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Color profile: Generic CMYK printer profile Composite Default screen 1678 REVIEW / SYNTHÈSE Developmental neurobiology of hydra, a model animal of cnidarians1 Osamu Koizumi Abstract: Hydra belongs to the class Hydrozoa in the phylum Cnidaria. Hydra is a model animal whose cellular and developmental data are the most abundant among cnidarians. Hence, I discuss the developmental neurobiology of hydra. The hydra nerve net is a mosaic of neural subsets expressing a specific neural phenotype. The developmental dynamics of the nerve cells are unique. Neurons are produced continuously by differentiation from interstitial multipotent stem cells. These neurons are continuously displaced outwards along with epithelial cells and are sloughed off at the extremities. However, the spatial distribution of each neural subset is maintained. Mechanisms related to these phenomena, i.e., the position-dependent changes in neural phenotypes, are proposed. Nerve-net formation in hydra can be examined in various experimental systems. The conditions of nerve-net formation vary among the systems, so we can clarify the control factors at the cellular level by comparing nerve-net formation in different systems. By large-scale screening of peptide signal molecules, peptide molecules related to nerve-cell differentiation have been identified. The LPW family, composed of four members sharing common N-terminal L(or I)PW, inhibits nerve-cell differentiation in hydra. In contrast, Hym355 (FPQSFLPRG-NH3) activates nerve differentiation in hydra. LPWs are epitheliopeptides, whereas Hym355 is a neuropeptide. In the hypostome of hydra, a unique neuronal structure, the nerve ring, is observed. This structure shows the nerve association of neurites. Exceptionally, the tissue containing the nerve ring shows no tissue displacement during the tissue flow that involves the whole body. The neurons in the nerve ring show little turnover, although nerve cells in all other regions turn over continuously. These associations and quiet dynamics lead me to think that the nerve ring has features similar to those of the central nervous system in higher animals. Résume : Hydra fait partie des hydrozoaires dans le phylum des cnidaires. Hydra est un animal modèle pour lequel, parmi les cnidaires, il existe le plus de données sur les cellules et le développement. Je présente donc ici le développement neurobiologique de l’hydre. Le réseau nerveux chez l’hydre est une mosaïque de sous-groupes neuraux qui compose un phénotype neural particulier. La dynamique du développement de ces cellules est inusitée. Des neurones sont produits de façon continue par différenciation de cellules souches interstitielles multipotentes. Les neurones sont continuellement repoussés vers l’extérieur en même temps que des cellules épithéliales et ils sont rejetés aux extrémités. Cependant, la répartition spatiale de tous les sous-groupes de nerfs est maintenue. Des mécanismes capables d’expliquer ces phénomènes, i.e. les changements dépendants de la position des phénotypes neuraux, sont proposés. La formation du réseau de nerfs chez Hydra peut être observée dans divers systèmes expérimentaux, mais les conditions peuvent varier d’un système à l’autre. Il est donc possible de comprendre les facteurs de contrôle à l’échelle des cellules en comparant la formation des nerfs dans les différents systèmes. Les molécules de peptides reliées à la différenciation des cellules ont été identifiées au cours d’un tri à grande échelle des molécules de peptides signalisateurs. La famille des LPW, composée de quatre unités qui ont en commun le N-terminal, L (ou I)PW, inhibe la différenciation des cellules nerveuses chez l’hydre. En revanche, Hym355 (FPQSFLPRG-NH3) active la différenciation. Les LPW sont des épithéliopeptides, alors que Hym355 est un neuropeptide. Il y a une structure neurale particulière, l’anneau nerveux, dans l‘hypostome de l’hydre. Cette structure a des associations nerveuses semblables à celles des neurites. Le tissu qui entoure l’anneau nerveux ne subit pas de déplacement, malgré le flux des tissus dans le reste du corps. Les neurones de l’anneau nerveux sont rarement remplacés, alors que les cellules nerveuses des autres régions sont continuellement changées. Ces associations et cette dynamique lente permettent de penser que l‘anneau nerveux ressemble par certains aspects au système nerveux central des animaux plus évolués. [Traduit par la Rédaction] Koizumi 1689 Received 30 November 2001. Accepted 15 July 2002. Published on the NRC Research Press Web site at http://cjz.nrc.ca on 15 November 2002. O. Koizumi. Neuroscience Laboratory, Department of Environmental Science, Fukuoka Women’s University, Kasumiga-oka 1-1-1, Higashi-ku, Fukuoka 813-8529, Japan (e-mail: [email protected]). 