Ultrastructural observations on gastrodermal regeneration in the planarian Dugesia japonica (Turbellaria)

The earliest detectable change during regeneration of the gastrodermis in Dugesia japonica was an aggregation of regenerative cells underneath the gastrodermis remaining at the wound margin. The gastrodermal cells in experimental regenerates retained some of their original characters and presented no indication of cell dedifferentiation. The regenerative cells came into contact with the basal surface of gastrodermal cells, forming stratified cell layers. Differentiation of these cells into gastrodermal cells was initiated by the development of synthetic organelles within their cytoplasm. These differentiating cells gave rise to two different types of gastrodermal cells, namely phagocytic cells and sphere cells. In later stages, there was an apparent movement of differentiated gastrodermal cells towards the parenchyma.     

Quantitative analysis of cell types during growth,degrowth and regeneration in the planarians Dugesia mediterranea and Dugesia tigrina

A method of tissue maceration (dissociation) of planarian tissues into single cells was used to characterize the basic cell types in the planarians Dugesia mediterranea and Dugesia tigrina, and to determine the total cell number and distribution of cell types during growth, degrowth and regeneration.Using this method, 13 basic cell types have been determined for both species. The total number of cells increases with body length and volume whereas the distribution of cell types is only slightly affected. Growth and degrowth occur mainly through changes in total cell number leaving cell distribution only moderately affected. During regeneration, an increase in neoblast density in the blastema followed later on by increases in nerve cells are the more significant changes detected.These results are discussed in relation to mechanisms of cell renewal, blastema formation and maintenance of tissue polarity.Abbreviations nb neoblasts - nv nerve cells - ep epidermal cells - fp fixed parenchyma cells - g gastrodermal cells     

Histological and ultrastructural studies of the basal disk of Hydra

Summary The gastrodermis and mesoglea of the basal disk of Hydra were investigated to conclude a three-part series of papers. The gastrodermis is composed of digestive cells (most predominant cell type), mucous and nerve cells (both immature and fully differentiated). The principal function of the digestive cells appears to be storage of protein, lipid and glycogen reserves which are utilized by neighboring cells. Mucous cells apparently use some of the reserves to synthesize their secretions which lubricate cells and prevent cell damage during egestion of waste through the aboral pore. The function of the gastrodermal nerve cells is uncertain.The mesoglea of the basal disk, contains the same structural components as seen in other regions of the polyp. It is reasonable to assume that it maintains the same function of cell adhesion and migration. As the mesoglea converges on the aboral pore, it loses its structural integrity and cells are sloughed off the column.This investigation was supported by The National Science Foundation, Grant Number GB-27395.     

FMRFamide-like immunoreactivity in the sea-fan <Emphasis Type="Italic">Eunicella cavolini</Emphasis> (Cnidaria: Octocorallia)

The presence of FMRFamide-related peptides (FaRPs) was investigated, by immunohistochemical methods with a polyclonal FMRFamide antiserum, in the sea-fan Eunicella cavolini (Van Koch 1887), a representative of the cnidarians (octocorallians). The identification of FaRP-immunoreactive elements as neuronal cells and a nerve net was performed by double immunohistochemical methods with the monoclonal anti-beta-tubulin antibody. A strong and widely distributed FaRPs immunoreactivity was detected: FaRPs-immunoreactive nerve cells were observed among and underlying gastrodermal epithelial cells, epidermal cells lining tentacles, muscular septs and gonophores. A diffuse FaRPs-immunoreactive nerve net was also found between epithelia and mesoglea and in the stalk of the gonophore. These results improve our knowledge of the gorgonian nervous system and demonstrate that most of the immunoreactive cells belong to neural elements.     

