but lacks papillae; this is remarkable given the presence of rostral nerves. The latter are presumed to be sensory, and can be compared with those in larvaceans and the larva of amphioxus. Sensory cells in the tail nerve, if present, lack cilia. The tail nerves are lateral in this species and presumed to be motor. © 1997 The Royal Swedish Academy of Sciences. Published by Elsevier Science Ltd     

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.     

Evolution of cerebral vesicles and their sensory organs in an ascidian larva

Sorrentino M., Manni L., Lane N. J. and Burighel P. 2000. Evolution of cerebral vesicles and their sensory organs in an ascidian larva. —Acta Zoologica (Stockholm) 81 : 243–258 The ascidian larval nervous system consists of the brain (comprising the visceral ganglion and the sensory vesicle), and, continuous with it, a caudal nerve cord. In most species two organs, a statocyst and an ocellus with ciliary photoreceptors, are contained in the sensory vesicle. A third presumptive sensory organ was sometimes found in an ‘auxiliary’ ganglionic vesicle. The development and morphology of the sensory and auxiliary ganglionic vesicles in Botryllus schlosseri and their associated organs was studied. The sensory vesicle contains a unique organ, the photolith, responding to both gravity and light. It consists of a unicellular statocyst, in the form of an expanded pigment cup receiving six photoreceptor cell extensions. Presumptive mechano‐receptor cells (S1 cells), send ciliary and microvillar protrusions to contact the pigment cup. A second group of distinctive cells (S2), slightly dorsal to the S1 cells, have characteristic microvillar extensions, resembling photoreceptor. We concur with the idea that the photolith is new and derived from a primitive statocyst and the S2 cells are the remnant of a primitive ocellus. In the ganglionic vesicle some cells contain modified cilia and microvillar extensions, which resemble the photoreceptor endings of the photolith. Our results are discussed in the light of two possible scenarios regarding the evolution of the nervous system of protochordates.     

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     

Serotonin immunoreactivity in the nervous system of the Pandora larva, the Prometheus larva, and the dwarf male of Symbion americanus (Cycliophora)

Cycliophora is a recently described phylum of enigmatic metazoans with a very complex life cycle that includes several sexual and asexual stages. Symbion pandora and Symbion americanus are the only two cycliophoran species hitherto described, of which morphological and genetic knowledge is still deficient to clarify the phylogenetic position of the phylum. Aiming to increase the database on the cycliophoran neural architecture, we investigated serotonin immunoreactivity in the free swimming Pandora larva, the Prometheus larva, and the adult dwarf male of S. americanus. In the larval forms, serotonin is mainly expressed in a ring-shaped pattern at the periphery of the antero-dorsal cerebral ganglion. Additionally, several serotonergic perikarya emerge from both sides of the cerebral ganglion. Thin neurites project anteriorly from the cerebral ganglion, while a pair of ventral longitudinal neurites emerges laterally and runs along the anterior-posterior body axis. Posteriorly, the ventral neurites fuse and extend as a posterior projection. In the dwarf male, serotonin is found mainly in the commissural neuropil of the large anterior cerebral ganglion. In addition, serotonin immunoreactivity is present in the most anterior region of the ventral neurites. Comparative analysis of spiralian nervous systems demonstrates that the neuroanatomy of the cycliophoran larval stages resembles much more the situation of adult rather than larval spiralians, which may be explained by secondary loss of larval structures and heterochronic shift of adult components into the nervous system of the Pandora and the Prometheus larva, respectively.     

FGF signaling establishes the anterior border of the Ciona neural tube

The Ciona tadpole is constructed from simple, well-defined cell lineages governed by provisional gene networks that have been defined via extensive gene disruption assays. Here, we examine the patterning of the anterior neural plate, which produces placodal derivatives such as the adhesive palps and stomodeum, as well as the sensory vesicle (simple brain) of the Ciona tadpole. Evidence is presented that the doublesex-related gene DMRT is expressed throughout the anterior neural plate of neurulating embryos. It leads to the activation of FoxC and ZicL in the palp placode and anterior neural tube, respectively. This differential expression depends on FGF signaling, which inhibits FoxC expression in the anterior neural tube. Inhibition of FGF signaling leads to expanded expression of FoxC, the loss of ZicL, and truncation of the anterior neural tube.     

