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PubMed1983

Neurulation in the Mexican salamander (Ambystoma mexicanum): a drug study and cell shape analysis of the epidermis and the neural plate.

We analysed the neurulation movements in the Mexican salamander Ambystoma mexicanum. Embryos were exposed to colchicine or nocodazole prior to neural fold formation. Exposure to these drugs prevented the anterior neural folds from closing. Neurulation however proceeded normally in the posterior regions of the embryo. We were unable to find apically constricted cells in the neural plate of colchicine-blocked neurulae. Only rounded-up neural plate cells were present (semithin sections). This situation was typical in embryos exposed to colchicine prior to neural fold formation. Concentrations of colchicine up to 2.5 x 10(-3) were not capable of blocking neurulation once the neural folds were formed. The wedge-shaped cells were present in similar numbers to those found in controls. We quantified the cell shape changes in the neural plate and in the epidermis in both controls and drug-arrested embryos. The comparison of these to classes of data shows that epidermal spreading is prevented by colchicine but only slightly affected by nocodazole. Embryos blocked in late neurulation by exposure to these drugs can resume neurulation following neural plate excision in nocodazole but not in colchicine. We conclude from this observation that the epidermis contributes to raising and closing of the neural folds. The presence of neural folds in absence of wedge-shaped cells in the neural plate is also taken as evidence that neurulation is not exclusively driven by forces generated in or acting on the neural plate. Our view on the concerted interplay of various embryonic components is illustrated in a summarizing diagram (Fig. 11).

Ambystoma mexicanum↗
PubMed1982

Spatial pattern of sensory cell terminals in the olfactory sac of the tiger salamander. I. A scanning electron microscope study.

The olfactory sac of adult tiger salamanders (Ambystoma tigrinum) was assessed by scanning electron microscopy and the hitherto unknown variety of the olfactory sac cells has been documented. Regional differences in respect to the composition of the apical epithelial surface border in the olfactory sac have been recorded. Transitions between the olfactory epithelium proper and epithelia with different surface structures have been described. It has been shown that both the roof and the floor of the olfactory sac are mainly covered by sensory epithelium with a surface of uniform appearance. However, the sensory area of the floor is interspaced with non-olfactory epithelial bands. Variations were also noted in regard to secretory cells, cells with conical profiles, microplicae arrangements and populations of microvilli and kinocilia. These differences have been discussed in relation to electrophysiological data which suggest a regional patterning in the response to odours.

Ambystoma↗
PubMed1980

Sarcomagenesis and myofibril maturation during limb regeneration and development of the newt, Triturus alpestris and salamander, Salamandra salamandra.

During limb regeneration of the salamander, Salamandra salamandra, muscles dedifferentiate and then mesenchymal cells appear, which subsequently, differentiate into muscle. Both in regenerates and in developing limbs of newts larvae mesenchymal cells begin to differentiate into myoblasts, which contain both thin (7 nm) and thick (14 nm) filaments, which are often associated with microtubules, developing a cytoskeletal network, which contribute directly to the shape of myoblasts and a linear alignment of the myofibrils. Both thin and thick filaments appear as a tandem near the periphery of the myoblasts, where myofibrilogenesis occurs. In the early stage of sarcomagenesis the appearance of "Z-bodies" was observed. They were intimately associated with the tandem of filaments thus forming the primary sarcomere units. Consequently the "Z-bodies" coalesce to form primitive Z-lines. The maturation of the primary sarcomeres occurs during fibrilogenesis and is manifested by the gradual thining of the Z-line and thickening of the M-line.

Animals↗
PubMed1994

Tectal activation of premotor and motor networks during feeding in salamanders.

In salamanders, three separate pathways, a crossed and two uncrossed ones, extend from the tectum mesencephali to the brain stem and spinal cord. These pathways arise from different types of tectal neurons; their dendrites arborize in different layers of retinal afferents and, thus, receive different types of retinal information. Collaterals of descending axons extend in regions, where motoneurons and interneurons related to prey capture are situated. The response properties of tectal neurons, interneurons and motoneurons related to prey capture were revealed by intracellular recording with subsequent dye (biocytin) injection. Most tectal neurons exhibit long latencies after optic-nerve stimulation, which indicates a complex processing of visual information inside the tectum. Our data show that no one-way connection exists between the tectum and motor nuclei; rather, these centres, together with a number of interneurons, exhibit a complex interaction during feeding.

Animals↗
PubMed1994

Depth perception in salamanders: the wiring of visual maps.