Ultrastructure of neurons and synapses in the tentacle gastrodermis of the sea anemone Calliactis parasitica

Little is known about gastrodermal neurons and synapses in the tentacles of sea anemones. Using transmission electron microscopy of serial thin sections of Calliactis parasitica, we have identified both a sensory cell and a ganglion cell with granular vesicles originating from the Golgi complex and have identified four types of synapses in the tentacular gastrodermal nerve plexus. The sensory cell has a recessed apical cilium with a basal body and a perpendicularly oriented centriole, below which are several strands of striated rootlets surrounded by mitochondria. The ganglion cell lacks a cilium and resembles a bipolar neuron, with oppositely directed processes lying parallel to the basally located circular smooth muscle. Both one-way and two-way interneuronal synapses are present with 60- to 90-nm granular vesicles of various densities aligned at the paired electron-dense membranes and fine cross filaments in the intervening 13-nm cleft. Two types of neuroeffector synapses have been located. Dense granular vesicles are present at neuromuscular synapses, whereas less dense vesicles are present at neuroglandular synapses. Most of the synaptic vesicles range from 60 to 120 nm in diameter. Two types of nerve cells and a variety of synaptic loci provide morphological substrates for the spontaneous SS2 conduction pulses in the tentacular gastrodermis of C. parasitica. J Morphol 231:217–223, 1997. © 1997 Wiley-Liss, Inc.     

Production of cell- and tissue-specific monoclonal antibodies in the freshwater planarian Phagocata vivida

To provide a tool for studying regeneration in planarians, we have produced monoclonal antibodies against a variety of cells and tissues of the freshwater planarian Phagocata vivida (Ijima et Kaburaki). We obtained five kinds of monoclonal antibodies specific, respectively, to 1) the excretory system, 2) nerve cells, 3) rhabdoid-forming cells and body-surface mucus, 4) gastrodermal and epidermal cells, and 5) male germ cells and epidermis.     

The fine structure of the perineural endothelium

Summary Fine strands of motor nerves were examined with the electron microscope using thin section as well as freeze-etching techniques. The specimens were taken from frog cutaneous pectoris nerve, rat sciatic nerve, mouse and shrew phrenic nerves and from human skin nerves. The perineural sheath (Henle, Ranvier, Key and Retzius) consists of one to several concentric laminae of endothelial cells; it encases nerve fascicles and eventually individual nerve fibers and terminals. The endothelial cells are extremely thin and fitted together smoothly by overlap and dove-tailing of their border zones. The cell contacts are formed by continuous zonulae occludentes, often reinforced by maculae adhaerentes, and in depth they comprise 3–15 strands with an average of 5–6 strands per junction. The membranes of endothelial cells are studded with attachment sites and stomata of plasmalemmal vesicles suggesting a high level of pinocytotic activity. This phenomenon is by no means restricted to the external laminae of the endothelial sheath. Each endothelial lamina is vested with basement membranes on both (epineural and endoneural) sides, and the spaces between laminae contain a few collagen fibers and fibroblasts. Occasionally, punctate tight junctions are seen between laminae. Cytological evidence supports the hypothesis that the perineural endothelium provides a relatively tight and highly selective barrier separating the peripheral nerves from surrounding tissue and its extracellular fluid spaces. This effect is achieved on the one hand by the sealing of pericellular spaces and on the other hand by a membrane controlled transcellular transport mechanism (pinocytosis), both of which are enhanced by their serial arrangement.Dedicated to Professor Wolfgang Bargmann, Kiel, on the occasion of his 70th birthday.The technical assistance of Dr. F. Dreyer, Mr. D. Savini, Miss H. Claassen and Miss R. Emch is gratefully acknowledged.Financial support was received by the following institutions: Swiss National Foundation for Scientific Research, grants Nrs. 3.368.0.74, 3.774.72, 3.259.74, 3.045.73. Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 38, Projekt N). The Dr. Eric Slack-Gyr Stiftung in Zürich and the Hartmann-Müller Stiftung for Medical Research in Zürich.     