1 This review is one of a series dealing with aspects of the phylum Cnidaria. This series is one of several virtual symposia on the biology of neglected groups that will be published in the Journal from time to time. Can. J. Zool. 80: 1678–1689 (2002) J:\cjz\cjz8010\Z02-134.vp Wednesday, November 13, 2002 11:06:08 AM DOI: 10.1139/Z02-134 © 2002 NRC Canada Color profile: Generic CMYK printer profile Composite Default screen Koizumi Introduction to the nerve net of hydra: unique features of the diffuse nervous system The freshwater coelenterate hydra has a simple body plan. Like all coelenterates, it is a diploblastic animal: it is composed of two layers of epithelial cells, the endoderm and the ectoderm. The apical end is the head and the basal end is the foot. The head has a hypostome with the mouth opening at the apex and several tentacles originating from the lower part of the hypostome. Hydra has a simple nervous system consisting of a nerve net that extends through the body (Hadñi 1909; Burnett and Diehl 1964; Lentz and Barrnett 1965; Lentñ, 1968). Nerve cells are interspersed among the epithelial cells of both layers (Fig. 1). No large concentrations of neurons such as ganglia are observed (Lentz and Barnett 1965; Lentz 1968; Bode et al. 1988b). The nerve net contains two types of nerve cells, ganglion cells and sensory cells (Davis et al. 1968; Koizumi and Bode 1991; Grimmelikhuijzen and Westfall 1995). Ganglion cells lie close to the muscle processes at the basal ends of the epithelial cells. Sensory cells have elongated cell bodies that extend from the level of the muscle processes in an apical direction and an elaborate ciliary cone at the apical end of the cell body (Fig. 1) (Westfall 1973; Westfall and Kinammon 1978; Bode et al. 1988b; Koizumi and Bode 1991; Grimmelikhuijzen and Westfall 1995). The nervous system of hydra has several unique features. The most remarkable of these is the multifunction of neurons. Each neuron in hydra possesses the entire repertory of nerve-cell functions (Westfall 1973; Westfall and Kinammon 1978), i.e., the neurons are all sensory–motor-interneurons with neurosecretory granules. For example, a sensory cell has sensory cilia as a sensory neuron, synaptic connections to the muscle layer as a motor neuron, synaptic connections to neurites or the cell body of a ganglion cell as an interneuron, and aggregations of granules in non-synaptic regions of proximal sites of the cell body as a neurosecretory cell (Fig. 2) (Westfall and Kinammon 1978). Ganglion cells have the same features (Westfall 1973). Moreover, as is shown in Fig. 2, sometimes a single neuron innervates two different types of effectors: muscle fibers and a nematocyte (Westfall et al. 1971; Grimmelikhuijzen and Westfall 1995). Immunohistochemistry using neuropeptide antisera and monoclonal antibodies specific to hydra neurons on whole mounts has made it feasible to study the nerve net of hydra (Grimmelikhuijzen 1985; Dunne et al. 1985). These studies have shown that the hydra nerve net contains numerous subsets of neurons and that the spatial distributions are highly position-specific (Fig. 3). Numerous subsets of neurons containing different neuropeptides and several subsets of neurons defined by monoclonal antibodies were noted (Grimmelikhuijzen et al. 1982, 1990, 1995; Grimmelikhuijzen 1985; Dunne et al. 1985; Koizumi and Bode 1986, 1991; Koizumi et al. 1988; Yaross et al. 1986). The regional distribution of each subset tends to be constant (Koizumi and Bode 1986; Koizumi et al. 1988; Bode et al. 1988b). Figure 4 is a diagram showing all types of ectodermal neurons. Ganglion cells, sensory cells, and unique nerve cells, including those of the nerve ring, are all localized together 1679 Fig. 1. Organization of a cnidarian epithelium and two types of nerve cells, a sensory cell and a ganglion cell (modified from Mackie and Passano 1968). on the muscle sheet of one layer of ectodermal epithelia, as shown in Fig. 1 (Mackie and Passano 1968). Developmental dynamics of neurons: other unique features of the diffuse nervous system Hydra has three types of cell lineages: an ectodermal epithelial cell lineage, an endodermal epithelial cell lineage, and a interstitial cell lineage. The interstitial cell lineage is composed of interstitial cells, nerve cells, nematocytes, gland cells, and gametes (Fig. 5). Interstitial cells are multipotent stem cells, committed precursors, and differentiating intermediates (Campbell and David 1974; David and Gierer 1974; David and Murphy 1977; Bode and David 1978; Bode 1996). In an adult hydra, nerve cells are produced continuously by constant differentiation from interstitial cells (Bode et al. 1988b; David and Hager 1994). Nerve-cell production in the nerve net is balanced by a loss of neurons at the extremities and by the supply of neurons to young buds. Therefore, neurons are continuously changing their axial location by moving with epithelial cells either towards the apical end (the apex of the hypostome or the tip of a tentacle) or towards the basal end (the basal