Most members of the salamander family Plethodontidae exhibit fast and precise prey localization and are likely to use stereopsis for this task. Tectal and isthmic neurons were stained iontophoretically with biocytin after intracellular recording in order to investigate the fine structure and arborization pattern of dendritic and telodendritic trees. The back-projection of isthmic neurons to both tectal hemispheres exhibits a conspicuous and spatially restricted connection with tectal neurons and the afferent fiber layers, which probably plays an important role in the context of stereoscopic depth perception and detection of retinal disparities.

Animals↗
PubMed1994

A novel GABA receptor modulates synaptic transmission from bipolar to ganglion and amacrine cells in the tiger salamander retina.

We investigated the mechanisms underlying the modulation of transmitter release from bipolar cells. Three currents, measured under whole-cell patch clamp in the tiger salamander retinal slice, were used to identify the sites of modulation and to establish their pharmacological profile. (1) A light-elicited inhibitory current was measured in bipolar cells that could be blocked by picrotoxin. This input probably arrives via GABAergic amacrine cells since there is high GABA sensitivity at the bipolar cell terminals and little GABA sensitivity at the dendrites. (2) Voltage-gated barium currents were elicited by depolarizing voltage ramps in bipolar cells. These currents most likely flowed through the calcium channels that are associated with transmitter release at the bipolar terminal. Bath-applied GABA suppressed the barium currents. (3) Puffs of potassium at bipolar dendrites depolarized bipolar cells and elicited an excitatory synaptic current measured in amacrine and ganglion cells. The excitatory synaptic currents, which reflect bipolar cell transmitter release, were also blocked by bath-applied GABA. For all three currents, the effects of GABA could be reversed by picrotoxin, but not by bicuculline or SR95531. The pharmacological profile of the receptors mediating GABA suppression of the barium currents and of excitatory synaptic transmission is characteristic of GABAC receptors (Cutting et al., 1991; Polenzani et al., 1991; Shimada et al., 1992). GABA receptors at bipolar terminals gate a chloride conductance, and most were found to have the pharmacological properties of GABAC receptors (Lukasiewicz et al., 1994). By contrast, the GABA receptors on ganglion cells have been found to be the GABAA subtype (Lukasiewicz and Werblin, 1990; Lukasiewicz et al., 1994). These results suggest that GABA acts presynaptically at GABAC receptors at the bipolar cell terminals. The GABAC receptors open chloride channels that can modulate the release of excitatory transmitter. In some experiments, bicuculline or SR95531 reversed a component of the GABA suppression of synaptic transmission. This indicates that GABAA receptors may also play a role in modulating transmission between bipolar and ganglion cells.

Animals↗
PubMed1993

Characterization of the glutamate transporter in retinal cones of the tiger salamander.

L-Glutamate elicits an inwardly rectifying current at hyperpolarized potentials in isolated retinal cones of the tiger salamander, as measured under whole-cell patch clamp. Evidence presented in this article supports the notion that cones possess a high-affinity glutamate transporter. This glutamate-elicited current shows no desensitization over a period of several minutes, and has an affinity (Km) of 10 microM. The inward current is mimicked by the amino acids L-aspartate, D-aspartate, L-cysteate, and to a lesser extent D-glutamate. It is neither blocked by the glutamate receptor antagonists kynurenic acid (1 mM), 6-cyano-7-nitroquinoxaline-2,3-dione (100 microM), or 2-amino-5-phosphonovalerate (100 microM), nor elicited by the glutamate receptor agonists (100 microM each) kainate, quisqualate, NMDA, or 2-amino-4-phosphonobutyrate. The glutamate-elicited current was reduced by the glutamate transport blockers dihydrokainate (DHKA), DL-threo-beta-hydroxyaspartate (beta HA), and L-trans-pyrrolidine-2,4-dicarboxylic acid. When glutamate was present on both sides of the membrane, the blockers reduced both uptake and release; the blocker-sensitive current as a function of membrane potential represents the transport current-voltage relation (I-V), and the reversal potential of the I-V represents the transporter equilibrium potential. This potential was a function of the equilibrium potential for glutamate. DHKA and beta HA depolarized horizontal cells in a retinal slice, and abolished their light responses, suggesting that in the absence of glutamate transport, glutamate concentrations in the cleft rise to a level that saturates the postsynaptic receptors. The high capacity of the cone glutamate transporter is well suited for the rapid removal of glutamate from the synaptic cleft required for the signaling of a light onset to postsynaptic cells.