Innervation patterns of the cerebral nerves in <Emphasis Type="Italic">Haminoea hydatis</Emphasis> (Gastropoda: Opisthobranchia): a test for intraspecific variability

This study describes the innervation patterns for the cerebral nerves which project to the cephalic sensory organs (CSOs) in the opisthobranch Haminoea hydatis (Linnaeus 1758) and uses axonal tracing techniques (backfilling) to reveal the central cellular origins for these cerebral nerves. Cell clusters projecting into the cerebral nerves can be defined by their positions in the ganglion relative to other clusters, nerve roots and lobes. The number of cell clusters and the relative sizes of somata are constant in a given cluster, whereas the absolute number of somata and absolute sizes of single somata in a given cluster increase with the size of the animal. Additionally, the invariable morphological characteristics of the cell clusters are used to define criteria for the assessment of possible homology for the clusters innervating the CSOs in Opisthobranchia. The data suggest two different strategies to accommodate the increasing body size; first, the additions of nerve cells and second, the growth of nerve cells. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

An immunocytochemical method for histamine: application to the planarians

Histamine-immunoreactivity was investigated in the planarians Dugesia tigrina and Polycelis nigra. Specific antisera against a histamine-protein conjugate were used, and 1-ethyl—3 (3-dimethyl-aminopropyl) carbodiimide was used both as coupling agent to prepare the antigen and as a tissue fixative. In D. tigrina, histamine-immunoreactivity was restricted to photoreceptor cells in the cerebral eye. In P. nigra, nerve fibers were found in the ventral nerve cord and nerves running laterally from these. The epidermal eyes did not display histamine-immunoreactivity. The results suggest that histamine may be a transmitter in some of the most primitive animals. They also suggest that the distribution of histamine may differ in planarians.     

Crustacean cardioactive peptide in the nervous system of the locust,Locusta migratoria: an immunocytochemical study on the ventral nerve cord and peripheral innervation

Summary Crustacean cardioactive peptide-immunoreactive neurons occur in the entire central nervous system of Locusta migratoria. The present paper focuses on mapping studies in the ventral nerve cord and on peripheral projection sites. Two types of contralaterally projecting neurons occur in all neuromers from the subesophageal to the seventh abdominal ganglia. One type forms terminals at the surface of the thoracic nerves 6 and 1, the distal perisympathetic organs, the lateral heart nerves, and on ventral and dorsal diaphragm muscles. Two large neurons in the anterior part and several neurons of a different type in the posterior part of the terminal ganglion project into the last tergal nerves. In the abdominal neuromers 1–7, two types of ipsilaterally projecting neurons occur, one of which gives rise to neurosecretory terminals in the distal perisympathetic organs, in peripheral areas of the transverse, stigmata and lateral heart nerves. Four subesophageal neurons have putative terminals in the neurilemma of the nervus corporis allati II, and in the corpora allata and cardiaca. In addition, several immunoreactive putative interneurons and other neurons were mapped in the ventral nerve cord. A new in situ whole-mount technique was essential for elucidation of the peripheral pathways and targets of the identified neurons, which suggest a role of the peptide in the control of heartbeat, abdominal ventilatory and visceral muscle activity.Abbreviations AG abdominal ganglia - AM alary muscle - AMN alary muscle nerve - CA corpus allatum - CC corpus cardiacum - dPSO distal perisympathetic organ - LHN lateral heart nerve - LT CCAP-immunoreactive lateral tract - NCA nervus corporis allati - NCC nervus corporis cardiaci - NM neuromer - PMN paramedian nerve - PSO perisympathetic organ - SOG subesophageal ganglion - VDM ventral diaphragm muscles - VNC ventral nerve cord     

A comparative study of neuropeptides in the intestine of two stomachless teleosts (Poecilia reticulata,Leuciscus idus melanotus) under conditions of feeding and starvation