disk) (Campbell 1967a, 1976b, 1973; Bode et al. 1986, 1988b; Bode 1992). However, the distribution of each subset of neurons expressing a certain neural phenotype is maintained (Bode et al. 1986, 1988b; Bode 1992). How is the constant nerve net maintained in spite of the active growth dynamics in hydra described above? In experiments related to this question, it was demonstrated that neurons can change the expression of FMRFamide-like peptide and vasopessin-like peptide depending upon their position in hydra (Koizumi and Bode 1986, 1991). Moreover, it was demonstrated that ganglion cells were converted to sensory cells when the the neurons were moved from the body column to the hypostome (Koizumi et al. 1988). These dynamic features of neurons in the adult hydra correspond to properties of developing nerve cells in embryos of higher animals. Mechanisms controlling nerve-net formation at the cellular level Hydra possesses several advantages for the study of nervenet formation, which can be examined in various unique © 2002 NRC Canada J:\cjz\cjz8010\Z02-134.vp Wednesday, November 13, 2002 11:06:09 AM Color profile: Generic CMYK printer profile Composite Default screen 1680 Can. J. Zool. Vol. 80, 2002 Fig. 2. Multifunctional nerve cells in hydra. A single sensory cell has synaptic connections to the muscle sheet of an epitheliomuscular cell, a nematocyte, and a ganglion cell. Moreover, it has sensory cilia and neurosecretory granules (drawing based on Westfall et al. 1971; Westfall 1973; Westfall and Kinnamon 1978). The arrows show synapses and their polarities. experimental systems: regenerating, repopulating, budding, and normal. Nerve-net formation progresses under different conditions depending on the system, therefore we can clarify important factors at the cellular level by comparing nervenet formation (Koizumi et al. 1990; Koizumi and Minobe 1992; Minobe et al. 1995). Regeneration system Hydra has a high capacity to regenerate, and a completely new nerve net appears when the new tissue regenerates. In this case, nerve-net formation progresses with morphogenesis of the tissue, which brings about changes in the epithelial environment of nerve precursor cells and nerve cells. In this system, the importance of epithelial cells for the formation of the nerve net was demonstrated (Bode et al. 1988a; Koizumi et al. 1990; Koizumi and Minobe 1992). Nerve-net formation during head regeneration was examined by means of immunohistochemistry, using an antiserum against the neuropeptide RFamide. In the head of an intact hydra, there are RFamide-like immunoreactive (RFamide+) sensory cells at the apex of the hypostome and RFamide+ ganglion cells in the lower part of the hypostome and the tentacle. The formation of the nerve net specific to the head progresses in two steps during head regeneration. The first step is the early appearance of ganglion cells at the apex. The second step is the late appearance of sensory cells at the apex and the simultaneous disappearance of ganglion cells from the apex (Fig. 6). This sequential patterning corresponds to the behavior of epithelial cells as defined by a monoclonal antibody, TS19, which binds only to ectodermal epithelial cells in the tentacle. The labeling pattern of TS19 progresses in two steps during head regeneration. First, TS19+ epithelial cells appear at the apex, and later TS19+ epithelial cells disappear from the apex and are present only in the ten- tacles. The similar two-step pattern of nerve-net formation and the appearance of tentacle-specific epithelial cells during head regeneration suggests that they have common control mechanisms (Bode et al. 1988a). Nerve-net formation was examined during head regeneration in three morphogenetic mutants, head-regeneration-deficient mutants, budding-deficient mutants, and multiheaded mutants, isolated by Dr. Sugiyama’s group in Mishima, Japan (Sugiyama and Fujisawa 1977). The results showed that the two steps of nerve-net formation are distinct processes that can change independently. Moreover, abnormalities in the behavior of tentacle-specific epithelial cells in the mutants correspond well to the formation of their nerve net (Koizumi et al. 1990). These data strongly suggest that a common mechanism controls the pattern of nerve-net formation and tentacle-specific epithelial cells. It is possible to make hydra chimeras in which all epithelial cells are from a wild type and all nerve cells are from a mutant or vice versa (Marcum and Campbell 1978a, 1978b; Sugiyama and Fujisawa 1978b, 1979). Nerve-net formation during head regeneration in these chimeras was examined to determine whether epithelial cells or neurons are the primary cause of abnormal nerve-net formation in mutants. The results show that the epithelial cells are responsible for abnormalities in nerve-net formation (Koizumi et al. 1990). Nerve-net formation in chimeric strains (produced between wild-type and mutant strains) demonstrated that