Mari-Luz Hernandez-Nicaise

Le Système Nerveux des Cténaires

SummaryA subectodermal nerve-net is demonstrated in ten species of Ctenophores by means of either Methylene Blue vital staining or silver impregnation. There is no evidence of subendodermal nerve-net. The ectodermal nerve-net displays a characteristic polygonal pattern which is thought to result from morphogenetic events. Both bipolar and tripolar neurones occur.Three types of concentrations of nervous tissue are described: a) a high accumulation of nervous perikarya at the aboral sensory pole; b) a noticeable densification of the net along the eight meridional ciliary strands of every species, and around the lips of Beroidea; c) in the Cydippids Pleurobrachia and Hormiphora two thick strands of fibers and neurones interconnecting the aboral organ and the tentacles: the tentacular nerves.Ultrastructural evidence is given for the presence of nervous elements under and amidst ectodermal epithelial cells. Three features allow the recognition and characterization of neurites and neurones: a) numerous clear and/or granulated vesicles; b) microtubules in variable amounts; c) frequent and highly differentiated synaptic contacts.The very peculiar arrangement of the presynaptic elements is observed in all the species of Ctenophores so far studied. These synaptic contacts suggest chemical transmission in the nerve-net.On the basis of ultrastructural evidence the author refutes the nervous nature previously attributed to the ciliated cells of the meridional grooves.A parallel is drawn between nervous systems of Cnidaria and Ctenophora.

Le systme nerveux des ctnaires@@@The nervous system of ctenophores: II. Les lments nerveux intra-msoglens des broids et des cydippids@@@II. The nervous elements of the mesoglea of beroids and cydippids

SummaryUltrastructural evidence is given of the occurrence of nervous elements in the mesoglea of Ctenophores based on the presence of the typical synapses of this phylum.In Beroids, nervous fibers from the ectodermal nerve-net cross the epithelial basal membrane and run through the mesoglea; they are devoid of any ensheathing cell. These neurites build highly differentiated synapses upon the muscles and upon peculiar cells, tentatively named mesenchymal cells.In Cydippids, nerve fibers and nerve cell-bodies have been observed in the mesoglea of the tentacles. The mesogleal core of each tentacle contains mesenchymal cells and a thick strand of neurons and neurites, forming a kind of elongated ganglion. Neurites of either the axial neurones or the epithelial nerve-net neurones form numerous radial nerve strands across the tentacular muscles. Interneural, neuro-muscular and neuro-mesenchymal junctions are very frequent in the tentacle.As far as the organization of the mesoglea is concerned, the Ctenophora thus appear closer to Turbellaria than to Cnidaria.

Ca(2+)-ATPase and Mg(2+)-ATPase in Aplysia glial and interstitial cells: an EM cytochemical study.

The localization of Ca(2+)- and Mg(2+)-ATPases was determined in Aplysia central and peripheral nervous system, using an electron microscopic cytochemical method. The enzyme activity appeared localized to the membrane of glial granules (gliagrana), particularly in the peripheral nervous system of the esophagus, and on the plasma membrane of central glial cells adjacent to neuronal cell bodies. No calcium- and/or magnesium-ATPase activity was detectable on the plasma membrane of glial cells surrounding nerve axons in the pleuro-visceral connectives. These findings are discussed along two main lines: (a) the calcium-ATPase of the gliagrana coincides with a high intragranular calcium and/or proton concentration; and (b) the presence of a calcium-ATPase activity at the glio-neuronal interface around the neuronal cell bodies coincides with the use of calcium ions as charge carriers of the action potential, and its absence at the level of the axon with the concurrent functional use of sodium ions.

GIANT SMOOTH MUSCLE FIBERS OF THE CTENOPHORE MNEMIOPSIS LEYDII: ULTRASTRUCTURAL STUDY OF IN SITU AND ISOLATED CELLS

The lobate ctenophore Mnemiopsis leydii possesses giant smooth muscle fibers grouped in two sagittal bundles. Functional isolated cells were obtained by an enzymatic digestion of mesoglea and epithelia.Each bundle is made of 30 to 50 multinucleated cylindrical cells which may reach 35 µm in diameter and 4 cm in length. The nuclei and non-contractile organelles (mitochondria, golgi, rough endoplasmic reticulum) are contained in a discontinuous axial core, surrounded by a thick sheath of myofilaments. Thin (actin) filaments, 5.9 nm in diameter, form irregular rosettes around the thick (myosin) filaments, 16.1 nm in diameter. An actin:myosin filament ratio of 7:2 and a myosin density of 249 filaments per µm2 were found in cross-sections of relaxed in situ cells. No dense bodies nor attachment plates were observed. From the coiled shape of contracted single cells and from the rearrangement of organelles in such coiled cells, we propose that myofilaments are organized in thin long myofibrils attached to the ce...

The Desheathed Periphery of Aplysia Giant Neuron. Fine Structure and Measurement of [Ca2+]o Fluctuations with Calcium‐selective Microelectrodes

The visceral ganglion of Aplysia was mechanically desheathed after protease softening of the connective tissue to permit the positioning of ion‐selective electrodes in the vicinity of the neuronal membrane. The effects of this treatment on satellite glia and neuronal cytology were observed by electron microscopy. The intracellular alterations were not suggestive of serious membrane damage but the cohesion between glial and neuronal membranes was affected—the glial processes appeared to retract from the trophospongium and in some cases the neuronal membrane was completely naked. The external calcium activity [Ca2+]o at the surface of identified giant neuron, R2, was measured using double‐barrelled calcium‐selective microelectrodes. A decrease of ∼1 mM in [Ca2+]o could be recorded only during trains of action potentials induced by intracellular depolarizing current injection, and when the electrode was pushed firmly against the neuron surface. A recovery from this decrease in [Ca2+]o could sometimes, but not always, be observed during the phase of induced neuronal activity.