Summary Endocrine cells containing bombesin-, enkephalin-, gastrin/CCK-, 5-HT-, and substance P-like material were demonstrated in the alimentary tract of Poecilia reticulata and Leuciscus idus melanotus. Endocrine cells with neuropeptide-Y-like immunoreactivity were found only in P. reticulata, those with VIP-like immunoreactivity only in L. idus melanotus. Gut nerves showing bombesin-, G/CCK-5-HT-, neurotensin-, substance P-and VIP-like immunoreactivity were observed in both species investigated, enkephalin- and neuropeptide Y-like immunoreactivity in P. reticulata alone. The distribution and amount of endocrine cells and nerves along the gut as visualized with the appropriate antisera varied in both teleosts. Histologically, the intestinal tract of these stomachless fish can be divided into three regions. A large number of endocrine cells with VIP-like immunoreactivity was noted in the rectum of L. idus melanotus. Endocrine cells containing bombesin-, enkepha-lin- and substance P-like material were found only in intestinal parts I and II in L. idus melanotus. Neuropeptide Y-like immunoreactivity was absent from intestinal part I of P. reticulata. The influence of starvation on the immunoreactivity of nerves and enteroendocrine cells in the teleost intestine was examined. After a starvation period of more than 6 weeks, no alterations were observed either in the appearance or amount of nerve and endocrine cell immunoreactivity.     

Expression of mRNAs for Neurotrophins (NGF,BDNF, and NT-3) and their Receptors (p75NGFR,Trk, TrkB,and TrkC) in Human Peripheral Neuropathies

The steady-state mRNA levels of NGF, BDNF and NT-3, and the mRNA levels of their receptors p75^NGFR, trk, trk,B and trkC were examined in various human peripheral neuropathies, to determine the correlation with myelinated fiber pathology and T cell and macrophage invasions in the diseased nerves. Steady state levels of p75^NGFR mRNAs were significantly elevated in nerves with axonal pathology. In contrast, steady state levels of trkB and trkC mRNA levels were diminished, trk mRNA was not detected in the human nerves. The NGF, BDNF, and NT-3 mRNA levels were elevated in the diseased nerves. The increase in BDNF and NT-3 mRNA levels were proportional to the extent of invasion of the nerves by T cells and macrophages, but did not directly correlate with axonal nor demyelinating pathology, thus suggesting that inflammatory cell invasions are involved in the regulation of BDNF and NT-3 mRNA expressions. These neurotrophin and their receptor gene expressions in the diseased human nerves would be regulated by an underlying pathology-related process, and could play a role in peripheral nerve repair.     

The morphology of suboesophageal ganglion cells innervating the nervus corporis cardiaci III of the locust

Summary The nervus corporis cardiaci III (NCC III) of the locust Locust migratoria was investigated with intracellular and extracellular cobalt staining techniques in order to elucidate the morphology of neurons within the suboesophageal ganglion, which send axons into this nerve. Six neurons have many features in common with the dorsal, unpaired, median (DUM) neurons of thoracic and abdominal ganglia. Three other cells have cell bodies contralateral to their axons (contralateral neuron 1–3; CN 1–3). Two of these neurons (CN2 and CN3) appear to degenerate after imaginal ecdysis. CN3 innervates pharyngeal dilator muscles via its anterior axon in the NCC III, and a neck muscle via an additional posterior axon within the intersegmental nerve between the suboesophageal and prothoracic ganglia. A large cell with a ventral posterior cell body is located close to the sagittal plane of the ganglion (ventral, posterior, median neuron; VPMN). Staining of the NCC III towards the periphery reveals that the branching pattern of this nerve is extremely variable. It innervates the retrocerebral glandular complex, the antennal heart and pharyngeal dilator muscles, and has a connection to the frontal ganglion.Abbreviations AH antennal heart - AN antennal nerves - AO aorta - AV antennal vessel - CA corpus allatum - CC corpus cardiacum - CN1, CN2, CN3 contralateral neuron 1–3 - DIT dorsal intermediate tract - DMT dorsal median tract - DUM dorsal, unpaired, median - FC frontal connective - FG frontal ganglion - HG hypocerebral ganglion - LDT lateral dorsal tract - LMN, LSN labral motor and sensory nerves - LN+FC common root of labral nerves and frontal connective - LO lateral ocellus - MDT median dorsal tract - MDVR ventral root of mandibular nerve - MVT median ventral tract - NCA I, II nervus corporis allati I, II - NCC I, II, III nervus corporis cardiaci I, III - NR nervus recurrens - NTD nervus tegumentarius dorsalis - N8 nerve 8 of SOG - OE oesophagus - OEN oesophageal nerve - PH pharynx - SOG suboesophageal ganglion - T tentorium - TVN tritocerebral ventral nerve - VLT ventral lateral tract - VIT ventral intermediate tract - VMT ventral median tract - VPMN ventral, posterior, median neuron - 1–7 peripheral nerves of the SOG - 36, 37, 40–45 pharyngeal dilator muscles     