Ambystoma↗
PubMed1993

Immunoreactive endocrine cells and nerve elements in the gut of the Italian cave salamander.

The presence and distribution of eleven different types of immunoreactive endocrine cells and nine types of immunoreactive nerve elements were immunohistochemically identified in the gut and pancreas of the italian cave salamander, Hydromantes ambrosii. The majority of gastrointestinal endocrine cells were of open-type, often presenting basal cytoplasmic processes. Gastrin- and substance P-immunoreactive cells in the fundus and bombesin-immunoreactive cells in the intestinal portion were instead of closed type. Immunoreactive nerve fibres were particularly numerous in the muscular layers and blood vessel wall; bombesin- and substance P-immunoreactive nerve fibres were also abundant beneath gastro-intestinal epithelium. Besides substance P-, caerulein- and cholecystokinin-immunoreactive nerve fibres, all the other immunopositive nerve fibres seemed to be of intrinsic types. By the use of four different gastrin/cholecystokinin antisera three variously distributed subpopulations of endocrine cells and nerve elements were detected. Most of the pancreatic endocrine cells were organised in chord-like islets, polarized in the direction of blood vessels. A sparse network of bombesin-immunoreactive fibres was also found in the pancreas. The distribution of bombesin- and of the gastrin/cholecystokinin-immunoreactive material in the stomach and the presence of closed type endocrine cells indicate a more evoluted organization of the gastroenteropancreatic neuroendocrine system thus confirming the position of Hydromantes ambrosii among the higher urodeles.

Animals↗
PubMed1993

Fine structure of the retinal epithelium of the tiger salamander (Ambystoma tigrinum).

The retinal pigment epithelium (RPE) of the tiger salamander (Ambystoma tigrinum) consists of a single layer of large squamous-shaped cells. These RPE cells are but minimally infolded basally (sclerally) but show numerous apical (vitreal) processes that interdigitate with the photoreceptor cells. A series of apically-located tight junctions joins the RPE cells to form part of the blood-ocular barrier. Internally the RPE nucleus is large, vesicular and flattened. Smooth endoplasmic reticulum predominates in these cells with only occasional small profiles of rough endoplasmic reticulum present. Polysomes and basally-located mitochondria are plentiful. Golgi zones, lipid droplets, lysosome-like bodies and phagosomes of outer segment material are also present. Myeloid bodies which may show ribosomes on their surface are also numerous. The round to oval melanosomes undergo retinomotor movements and in the light-adapted state reach almost to the external limiting membrane. Bruch's membrane or complexus basalis shows a pentalaminate structure with the central elastic lamina poorly represented. The choriocapillaris is a single layer of large anastomosing capillaries which are but minimally fenestrated.

Ambystoma↗
PubMed1993

Fine structure of the retinal photoreceptors of the tiger salamander (Ambystoma tigrinum).

The retinal photoreceptors of the tiger salamander (Ambystoma tigrinum) have been studied by light and electron microscopy in both light- and dark-adaptation. Rods and cones are present in this duplex retina in a ratio of about 20:1. As in other urodele species these photoreceptors are very large cells. The rod outer segment is composed of bi-membranous discs of uniform diameter displaying several very deep incisures. The rod inner segment displays an ellipsoid of mitochondria and a myoid region which changes in diameter during the lighting cycle indicating that rods undergo photomechanical movements. Rod nuclei are located at all levels of the outer nuclear layer and rod spherules are large and display several invaginated and superficial synaptic sites. Cone photoreceptors while large cells are smaller than the rods. They show a smaller tapering outer segment, a large distal ellipsoid of mitochondria and a prominent paraboloid of glycogen but no oil droplet within the inner segment. Judging by the width of the myoid region which remains similar throughout the lighting cycle, cones in this species show no retino-motor responses. Cone nuclei are less electron dense than rods and are also located at all levels of the outer nuclear layer. The cone synaptic pedicle is larger than that of the rods and also shows several invaginated and superficial synaptic sites.

Adaptation, Ocular↗
PubMed1995

[Oswaldocruzia bonai n. sp. (Nematoda, Trichostrongylina, Molineoidea),a parasite of a Sardinian cave salamander].

The Oswaldocruzia parasites of amphibians in Europe are closely related. Contrary to this, the new species Oswaldocruzia bonai discovered in the cave salamander from Sardinia shows particular and archaic characters: dorsal ray of the caudal bursa well developed and tips of the rays 4 equidistant from those of the 3 and 5; female with few eggs of large size. The lack of ridges in front of each lateral cord is a character shared with Ethiopian species but which differentiated the Sardinian species from the European species.