it is controlled by the environment provided by epithelial cells during head regeneration. All of the above experimental results suggest that interactions between epithelial cells and nerve precursor cells are important for nerve-cell differentiation and nerve-net formation in hydra (Bode et al. 1988a; Koizumi et al. 1990; Koizumi and Minobe 1992). © 2002 NRC Canada J:\cjz\cjz8010\Z02-134.vp Wednesday, November 13, 2002 11:06:12 AM Color profile: Generic CMYK printer profile Composite Default screen Koizumi 1681 Fig. 3. Nerve net in hydra, visualized immunohistochemically. The patterns of the nerve nets differ depending on their position. (A) RFamide-like immunoreactive (RFamide+) nerve net in the hypstome. View from above the hypostome, showing the dark mouth region in the center surrounded by a large number of epidermal sensory cells. (B) RFamide+ nerve net of the upper body column. Scale bars = 50 µm. Repopulation system We can examine nerve-net formation in a unique system called the repopulation system. Many of hydra’s highly specific cell types (neurons, nematocytes, gland cells, and gametes) are part of a single lineage of cells that is continually being renewed by proliferation and differentiation of stem cells called interstitial cells (Fig. 5). The entire interstitialcell lineage can be removed from a hydra by various means (Campbell 1976; Sugiyama and Fujisawa 1978a). The resulting animal is termed an epithelial hydra and is composed of only ectodermal and endodermal epithelial cells. This viable epithelial shell can then be repopulated by interstitial cells, since they migrate into the depleted animal from a small temporary graft of normal tissue (Marcum and Campbell 1978a, 1978b; Sugiyama and Fujisawa 1978b, 1979). Using the repopulation system, therefore, neuronal differentiation and nerve-net formation can be observed in neuronfree tissue. Figure 7 shows the sequential appearance of nerve cells in the head that was observed. Various behavioral responses corresponding to the sequences were recovered sequentially. This “epithelial hydra host” provides an excellent experimental system for examining the relative roles of the epithelial factors described in the previous section (Regeneration system). Nerve-net formation in this system, in contrast to the regeneration system, occurs without morphogenesis of © 2002 NRC Canada J:\cjz\cjz8010\Z02-134.vp Wednesday, November 13, 2002 11:06:17 AM Color profile: Generic CMYK printer profile Composite Default screen 1682 Can. J. Zool. Vol. 80, 2002 Fig. 4. All types of ectodermal nerve cells in hydra. Neurons localized in the same place are illustrated separately in A–C. (A) Ganglion cells. (B) Nerve ring and other unique neurons. (C) Sensory cells. Fig. 5. The three cell lineages in hydra. (A) The ectodermal epithelial cell lineage. (B) The endodermal epithelial cell lineage. (C) The interstitial cell lineage, containing interstitial cells and their differentiation products. Interstitial cells consist of multipotent stem cells (I stem), committed precursor cells, and differentiating intermediates. Nv, nerve cells; GC, ganglion cells; SC, sensory cells; Nc, nematocyte; GL, gland cells. epithelial tissue because the epithelial hydra host maintains the established morphology (Marcum and Campbell 1978a; Sugiyama and Fujisawa 1978b). If epithelial cells indeed play an important role in producing region-dependent nerve-cell differentiation, we would expect nerve-cell differentiation in the epithelial hydra host to have similar regional specificity to that of a normal hydra. However, such specificity would not be expected if neurons instead of epithelial cells are the major factor determining the regional distribution of neurons. Instead, a random or totally different neuron-differentiation pattern would be expected. It has been reported that nerve-cell differentiation in epithelial hydra hosts occurs in the same region-dependent manner as in a normal hydra, thus providing direct evidence for the role of epithelial cells in regulating nerve-cell differentiation (Minobe et al. 1995). Normal (and adult) system In an adult hydra, neurons arise continuously by differentiation from multipotent stem cells among the interstitial cells (David and Gierer 1974; Bode and David 1978; Bode 1992). Interstitial cells committed to neuron differentiation divide to form a pair of small interstitial cells, which in turn divide and subsequently form neurons (Bode et al. 1990; David and Hager 1994). To compare the neuron-production rates, animals were pulse-labeled with BrdU, and the labeling index of various subsets of neurons was measured periodically. A particular subset was identified by doublelabeling cells with an antibody against BrdU and with an antibody against a particular type of neuron. To analyze the kinetics of neuronal differentiation, animals initially labeled with BrdU were subsequently labeled with an antibody against BrdU (Plickert and