Distribution of catecholamine-containing,serotonin-like and neuropeptide FMRFamide-like immunoreactive neurons and processes in the nervous system of the actinotroch larva ofPhoronis muelleri (Phoronida)

Summary Glyoxylic-acid-induced fluorescence of catecholamines and antibodies against serotonin and FMRFamide were used to study the distribution of putative neurotransmitters in the actinotroch larva ofPhoronis muelleri Selys-Longchamps, 1903. Catecholamines occur in the neuropile of the apical ganglion, in the longitudinal median epistome nerves, in the epistome marginal nerves, and in the nerve along the bases of the tentacles. The tentacles have laterofrontal and latero-abfrontal bundles of processes that form two minor nerves along the lateral ciliary band of the tentacles, and a medio-frontal bundle of processes. Monopolar cells are located on the ventro-lateral part of the mesosome. Processes are located along the posterior ciliary band and as a reticulum in the epidermis. Serotonin-like immunoreactive cells and processes are located in the apical ganglion, in the longitudinal median epistome nerves, and as a dorsal and ventral pair of bundles along the tentacle bases. Processes from the latter extend into the tentacles as the medioabfrontal processes. The latero-abfrontal processes form a minor nerve along the ciliary band. The dorsal bundles forms the major nerve ring along the tentacles and processes extend from it to the metasome. Processes are located along the posterior ciliary band. FMRFamide-like immunoreactive cells and processes are found in the apical ganglion, in the longitudinal median epistome nerves and as a pair of lateral epistome processes projecting towards the ring of tentacles. In the tentacles, a pair of latero-frontal processes are found; these form a minor nerve along the ciliary band. A band of cells can be seen along the tentacle ring.     

Distribution of PACAP (pituitary adenylate cyclase-activating polypeptide)-like and helospectin-like peptides in the teleost gut

Pituitary adenylate cyclase-activating polypeptide (PACAP) and helospectin are two vasoactive intestinal polypeptide (VIP)-related neuropeptides that have recently been demonstrated in the mammalian gut; the aim of this study was to reveal their occurrence and localisation in the gastrointestinal tract, swimbladder, urinary bladder and the vagal innervation of the gut of teleosts, using immunohistochemical methods on whole-mounts and sections of these tissues from the Atlantic cod, Gadus morhua and the rainbow trout, Oncorhynchus mykiss. Both PACAP-like and helospectin-like peptides were present in the gut wall of the two species. Immunoreactive nerve fibres were found in all layers but were most frequent in the myenteric plexus and along the circular muscle fibres. Immunoreactivity was also demonstrated in nerves innervating the swimbladder wall, the urinary bladder and blood vessels to the gut. Immunoreactive nerve cell bodies were found in the myenteric plexus of the gut and in the muscularis mucosae of the swimbladder. In the vagus nerve, non-immunoreactive nerve cells were surrounded by PACAP-immunoreactive fibres. Double staining revealed the coexistence of PACAP-like and helospectin-like peptides with VIP in all visualized nerve fibres and in some endocrine cells. It is concluded that PACAP-like and helospectin-like peptides coexist with VIP in nerves innervating the gut of two teleost species. The distribution suggests that both PACAP and helospectin, like VIP, are involved in the control of gut motility and secretion.     