Amphibians↗
PubMed1996

Salamander pigment patterns: how can they be used to study developmental mechanisms and their evolutionary transformation?

Neural crest cells of ectothermic vertebrates give rise to three types of pigment cells: melanophores, xanthophores, and iridophores. In early larval salamanders, these cells can combine to generate a variety of pigment patterns across taxa, including vertical bars and horizontal stripes. Such patterns offer an opportunity to study neural crest morphogenesis and differentiation, as well as the evolution of these processes and the morphologies that arise from them. This review examines the phylogenetic distribution of specific pigment patterns, our current understanding of the mechanisms underlying these patterns, and how evolutionary transformations of these mechanisms may have resulted in alternative pigment patterns across taxa.

Animals↗
PubMed1996

Pretecto-tectal interactions: effects of lesioning and stimulating the pretectum on field potentials in the optic tectum of salamanders in vitro.

Interactions between pretectum and optic tectum of salamanders were analyzed by recording evoked potentials (EPs) in the optic tectum in vitro in response to stimulation of the contralateral optic nerve. Neither lesioning of the pretectum nor ablation of the medulla oblongata including the nucleus isthmi altered the shape of tectal EPs, suggesting that the tectal EP in this preparation reflects activation of tectal circuitry by retinal afferents without a major contribution from non-retinal afferents. To analyze the effect of stimulation of the pretectum on the tectal EP, we stimulated the pretectal area pharmacologically. Amplitudes of tectal EPs decreased rapidly after stimulation of the pretectum and recovered within minutes (glutamate) or hours (kainic acid). The pretectal influence on the tectal EP might act presynaptically on retinal afferents or by modulating the response of inhibitory interneurons.

Animals↗
PubMed1997

Hemoglobin transition from larval to adult types occurs within a single erythroid cell population during metamorphosis of the salamander Hynobius retardatus.

In amphibians transitions of hemoglobins (Hbs) and red blood cells (RBCs) from larval to adult types have been reported to occur at metamorphosis. The transition of Hbs in the salamander Hynobius retardatus also occurs during metamorphosis, but almost independently of thyroid activity. Changes in several properties of RBCs, including their morphology, buoyant density and Hb phenotypes, were analyzed during the normal development of Hynobius retardatus. Typical larval RBCs were distinguished from typical adult ones by their different morphology and different buoyant density, while RBCs from metamorphosing animals had a single buoyant density and thus could not be separated into two populations on a Percoll density gradient. When RBCs from metamorphosing animals were examined immunohistochemically using larval or adult globin-specific antibodies, all the RBCs from any developmental stages from early larvae (36 days after hatching) to metamorphosed juveniles contained varying quantities of both antigens recognized by these antibodies. Immunohistochemical observation also demonstrated that the erythropoietic organs were the liver and spleen at early larval stages, but limited to the spleen in metamorphosing larvae and metamorphosed adults. These findings support the idea that the Hb switching in Hynobius retardatus occurs in a single RBC population, rather than the concept that larval RBCs are replaced by new, adult RBCs, as is known to occur in many amphibians.

Animals↗
PubMed1998

Isthmotectal connections in plethodontid salamanders.

In the plethodontid salamander species Plethodon jordani and Hydromantes italicus, the morphology and connectivity of isthmic cells were investigated by means of intracellular staining with biocytin. Dendritic arborization, axonal pathways, and size and morphology of telodendritic structures in the fiber layers of the optic tectum were determined. The latter were studied electron microscopically. The majority of isthmic neurons project to both tectal hemispheres, each cell forming telodendritic structures of different extent in the ipsilateral and contralateral hemisphere. These structures are column-like with diameters of about 60 microm. The ipsilateral terminals extend through layers 1-3 of the tectal white matter and intermingle with terminals of contralateral retinal afferents in layers 1-3, as well as with ipsilateral retinal afferents in layer 3. The corresponding contralateral telodendritic structures are confined to layer 1 and are not in direct contact with ipsilateral retinal afferents in layer 3. Both telodendritic structures are located in the rostral two-thirds of the optic tectum, which are binocularly innervated. The rostrocaudal and mediolateral sites of isthmic telodendra are in register with the direct retinal map in each tectal hemisphere. Few isthmic cells project to the caudal one-third of the optic tectum, which is monocularly driven. These cells form only one axonal terminal in the ipsilateral tectal hemisphere. Size and structure of these ipsilateral terminals are similar to the ipsilateral telodendra of bilaterally projecting isthmic neurons.