Kroiher 1988) and either DB5 or RC9. BrdU (1 mM) dissolved in hydra © 2002 NRC Canada J:\cjz\cjz8010\Z02-134.vp Wednesday, November 13, 2002 11:06:20 AM Color profile: Generic CMYK printer profile Composite Default screen Koizumi 1683 Fig. 6. Reappearance of the RFamide+ nerve net during head regeneration in hydra. The numbers indicate days after decapitation. culture medium was injected into the gastric cavity of a hydra. To measure the labeling index of neurons, labeled nuclei were visualized with a monoclonal antibody against BrdU (mouse IgG) and the cytoplasm of neurons with the monoclonal antibody DB5 or RC9 (both mouse IgMs). The difference in isotypes as well as the sequence of steps reduced cross-reactivity to background levels in the doublelabeling procedure (Fig. 8) (Koizumi et al. 1992). The epithelial hydra host is also an excellent system for examining the relative roles of neuronal factors. Since an epithelial hydra is completely devoid of nerve cells, neuron differentiation occurs in the absence of any influence from existing neurons. In contrast, nerve-cell differentiation occurs in the presence of fully matured nerve cells in the normal (and adult) system. If there are differences in nerve-cell differentiation and nerve-net formation between the repopulation system and the normal system, we can assess the role of mature nerve cells in nerve-cell differentiation. Other systems that can be used in the study of nerve-net formation are the budding system and dissociation/reaggregation system. Budding is the mode of asexual reproduction used by hydra. A second axis formed by simple protrusion from the body column produces the head, body column, and foot sequentially during budding (Otto and Campbell 1977). In the dissociation/reaggregation system, a normal hydra can regenerate from a cell mass after being dissociated into single cells and then reaggregated by centrifugation (Gierer et al. 1972). In both cases a normal nerve net eventually develops. Mechanisms controlling nerve-net formation at the molecular level To examine the molecular mechanisms of nerve-net formation, a joint project, “Large-scale non-targeting screening of peptide signal molecules in hydra”, has started (Takahashi et al. 1997; Bosch and Fujisawa 2001). Takahashi et al. (1997) developed a novel procedure for systematically isolating peptide signal molecules from hydra. Peptides were extracted from large numbers of hydra, purified to homogeneity using high-performance liquid chromatography (HPLC) without any biological assays. The isolated peptides were subjected to structural analysis using automated amino-acid analysis and then synthesized chemically, and the identity of synthetic peptides with native peptides was confirmed using HPLC. The synthetic peptides were then subjected to a series of biological test to examine their functions in hydra. Using this approach, a number of peptides have been identified that regulate development in hydra in addition to neuropeptides controlling synaptic transmission and muscle contraction (Table 1) (Takahashi et al. 1997, 2000; Yum et al. 1998; Grens et al. 1999; Bosch and Fujisawa 2001; Harafuji et al. 2001). Among these peptides, some that control nerve differentiation in hydra were identified. Some belong to the LPW family, and another peptide is Hym355 (FPQSFLPRGamide). A group of 4 peptides belong to the LPW family, which have 5–8 amino-acid residues and share the common Nterminal structure of L(or I)PW. All LPW peptides inhibit nerve-cell differentiation in hydra (Takahashi et al. 1997). In contrast, Hym355 activates nerve differentiation in hydra (Takahashi et al. 2000). Immunohistochemical analysis using antibodies to these peptides shows that Hym355 is a neuropeptide localized in nerve cells but LPW peptides are epitheliopeptides which are localized in epithelial cells. Cotreatment with a LPW peptide and Hym355 nullified the effect of both peptides, which suggests that they act in an antagonistic manner (Takahashi et al. 2000). Figure 9 illustrates the sequence of nerve differentiation and the effects of peptides. Interstitial multipotent stem cells © 2002 NRC Canada J:\cjz\cjz8010\Z02-134.vp Wednesday, November 13, 2002 11:06:24 AM Color profile: Generic CMYK printer profile Composite Default screen 1684 Can. J. Zool. Vol. 80, 2002 Fig. 7. Reappearance of the nerve net in the repopulation system. (A) RFamide+ nerve net. (B) RC9+ nerve net. RC9 is a monoclonal antibody specific to nerve precursor cells (interstitial cells) and ganglion cells. The time (hours or days) elapsed after nerve precursor cells were grafted into the epithelial host is indicated. Both are views from above the hypostome. (I stem) are committed to nerve differentiation in the body column; the committed nerve precursor cells (I Nv) migrate into the head and foot and then differentiate into nerve cells (Nv) (Bode et al. 1990; Teragawa and Bode 1990; David and Hager 1994; Bode 1996). At