Adrenocorticotropin-like immunoreactivity in the granules of neural complex cells of the ascidian Halocynthia roretzi

Immunohistochemical studies on the neural complex (neural gland, dorsal strand, and cerebral ganglion) of an ascidian, Halocynthia roretzi, were performed by using an antiserum against porcine ACTH. The antiserum recognized a considerable number of the cells scattered along the tubular structure of the dorsal strand and a few cells in the cerebral ganglion. Immunoelectron microscopic studies revealed that the ACTH-like substance resided within secretory granules with diameter of 300-500 nm. Furthermore, those ACTH-immunoreactive cells were demonstrated to be different from PRL-immunoreactive cells, the presence of which had previously been reported.     

Novel, posterior sensory organ in the trochophore larva of Phyllodoce maculata (Polychaeta)

A new posterior sensory organ (PSO), located at the dorsal midline of the hyposphere, is described by immunocytochemical detection of acetylated alpha tubulin and serotonin (5-HT) in a laser-scanning microscope, as well as three-dimensional reconstructions after optical serial sectioning in the trochophore larva of the polychaete Phyllodoce maculata (Phyllodocidae). The unpaired PSO consists of five bipolar sensory cells, two of them being 5-HT immunopositive, which send axons to the cerebral ganglion and prototroch nerve. The dendrites of these cells project to the surface and bear one cilium each. A single neuronal fibre from the apical sensory organ innervates the PSO.     

Structure of the caudal neural tube in an ascidian larva: Vestiges of its possible evolutionary origin from a ciliated band

Ultrastructural analysis and differential immunocytochemical staining with two antitubulin monoclonal antibodies were used to reexamine the organization and development of the neural tube in the larva of an ascidian, Ciona intestinalis, in appraisal of a theory that the dorsal tubular nervous system of the chordates evolved from two halves of a ciliated band in an auricularia-like larva of the kind found in echinoderms and hemichordates. One of the antibodies stained cilia in the nervous system and elsewhere; the other reacted primarily with neuronal axons. The caudal neural tube consists of four rows of large ciliated ependymal-glial cells enclosing an axial neural canal into which their single cilia extend. Two ventrolateral nerve tracts, containing axons, arise in the posterior brain region and extend along the length of the caudal tube, partially surrounded by the ependymal cells. The nonnervous, ciliated, ependymal neural tube of the ascidian larva with its two associated nerve tracts survives as a primitive early condition that could result from a ciliated band transformation. Tissues in the distal-most part of the ascidian larval tail have cell lineage origins that indicate an evolutionary history different from those in the proximal majority of the tail. The ependymal cells in this presumed later addition to the tail are not ciliated, although all of the others in the caudal ependymal tube appear to be.     

Structure of the caudal neural tube in an ascidian larva: vestiges of its possible evolutionary origin from a ciliated band.

Ultrastructural analysis and differential immunocytochemical staining with two antitubulin monoclonal antibodies were used to reexamine the organization and development of the neural tube in the larva of an ascidian, Ciona intestinalis, in appraisal of a theory that the dorsal tubular nervous system of the chordates evolved from two halves of a ciliated band in an auricularia-like larva of the kind found in echinoderms and hemichordates. One of the antibodies stained cilia in the nervous system and elsewhere; the other reacted primarily with neuronal axons. The caudal neural tube consists of four rows of large ciliated ependymal-glial cells enclosing an axial neural canal into which their single cilia extend. Two ventrolateral nerve tracts, containing axons, arise in the posterior brain region and extend along the length of the caudal tube, partially surrounded by the ependymal cells. The nonnervous, ciliated, ependymal neural tube of the ascidian larva with its two associated nerve tracts survives as a primitive early condition that could result from a ciliated band transformation. Tissues in the distal-most part of the ascidian larval tail have cell lineage origins that indicate an evolutionary history different from those in the proximal majority of the tail. The ependymal cells in this presumed later addition to the tail are not ciliated, although all of the others in the caudal ependymal tube appear to be.     