Animals↗
PubMed1998

Spiroxys hanzaki n. sp. (Nematoda: Gnathostomatidae) collected from the giant salamander, Andrias japonicus (Caudata: Cryptobranchidae), in Japan.

Spiroxys hanzaki n. sp. (Nematoda: Gnathostomatidae) taken from the giant salamander, Andrias japonicus (Temminck, 1836) (Caudata: Cryptobranchidae) from Hyogo Prefecture, Japan, is described. It closely resembles Spiroxys allegheniensis Walton, 1930, from the hellbender, Cryptobranchus alleganiensis (Daudin, 1803) (Cryptobranchidae), of North America, in that it has minute dorsal and ventral lobes of the pseudolabium and a reduced gubernaculum but is readily distinguished by a pseudolabium with 2 apical, 1 dorsal, and 1 ventral tooth on the median lobe.

Animals↗
PubMed1998

Transformation of skin from larval to adult types in normally metamorphosing and metamorphosis-arrested salamander, Hynobius retardatus.

Transformation of skin from larval to adult types in a salamander, Hynobius retardatus, which had been reported to show neotenic reproduction in a specific environment, was examined morphologically in normally metamorphosing, precociously metamorphosing and metamorphosis-arrested larvae. Typical larval skin was composed of an epidermis constituted by three types of cells such as apical, Leydig, and basal cells. The Leydig cells were larval specific, and thus disappeared and were replaced by adult epidermal cells during the metamorphosis. Disappearance of the Leydig cells was accomplished by apoptosis as confirmed by the terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick-end-labeling method and electron microscopy during the normal metamorphosis and precocious metamorphosis induced by exogenously applied triiodothyronine. Typical adult skin was composed of epidermis constituted by stratified squamous cells and of dermis mainly occupied with two types of dermal glands, mucous and serous glands. When the metamorphosis was arrested by different procedures (thyroidectomy, hypophysectomy, goitrogen treatment, and rearing at low temperature), the larval-specific Leydig cells fully remained in the epidermis, suggesting that the disappearance of these depended on the thyroid activity. Contrary to this, dermal glands behaved differently from the Leydig cells, though they developed and differentiated from epidermal basal cells and constituted the same skin. Those in the metamorphosis-arrested (thyroidectomized, hypophysectomized, or goitrogen-treated) larvae, except in the larvae reared at 4 degrees C, appeared a little later than in the controls. Thus, the aged, metamorphosis-arrested larvae had skin which consisted of larval type epidermis (Leydig cells) and adult type dermis (mucous and serous glands).

Animals↗
PubMed1999

Tenascin-R inhibits the growth of optic fibers in vitro but is rapidly eliminated during nerve regeneration in the salamander Pleurodeles waltl.

Tenascin-R is a multidomain molecule of the extracellular matrix in the CNS with neurite outgrowth inhibitory functions. Despite the fact that in amphibians spontaneous axonal regeneration of the optic nerve occurs, we show here that the molecule appears concomitantly with myelination during metamorphosis and is present in the adult optic nerve of the salamander Pleurodeles waltl by immunoblots and immunohistochemistry. In vitro, adult retinal ganglion cell axons were not able to grow from retinal explants on a tenascin-R substrate or to cross a sharp substrate border of tenascin-R in the presence of laminin, indicating that tenascin-R inhibits regrowth of retinal ganglion cell axons. After an optic nerve crush, immunoreactivity for tenascin-R was reduced to undetectable levels within 8 d. Immunoreactivity for the myelin-associated glycoprotein (MAG) was also diminished by that time. Myelin was removed by phagocytosing cells at 8-14 d after the lesion, as demonstrated by electron microscopy. Tenascin-R immunoreactivity was again detectable at 6 months after the lesion, correlated with remyelination as indicated by MAG immunohistochemistry. Regenerating axons began to repopulate the distal lesioned nerve at 9 d after a crush and grew in close contact with putative astrocytic processes in the periphery of the nerve, close to the pia, as demonstrated by anterograde tracing. Thus, the onset of axonal regrowth over the lesion site was correlated with the removal of inhibitory molecules in the optic nerve, which may be necessary for successful axonal regeneration in the CNS of amphibians.

Animals↗