the same time they differentiate into the type of neuron appropriate for the final location (Bode 1996). Hym355 released from nerve cells activates nerve-cell differentiation, but LPW peptides from © 2002 NRC Canada J:\cjz\cjz8010\Z02-134.vp Wednesday, November 13, 2002 11:06:29 AM Color profile: Generic CMYK printer profile Composite Default screen Koizumi 1685 Fig. 8. Double-labeling of BrdU-labeled nuclei and nerve cells using the BrdU antibody and the neuron-specific monoclonal antibody RC9. BurU-labeled nuclei were visualized by fluorescein (green fluorescence) and nerve cells were labeled by Texas red (red fluorescence). BrdU-labeled nuclei of double-stained nerve cells appear yellow with a dual-band filter. (A) Hypostome. (B) Tentacles. Scale bar = 50 µm. epithelial cells inhibit it. Both might interact and act antagonistically (Takahashi et al. 2000; Bosch and Fujisawa 2001). The next step of the study is to clarify the exact site of action. Is it commitment, migration, or differentiation? A unique neural structure: the nerve ring in the perihypostomal region of hydra The nerve ring in the hypostome of hydra was observed immunocytochemically using an antiserum against neuropeptides and neuron-specific monoclonal antibodies (Fig. 10). The nerve ring in the mesh-like nerve net of hydra is unique. It is a distinct neuronal complex consisting of a thick nerve bundle running circumferentially at the border between the hypostome and the tentacle zone. Immunolabeling showed that the nerve ring is heterogeneous and contains at least four different subsets of neurons. During head regeneration and budding, the nerve ring appeared only after the nerve © 2002 NRC Canada J:\cjz\cjz8010\Z02-134.vp Wednesday, November 13, 2002 11:07:01 AM Color profile: Generic CMYK printer profile Composite Default screen 1686 Can. J. Zool. Vol. 80, 2002 Table 1. Hydra peptides whose function was identified. Peptide Structure Function Reference Hym323 KWVQGKPTGEVKQIKF Morphogenesis Foot activation Harafuji et al. 2001 Hym346 Hym301 AGEDVSHELEEKEKALANHS KPPRRCYLNGYCSPa LPW family Hym33H Hym35 Hym37 Hym310 Hym355 Hym176 AALPW EPSAAIPW SPGLPW DPSALPW FPQSFLPRGa APFIFPGPKVa GLWamide family Hym53 Hym54 Hym248 Hym249 Hym331 Hym338 Hym370 NPYPGLWa GPMTGLWa EPLPIGLWa KPIPGLWa GPPPGLWa GPPhPGLWa KPNAYKGKLPIGLWa Grens et al. 1999 Morphogenesis Tentacle formation Cell differentiation Inhibition of nerve differentiation Activation of nerve differentiation Muscle contractions Ectoderm Takahashi et al. 1997 Takahashi et al. 2000 Yum et al. 1998 Takahashi et al. 1997 Fig. 9. Effects of peptide signal molecules on nerve differentiation in hydra. A neuropeptide, Hym355, activates nerve differentiation, while epithelial peptides, the LPW family, inhibit it. I stem, interstitial stem cell; I Nv, committed nerve precursor cells; Nv, differentiated nerve cells. Bud detachment These associations and quiet dynamics lead us to think the nerve ring has features that closely resemble those of the central nervous system in higher animals. Conclusion net of ganglion and sensory cells had formed (Koizimi et al. 1992). The ectoderm in the immediate vicinity of and including the nerve ring constitutes a stationary zone that is not displaced. Tissue immediately above this zone is displaced towards the tip of the hypostome, while tissue below is displaced along the tentacles. Correspondingly, the production of new neurons in the ring, measured by their differentiation kinetics, is much slower than in surrounding areas. Thus, the nerve ring is static and stable in contrast to the dynamic features of the nerve net of hydra (Koizumi et al. 1992). To understand a certain nervous system, both interdisciplinary and overall neurobiological study is essential. Study of the formation of the nervous system (developmental neurobiology) is essential in addition to studies of structure (neuroanatomy) and function (neurophysiology, neuroethology, and behavioral physiology). In addition, studies at various levels are desirable, from the molecular level to the whole-animal level. According to current overall neurobiological studies of the nervous system of hydra, unique features of the neurons in this primitive nervous system have appeared. Each neuron in hydra has the general properties of a cell, while neurons in higher animals are highly specialized. Each neuron in hydra has a complete set of nerve functions. Each neuron has neurites as nerve fibers, but the differences between dendrites and axons present in higher animals are not observed in hydra. Neurons in hydra show constant birth and death and constant displacement. They show considerable changes in phenotype under the influence of the environment. Hence, they show active developmental dynamics and plastic properties. Because of these properties we can use unique experimental systems, such as the regeneration, budding, repopulation, normal, and dissociation–reaggregation systems, for studying nerve-net formation. In the near future, descriptions of nerve-net formation in hydra will be made possible by combining studies at the molecular, cellular, and system levels. © 2002 NRC Canada J:\cjz\cjz8010\Z02-134.vp Wednesday, November 13, 2002 11:07:02 AM Color profile: Generic CMYK printer profile Composite Default screen Koizumi 1687 Fig. 10. RFamide+ nerve ring observed in the hypostome of Hydra oligactis. Scale bar = 100 µm. References Bode, H.R. 1992. Continuous conversion of neuron phenotype in hydra. Trends Genet. 