Expression of an Otx gene in the adult rudiment and the developing central nervous system in the vestibula larva of the sea urchin Holopneustes purpurescens

Expression of the Otx gene, HprOtx, from the sea urchin Holopneustes purpurescens, is described during the development of the adult echinoid rudiment in the vestibula larva of this species. The adult rudiment forms directly after gastrulation in the vestibula larva since, unlike the pluteus larva of most other sea urchin species, it is not a feeding larva. The expression is described during the period from hatching to a late vestibula larva. At hatching, HprOtx is expressed throughout the ectoderm of the gastrula. A short time later, expression is absent from the ectoderm on the oral side of the gastrula where the vestibule will form. In an early vestibula larva, HprOtx is not expressed in the ectodermal floor of the vestibule but is expressed in an asymmetric pattern in the aboral ectoderm. As the vestibule invaginates, HprOtx is newly expressed in the ectodermal floor of the vestibule as it develops into the neuroectoderm that is the anlage of the circum-oral central nervous system. The expression is at first in the central part of the floor, then it extends outwards to the ectoderm around the five primary podia and to the epineural folds between the podia. The epineural folds later close to form the radial nerves and the circum-oral nerve ring. In a late vestibula larva, HprOtx is expressed in the radial nerves and the nerve ring. The expression of an Otx gene in the developing echinoid central nervous system is interpreted as an instance of conserved gene expression in echinoderm development.     

Dorsal–ventral patterning of the neural tube: A tale of three signals

Development of the vertebrate nervous system begins with the acquisition of neural identity from the midline dorsal‐ectodermal cells of the gastrulating embryos. The subsequent progressive specification of the neural plate along its anterior–posterior and dorsal–ventral (DV) axes allows the generation of the tremendous variety of neuronal and glial cells that compose the vertebrate central nervous system (CNS). Studies on the development of the spinal cord, the anatomically simplest part of the CNS, have generated most of our current knowledge on the signaling events and the genetic networks that orchestrate the DV patterning of the neural plate. In this review, we discuss the recent advances in our understanding of these events and highlight unresolved questions. We focused our attention on the activity and the integration of the three main instructive cues: Sonic hedgehog, the Wnts and the Bone Morphogenetic Proteins, giving particular attention to the less well understood dorsal signaling events. © 2012 Wiley Periodicals, Inc. Develop Neurobiol, 2012     

粘虫的胚胎发育

粘虫(Mythimna separata)胚胎发育经过卵裂及胚盘形成、胚带及原肠发生、胚带分节及附肢形成、体壁形成及背向闭合、胚胎反转和器官发生与形成6个时期。粘虫卵在25℃,胚胎发育至12h,胚带呈新月形或“C”字形。随着原肠发生,首先出现口陷与肛陷,与此同时,胚带逐渐伸长并开始分节。胚胎发育至32h,胚带头尾相接并呈波浪形弯曲,在胚胎反转前,胚胎发育至42h,前肠、后肠及马氏管已经形成。胚胎发育至54h时,胚动完成之 后,中肠才明显可见。同时将大量卵黄包围起来。神经系统的发生与气管形成始于原肠发生之后,至胚胎反转之前,神经节索才出现,随着胚动发生,神经节体积不断增大,腹神经索逐渐形成,纵走气管明显可见。     

Early stages of spinal ganglion formation during tail regeneration in the newt, Notophthalmus viridescens

Stages in the development of sensory ganglia in the regenerating newt tail after amputation are described by taking advantage of the rostrocaudal developmental gradient of the regenerating tail. A series of ganglia, beginning at the tip of the regenerate and progressing rostrally, were examined. Eight-week regenerates were used because they showed the most complete array of stages. The first recognizable ganglia appear as small clusters of cells sitting dorsally on the already established ventral roots. The cluster of ganglionic cells steadily expands with the addition of many new cells. Signs of cell differentiation within the ganglion precede the formation of the dorsal root rudiment, which assumes several different configurations but most commonly enters the cord close to the ventral root. Our material suggests that ganglion precursor cells originate in the ventral region of the developing spinal cord and migrate out of the cord by travelling along the ventral root until, at a suitable distance from the cord, they halt, proliferate, and eventually differentiate. In the regenerate, we saw no evidence of neural crest cells--such as those that give rise to ganglia in the trunk region during development--forming at the dorsal region of the regenerated neural tube. Nor was there any morphological evidence of mesenchymal contribution to the ganglion cell clusters.