The nervous system of the actinotroch larva of Phoronis muelleri (Phoronida)

Summary The nervous system of the actinotroch larva of Phoronis muelleri has been investigated with the transmission electron microscope (TEM). Attempts have been made to localize all of the major nerves and to reveal the cytoarchitecture of the apical ganglion. The nervous system is intraepithelial in position and consists of an apical ganglion, located on the epistome, with at least four different cell types, including monopolar sensory cells and mono- or multipolar neuron-like cells. From the anterior part of the apical ganglion three median nerves extend to the edge of the epistome; two of these nerves connect to nerves which follow the edge of the epistome all the way to the junction of the epistome and the mesosome. From the posterior part of the ganglion extend two lateral nerves which continue along the tentacular ring. Each tentacle has three nerves located on the frontal side which connect to the nerve ring along the tentacles. Along the posterior ciliary band is a minor nerve ring. In addition, a nerve net is found on the epistome, mesosome, and metasome, but no longitudinal nerves were observed between the posterior ciliary band and the apical ganglion. All nerve cells were found in the apical ganglion and none was observed along the nerves. Sensory cells (probably mechanoreceptors) are located in two rows on each tentacle; sensory organs such as eyes and statocysts were not observed.Abbreviations ac accessory centricle - aen anterior epistome edge nerve - af abfrontal cells - bl basal lamina - bl.c blastocoel coelomocyte - ci cilium - co collar - cp cell process - cr ciliary root - ec ₁ epistome edge cell type 1 - mne mouth nerve ring - mo mouth - mp metasomal pouch - ms mesosome - mt metasome - mu muscle - n nerve process - ne nerve - np neuropil - nu nucleus - pc ₁ posterior ciliary band cell type 1 - ec ₂ epistome edge cell type 2 - ec ₃ epistome edge cell type 3 - epi epidermis - es epistome - ese epistome edge - fc frontal cell - gc ₁ type 1 ganglion cells - gc ₂ type 2 ganglion cells - gc ₃ type 3 ganglion cells - ge gut epithelium - ij intermediate junction - laf lateroabfrontal cell - lc lateral cell - lfc laterofrontal cell - lgc lateral ganglion cell - me metacoel epithelium - lne longitudinal median epistome nerves - pc ₂ posterior ciliary band cell type 2 - pc procoel - pe procoel epithelium - pen posterior epistome edge nerve - pr posterior ciliary band - p.rec proximal recess of procoel epithelium - prne nerve ring along posterior ciliary band - sj septate junction - sne secondary nerve along the tentacular ring - t tentacle - tr tentacular ring - trne horseshoe-shaped nerve along the tentacular ring     

Commissural ring nerve: A female-specific neurosecretory tract supplied by bifurcating median neurons in the cockroach Periplaneta americana (L.) and the cricket Teleogryllus commodus (Walker)

Summary In the American cockroach, Periplaneta americana, and the Australian field cricket, Teleogryllus commodus, the two nerves supplying the bases of the cerci are joined by a branch that crosses behind the last abdominal ganglion. This commissural ring nerve is restricted to females, and it contains many axons filled with granular and agranular vesicles. The axons stem from somata located within the ganglion. There are one (Periplaneta) or two (Teleogryllus) groups of median neurons with bilaterally symmetrical bifurcations, and a group of postero-ventral neurons on each side. In T. commodus, these neurons are distinct from others associated with the cerci. In the two species, the ring nerve neurons contribute to a neuropile near the root of each cereal nerve. The bifurcating median neurons arborize on both sides before entering the ring nerve, while the postero-ventral ones branch more extensively ipsilateral to their somata. The possibilities are discussed that the bifurcating neurons may be homologous to dorsal unpaired median neurons, and that the ring nerve may be a neurohemal area.     