8: 279–284. Bode, H.R. 1996. The interstitial cell lineage of hydra: a stem cell system that arose early in evolution. J. Cell Sci. 109: 1155– 1164. Bode, H.R., and David, C.N. 1978. Regulation of a multipotent stem cell, the interstitial cell of hydra. Prog. Biophys. Mol. Biol. 33: 189–206. Bode, H.R., Dunne, J., Heimfeld, S., Huang, L., Javois, L., Koizumi, O., Westerfield, J., and Yaross, M., 1986. Transdifferentiation occurs continuously in adult hydra. Curr. Top. Dev. Biol. 20: 257–280. Bode, H.R., Gee L.W., and Chow, M. 1990. Neuron differentiation in hydra involves dividing intermediates. Dev. Biol. 139: 231– 243. Bode, P.M., Awad, T.A., Koizumi, O., Nakashima, Y., Grimmelikhuijzen, C.J.P, and Bode, H.R. 1988a. Development of the two part pattern during regeneration of the head in hydra. Development, 102: 223–235. Bode, H.R., Heimfeld, S., Koizumi, O., Littlefield, C.L., and Yaross, M.S. 1988b. Maintenance and regeneration of the nerve net in hydra. Am. Zool. 28: 1053–1063. Bosch, T.C.G., and Fujisawa, T. 2001. Polyp, peptides and patterning. BioEssays, 23: 420–427. Burnett, A.L., and Diehl, N.A. 1964. The nervous system of Hydra. I. Types, distribution and origin of nerve elements. J. Exp. Zool. 157: 217–226. Campbell, R.D. 1967a. Tissue dynamics of steady-state growth in Hydra littoralis. I. Patterns of cell division. Dev. Biol. 15: 487– 502. Campbell, R.D. 1967b. Tissue dynamics of steady state growth in Hydra littoralis. II. Patterns of tissue movement. J. Morphol. 121: 19–28. Campbell, R.D. 1973. Vital marking of single cells in developing tissues: India ink injection to trace tissue movements in Hydra. J. Cell Sci. 23: 651–661. © 2002 NRC Canada J:\cjz\cjz8010\Z02-134.vp Wednesday, November 13, 2002 11:07:07 AM Color profile: Generic CMYK printer profile Composite Default screen 1688 Campbell, R.D. 1976. Elimination of hydra interstitial and nerve cells by means of colchicine. J. Cell Sci. 21: 1–13. Campbell, R.D., and David, C.N. 1974. Cell cycle kinetics and development of Hydra attenuata. II. Interstitial cells. J. Cell Sci. 16: 349–358. David, C.N., and Gierer, A. 1974. Cell cycle kinetics and development of Hydra attenuata. III. Nerve and nematocyte differentiation. J. Cell Sci. 16: 359–375. David, N.D., and Hager, G. 1994. Formation of a primitive nervous system: nerve cell differentiation in the polyp hydra. Perspectives Dev. Neurobiol. 2: 135–140. David, C., and Murphy, S. 1977. Characterization of interstitial stem cells in hydra by cloning. Dev. Biol. 58: 372–383. Davis, L.E., Burnett, A.L., and Haynes, J.F. 1968. Histological and ultrastructural study of the muscular and nervous system in Hydra. II. Nervous system. J. Exp. Zool. 162: 295–332. Dunne, J., Javois, L.C., Huang, L.W., and Bode, H.R. 1985. A subset of cells in the nerve net of Hydra oligactis defined by a monoclonal antibody: its arrangement and development. Dev. Biol. 109: 41–53. Gierer, A., Berking, S., Bode, H., David, C.N., Flick, K., Hansmann, G., Schaller, H., and Trenkner, E. 1972. Regeneration of hydra from reaggregated cells. Nature (London) New Biol. 239: 98–101. Grens, A., Shimizu, H., Hoffmeister, S.A.H., Bode, H.R., and Fujisawa, T. 1999. The novel signal peptides, Pedibin and Hym346, lower positional value thereby enhancing foot formation in hydra. Development, 126: 517–524. Grimmelikhuijzen, C.J.P. 1985. Antisera to the sequence Arg-Pheamide visualize neuronal centralization in hydroid polyps. Cell Tissue Res. 241: 171–182. Grimmelikhuijzen, C.J.P. and Westfall, J.A. 1995. The nervous systems of cnidarians. In The nervous systems of invertebrates: an evolutionary and comparative approach. Edited by O. Bredbach and W. Kutsch. Birkhauser Verlag, Basel, Switzerland. pp. 7–24. Grimmelikhuijzen, C.J.P., Dierickx, K., and Boer, G.J. 1982. Oxytocin/vasopression-like immunoreacitivity in the nervous system of Hydra. Neuroscience, 7: 3191–3199. Grimmelikhuijzen, C.J.P., Graff, D., Koizumi, O., and McFarlane, I.D. 1990. Neurons and their peptide transmitters in coelenterates. In Evolution of the first nervous systems. Edited by P.A.V. Anderson. Plenum Press, New York. pp. 95–109. Hadñi, H. 1909. Über das Nervensystem von Hydra. Arb. Zool. Inst. Univ. Wien, 17: 225–268. Harafuji, N., Takahashi, T., Hatta, M., Tezuka, H., Morishita, F., Matsushima, O., and Fujisawa, T. 2001. Enhancement of foot formation in Hydra by a novel epitheliopeptide, Hym323. Development, 128: 437–446. Koizumi, O., and Bode, H.R. 1986. Plasticity in the nervous system of adult hydra. I. The position-dependent expression of FMRFamide-like immunoreactivity. Dev. Biol. 116: 407–421. Koizumi, O., and Bode, H.R. 1991. Plasticity in the nervous system of adult hydra. III. Conversion of neurons to expression of a vasopressin-like immunoreactivity depends on axial location. J. Neurosci. 