Intercellular Junctions in Ctenophore Integument

The ctenophoran integument consists basically of a single-layered epidermis which covers the entire body, including appendages such as the tentacular apparatus, the lobes and auricles, and Unes the stomodeal cavity (generally referred to as the gastric cavity). This integument is always devoid of any cuticle or hard secretion, but is permanently covered by a film of mucus. It rests on a gelatinous mesoglea, which is an unusual connective tissue devoid of collagen and elastin fibers (Franc et al., 1976), and harboring mesenchymal cells and numerous true muscle cells. The mesoglea may be considered as the internal milieu of the ctenophore; it is fed and oxygenated by a system of gastrovascular channels, which may be very elaborate in large species. Franc (1972) demonstrated that specialized ciliated cells, located in the canal walls and grouped in ciliated rosettes, regulated the relative ionic and osmotic composition of the mesoglea.

TWO DISTINCT DISTRIBUTION PATTERNS OF SARCOPLASMIC RETICULUM IN TWO FUNCTIONALLY DIFFERENT GIANT SMOOTH MUSCLE CELLS OF BEROE OVATA

The sarcoplasmic reticulum has been studied in radial and longitudinal giant smooth muscle fibres of the marine planktonic invertebrate Beroe. Impregnation with heavy metals has revealed that the smooth component is organised in a longitudinally oriented three-dimensional network of tubules running along the myofilaments, An ultrastructural morphometric analysis has shown that the relative volume of the sarcoplasmic reticulum is the same (1% of the myofilament volume) in both fibres but that the size, number and distribution of the sarcoplasmic reticulum tubules differ significantly. The longitudinal fibres are characterised physiologically by an action potential with a short calcium-dependent plateau that can trigger a short contraction; radial fibres produce action potentials without a plateau and their contraction requires a train of spikes. The sarcoplasmic reticulum tubules in longitudinal fibres are thinner (132 nm in diameter) and more numerous than those in radial fibres (160 nm in diameter). Moreover, the tubules are homogeneously distributed among the myofilaments in radial fibres, whereas they are more numerous in the centre of longitudinal muscles.

Cytochemical localization of Ca2+-ATPases and demonstration of ATP-dependent calcium sequestration in giant smooth muscle fibres of Beroe

SummaryA cytochemical analysis of the mechanisms underlying cytosolic calcium regulation was undertaken in the giant smooth muscle fibres of the marine invertebrate Beroe. The ability of the sarcoplasmic reticulum to accumulate Ca2+ was demonstrated on living skinned single cells. In the presence of oxalate, and physiological concentrations of Ca2+, calcium oxalate crystals were formed in the lumen of tubules and cisternae of the sarcoplasmic reticulum. The subcellular distribution of Ca2−-ATPase was studied with a cytochemical technique; a dense precipitate resulting from Ca2+-ATPase activity was found on the plasma membrane, on the membranes of tubules and cisternae of the sarcoplasmic reticulum, and in mitochondria.

Quenching of a proton gradient and concomitant increase of intragranular calcium in interstitial cells of Mytilus retractor muscle

SummaryThe content of specific glio-interstitial granules in situ was studied in Mytilus retractor muscle using fluorescent probes and X-ray microanalysis. The granules readily take up the fluorescent monoamine dye acridine orange added to sea water (2.7×10-6 M) and appear red in fluorescence microscopy. The addition of ammonium chloride (10 mM) or various proton ionophores results in extinction of the granule fluorescence. In addition, a step-wise decrease in granule fluorescence is observed when the tissue is perfused with artificial sea water of decreasing pH. These granules thus appear to be acidic inside. The animals were maintained in artificial sea water containing 8.36 mM Ca2+ and 528.90 mM Na+, the ratio R=[Ca2+]0/[Na+]20 being thus equal to 3x10-5. Perfusions of the tissue with artificial sea water containing a higher calcium concentration (12.2 mM) and/or a higher [Ca2+]0/[Na+]20 ratio (R=4.5×10-5) result in a drastic reduction of the proton gradient, evidenced by a quenching of the acridine orange fluorescence. Under the same conditions, a significant increase of the total intragranular calcium concentration was demonstrated by quantitative X-ray micro-analysis of the tissue processed by quick freezing and freeze-substitution in the presence of oxalic acid. The fluorescence of the probe Fluo-3/AM, indicative of ionized calcium, is higher in the granules than in the surrounding cytoplasm; this suggests that calcium is accumulated in the granule against its concentration gradient. The acidic gradient of specific glio-interstitial cell granules could provide the energy needed for this calcium accumulation through a Ca2+/H+ exchange. These results are discussed with regard to the hypothesis that the glio-interstitial tissue can regulate pericellular calcium and/or hydrogen ion ioncentration in the vicinity of nerve and muscle cells.

Ionic Currents in Ctenophore Muscle Cells

The term Coelenterate includes two phylla, the Cnidaria and the Ctenophora. Typically, they are diploblastic organisms in which an ectoderm is separated from an endoderm by a gelatinous layer, the mesoglea. Ctenophora appear phylogenetically more advanced than Cnidaria. In Cnidaria the mesoglea is generally acellular and the muscle system consists mainly of myoepthelial cells. Ctenophores have differentiated true muscle cells embedded in the mesoglea. The phylogenetic significance of the presence of muscle cells in the mesoglea of Ctenophores is still debated.