Stem cells in microturbellarians

Summary A combination of microscopical, immunocytochemical, and autoradiographic techniques were employed to study stem cells and their fates during asexual reproduction and regeneration in two microturbellarians,Microstomum lineare (Macrostomida) andStenostomum leucops (Catenulida). Special attention was paid to the development of the immunoreactivity (IR) to FMRF/RF-amide and 5-HT in differentiating nerve cells.Asexual reproduction inM. lineare andS. leucops occurs by paratomy, i.e., fragmentation after completed differentiation of the new organs. Regeneration, on the other hand, involves a combination of morphallactic and epimorphic processes without the formation of a regeneration blastema. The only cells incorporating tritiated thymidine ([³H]T) were the mesenchymal and gastrodermal neoblasts, which proliferate continuously replenishing the population of stem cells available for growth, asexual reproduction and regeneration. These proliferative cells occurred in two ultrastructurally different forms, differing from each other only by the presence or absence of ciliar basal bodies in the cytoplasm. Few differentiated cells were labeled in the head piece after completed regeneration. A greater amount of labeled differentiated cells were, however, observed postpharyngeally in the first zooid as well as in zooids having developed during the same time (i.e., 20–45 h after the treatment with [³H]T). Furthermore, many labeled cells were still undifferentiated at that time or just in the beginning of the differentiation process. It can therefore be concluded that neoblasts function both as reserve cells and as functional stem cells for all differentiated cell types in these worms. IR to FMRF/RF-amide neuropeptides was not observed in nerve cells differentiating from neoblasts until the occurrence of dense-core vesicles in their cytoplasm. Due to methodological difficulties only weak or no IR to 5-HT could be traced in the nervous system of the asexual and regenerating worms.Abbreviations ICC Immunocytochemical - IR immunoreactivity - [³H]T tritiated thymidine     

Rhopalia are integrated parts of the central nervous system in box jellyfish

In cubomedusae, the central nervous system (CNS) is found both in the bell (the ring nerve) and in the four eye-bearing sensory structures (the rhopalia). The ring nerve and the rhopalia are connected via the rhopalial stalks and examination of the structure of the rhopalial stalks therefore becomes important when trying to comprehend visual processing. In the present study, the rhopalial stalk of the cubomedusae Tripedalia cystophora has been examined by light microscopy, transmission electron microscopy, and electrophysiology. A major part of the ring nerve is shown to continue into the stalk and to contact the rhopalial neuropil directly. Ultrastructural analysis of synapse distribution in the rhopalial stalk has failed to show any clustering, which indicates that integration of the visual input is probably spread throughout the CNS. Together, the results indicate that cubomedusae have one coherent CNS including the rhopalia. Additionally, a novel gastrodermal nerve has been found in the stalk; this nerve is not involved in visual processing but is likely to be mechanosensory and part of a proprioceptory system.This work was supported by grants 621-2002-4873 from the Swedish Research Council to D.-E. Nilsson and 21-2204-04 from the Danish Research Council to A. Garm.     

Ultrastructure of the tentacle nerve plexus and putative neural pathways in sea anemones

Abstract. Neurons of sea anemone tentacles receive stimuli via sensory cells and process and transmit information via a plexus of nerve fibers. The nerve plexus is best revealed by scanning electron microscopy of epidermal peels of the tentacles. The nerve plexus lies above the epidermal muscular layer where it appears as numerous parallel longitudinal and short interconnected nerve fibers in Calliactis parasitica . Bipolar and multipolar neurons are present and neurites form interneuronal and neuromuscular synaptic contacts. Transmission electron microscopy of cross sections of tentacles of small animals, both C. parasitica and Aiptasia pallida , reveals bundles of 50–100 nerve fibers lying above groups of longitudinal muscle fibers separated by intrusions of mesoglea. Smaller groups of 10–50 slender nerve fibers are oriented at right angles to the circular muscle formed by the bases of the digestive cells. The unmyelinated nerve fibers lack any glial wrapping, although some bundles of epidermal fibers are partially enveloped by cytoplasmic extensions of the muscle cells; small gastrodermal nerve bundles lie between digestive epithelial cells above their basal myonemes. A hypothetical model for sensory input and motor output in the epidermal and gastrodermal nerve plexuses of sea anemones is proposed.