11: 2011–2020. Koizumi, O., and Minobe, S. 1992. Important role of epithelial cells in the formation and maintenance of nerve net in the primitive nervous system of hydra. In Molecular basis of neural connectivity. Edited by M. Satake, K. Obata, H. Hatanaka, E. Miyamato, and T. Okuyama. Kohko-Do, Niigata, Japan. pp. 11–14. Koizumi, O., Heimfeld, S., and Bode, H.R. 1988. Plasticity in the nervous system of adult hydra. II. Conversion of ganglion cells of the body column into epidermal sensory cells of the hypostome. Dev. Biol. 129: 358–371. Can. J. Zool. Vol. 80, 2002 Koizumi, O., Mizumoto, H., Sugiyama., T., and Bode, H.R. 1990. Nerve net formation in the primitive system of hydra: an overview. Neurosci. Res. Suppl. 13: 165–170. Koizumi, O., Itazawa, M., Mizumoto, H., Minobe, S., Javois, J.C., Grimmelikhuijzen, C.J.P., and Bode, H.R. 1992. The nerve ring of the hypostome in hydra. I. Its structure, development and maintenance. J. Comp. Neurol. 326: 7–21. Lentz, T. 1968. Primitive nervous systems. Yale University Press, New Haven, Conn., and London. Lentz, T.L., and Barrnett, R. 1965. Fine structure of the nervous system of Hydra. Am. Zool. 5: 341–356. Mackie, G.O., and Passano, L.M. 1968. Epithelial conduction in hydromedusae. J. Gen. Physiol. 52: 600–621. Marcum, B.A., and Campbell, R.D. 1978a. Development of hydra lacking nerve and interstitial cells. J. Cell Sci. 29: 17–33. Marcum, B.A., and Campbell, R.D. 1978b. Developmental roles of epithelial and interstitial cell lineages in hydra: analysis of chimeras. J. Cell Sci. 32: 233–247. Minobe, S., Koizumi, O., and Sugiyama, T. 1995. Nerve cell differentiation in nerve-free tissue of epithelial hydra from precursor cells introduced by grafting. I. Tentacles and hypostome. Dev. Biol. 172: 170–181. Otto, J.J., and Campbell, R.D. 1977. Budding in Hydra attenuata: bud stages and fate map. J. Exp. Zool. 200: 417–428. Plickert, G., and Kroiher, M. 1988. Proliferation kinetics and cell lineage can be studied in whole mounts and macerates by means of BrdU/ anti BrdU technique. Development, 103: 791–794. Sugiyama, T., and Fujisawa, T. 1977. Genetic analysis of developmental mechanisms in hydra. I. Sexual reproduction of Hydra magnipapillata and isolation of mutants. Dev. Growth Differ. 19: 187–200. Sugiyama, T., and Fujisawa, T. 1978a. Genetic analysis of developmental mechanisms in hydra. II. Isolation and characterization of an interstitial cell-deficient strain. J. Cell Sci. 29: 35–52. Sugiyama, T., and Fujisawa, T. 1978b. Genetic analysis of developmental mechanisms in hydra. V. Cell lineage and development of chimera hydra. J. Cell Sci. 32: 215–232. Sugiyama, T., and Fujisawa, T. 1979. Genetic analysis of developmental mechanisms in hydra. VI. Cellular composition of chimera hydra. J. Cell Sci. 35: 1–15. Takahashi, T., Muneoka, Y., Lohmann, J., deHaro, L.M., Solleder, G., et al. 1997. Systematic isolation of peptide signal molecules regulating development in hydra: LWamide and PW families. Proc. Natl. Acad. Sci. U.S.A. 94: 1241–1246. Takahashi, T, Koizumi, O., Ariura, Y., Romanovitch, A., Bosch, T.C.G., Kobayakawa, Y., Mohri, S., Bode, H., Yum, S., Hatta, M., and Fujisawa, T. 2000. A novel neuropeptide, Hym355, positively regulates neuron differentiation in hydra. Development, 127: 997–1005. Teragawa, C.K., and Bode, H.R. 1990. Spatial and temporal patterns of interstitial cell migration in Hydra vulgaris. Dev. Biol. 138: 63–81. Westfall, J. 1973. Ultrastructural evidence for a granule-containing sensory-motor-interneuron in Hydra littoralis. J. Ultrastruct. Res. 42: 268–282. Westfall, J., and Kinammon, J.C. 1978. A second sensory-motorinterneuron with neurosecretory granules in Hydra. J. Neurocytol. 7: 365–379. Westfall, J., Yamataka, S., and Enos, P.D. 1971. Ultractructural evidence of polarized synapses in the nerve net of hydra. J. Cell Biol. 51: 318–323. Yaross, M.S., Westerfield, J., Javois, L.C., and Bode, H.R. 1986. Nerve cells in hydra: monoclonal antibodies identify two lin© 2002 NRC Canada J:\cjz\cjz8010\Z02-134.vp Wednesday, November 13, 2002 11:07:07 AM Color profile: Generic CMYK printer profile Composite Default screen Koizumi eages with distinct mechanisms for their incorporation into head tissue. Dev. Biol. 114: 225–237. Yum, S, Takahashi, T., Koizumi, O., Ariura, Y., Kobayakawa, Y., 1689 Mohri, S., and Fujisawa, T. 1998. A novel neuropeptide, Hym176, induces contraction of the ectodermal muscle in Hydra. Biochem. Biophys. Res. Comm. 248: 584–590. © 2002 NRC Canada J:\cjz\cjz8010\Z02-134.vp Wednesday, November 13, 2002 11:07:07 AM