Die Ultrastruktur der Photorezeptoren von Lithobius forficatus L. (Chilopoda: Lithobiidae)

Zusammenfassung Die Untersuchung der feinstrukturellen Organisation der Ocellen von Lithobius forficatus L. ergab, daß der dioptrische Apparat aus einer lamellenartig gebauten, ungleichseitig bikonvexen Cornealinse besteht. Ein Glaskörper und spezielle Pigmentzellen fehlen. Der Augenbecher wird von 35–110 Sinneszellen gebildet, unter denen 2 morphologisch distinkte Typen unterschieden werden können. Die großen Sehzellen des distalen Bereiches besitzen einseitig inserierende Rhabdomere, die in radiärsymmetrischer Anordnung ein umfangreiches geschlossenes Rhabdom bilden. Der proximale Teil des Augenbechers wird von kleineren, konischen Basalzellen in Form einer undeutlich abgesetzten Retinula eingenommen. Durch enge Verzahnung ihrer zirkumapikal oder zweiseitig angeordneten Mikrovilli entstehen stelzenförmige Doppel- und Mehrfachrhabdomere, die mit dem zentralen Rhabdom in Verbindung stehen. Alle Sehzellen sind durch eine Gliederung in verschiedene Zonen gekennzeichnet. Sie sind bei distalen Rezeptoren senkrecht zur optischen Achse, bei Basalzellen transversal zur Längsachse der Zelle angeordnet. Auf die rhabdomerischen Mikrovilli des Augenzentrums folgt nach außen eine Schaltzone aus Elementen des ER und anderen vesilukären Bildungen. Diese Schaltzone stellt wahrscheinlich eine mit dem Adaptationszustand des Auges korrelierte Funktionsstruktur dar. In der cytoplasmatischen Zone fällt die Zahl verschiedenartiger multivesikulärer Korpuskel neben wenigen großen multilamellären Körpern auf. Die funktionelle Bedeutung des Ocellusaufbaus bei Lithobius wird diskutiert.

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Ultrastructure of the Photoreceptors of Lithobius forficatus L. (Chilopoda: Lithobiidae)

Summary The ultrastructure of the ocelli of Lithobius forficatus L. was investigated by means of conventional electron microscopy. The dioptric apparatus consists of an unequal biconvex corneal lens which has a lamella-like fine structure. Crystalline cones and special pigment cells are lacking. The eye cup is composed of 35 to 110 sense cells of two different morphological types. The large visual cells of the distal region are characterized by unilaterally inserted rhabdomeres which form in a radial symmetrical arrangement the extended closed rhabdome. The proximal part of the eye cup is occupied by somewhat smaller basal cells of conical shape, showing an indistinct retinula. These cells have numerous microvilli either in the apex region or laterally which interdigitate to form stiltlike double or multiple rhabdomeres in close communication to the central rhabdome. In both types of sense cells particular zones appear because of the characteristic distribution of certain cell elements. They are arranged perpendicularly to the optical axis in the large receptors and vertical in the main axis of the basal cells. Elements of ER and other vesicles constitute a Schaltzone which borders the microvilli of the rhabdomer. This Schaltzone probably is a functional structure correlated to the adaptional state of the eye. The cytoplasmatic zone contains numerous different multivesicular and few large multilamellar bodies. The functional meaning of the organization of the Lithobius ocellus is discussed.

Mit dankenswerter Unterstützung durch die Deutsche Forschungsgemeinschaft.     

PERIANTH DEVELOPMENT OF POTAMOGETON RICHARDSONII

Interpretation of the Potamogeton flower is complicated by the attachment of the “perianth segment” to the stamen connective. Developmental studies show that the perianth segments are not outgrowths of the stamen connectives. They are initiated on the floral apex acropetally before the (superposed) primordia of the stamens. After the inception of the stamen primordia, growth occurs in the regions between the primordia of each perianth segment and stamen. Thereby the bases of the developing perianth segment and stamen become united, and in the adult flower eventually the perianth segment is inserted on the connective of the stamen. The primordium of the perianth segment develops from the 2 outer layers (tunica) of the floral apex, in contrast to the stamen primordium which originates from the 3 outer layers. The vascular bundles for each perianth segment–stamen region develop acropetally from 1 common bundle which bifurcates into 1 bundle for the perianth segment and 1 for the stamen. The bundle leading into the perianth segment branches in a more or less dichotomous manner. The veins form none or only 1 or 2 anastomoses at the base of the lamina, whereas the vein endings remain free. The interpretation of the perianth segments is discussed in terms of the classical and the gonophyll theory. Since both theories rest on an ambiguous methodological basis, interpretation is postponed until a new approach to comparative morphology has been worked out and until the floral development of other Helobiales has been studied.     

THE EFFECTS OF MAGNESIUM ON NUCLEOSIDE PHOSPHATASE ACTIVITY IN FROG SKIN

Histochemical tests, employing the Wachstein-Meisel medium, indicate that nucleoside triphosphatase activity is found predominantly in two areas of the frog skin epidermis: (1) in mitochondria, where activity is enhanced by dinitrophenol, Mg²⁺ dependent, but inhibited by fixation; and (2) apparently associated with cell membranes of the middle and outer portions of the epidermis, where activity is inhibited by Mg²⁺, unaffected by dinitrophenol, and only slightly reduced by fixation. Spectrophotometric analysis shows that Mg²⁺ in the medium does not increase spontaneous hydrolysis of ATP, thus obviating the possible explanation that changes in substrate concentrations in the medium lead to alterations in the "staining" distributions. It is postulated that perhaps the two enzymes differ in their requirements for substrate—one requiring the polyphosphate to be in complexed form with Mg²⁺, the other uncomplexed. Concentrations of Mg²⁺ required to inhibit cell membrane nucleoside triphosphatase activity also inhibit the electrical potential difference and short-circuit current of the frog skin. Although these observations might be taken as presumptive evidence of the cell membrane enzyme as a component of the ion pump system, because of certain dissimilarities with respect to the biochemists' "transport ATPase" and for other reasons discussed in the paper, any definite conclusions in this regard are premature.     

Replication of SV40 minichromosomes in vitro

We operationally define two forms of SV40 minichromosomes, a 75S-form, prepared at low salt concentration, referred to as native minichromosomes, and a 50S-form, obtained after treatment with 0.5M potassium acetate, the salt-treated minichromosomes. Both preparations of minichromosomes serve well as templates for replication in vitro. Their respective replication products are strikingly different: replicated native minichromosomes contain a densely packed array of the maximal number of nucleosomes whereas replicated salt-treated minichromosomes carry, on average, half of the maximal number. We conclude that in both cases parental nucleosomes are transferred to progeny DNA, and, in addition, that an assembly of new nucleosomes occurs during the replication of native minichromosomes. This is apparently due to the presence of a nucleosome assembly factor as a constituent of native minichromosomes that dissociates upon treatment with salt. We further show that preparations of minichromosomes usually contain significant amounts of copurifying hnRNP particles and SV40 virion precursor particles. However, these structures do not detectably affect the replication and the chromatin assembly reactions.     

Identification of a common cyanobacterial symbiont associated with Azolla spp. through molecular and morphological characterization of free-living and symbiotic cyanobacteria.

Symbiotically associated cyanobacteria from Azolla mexicana and Azolla pinnata were isolated and cultured in a free-living state. Morphological analyses revealed differences between the free-living isolates and their symbiotic counterparts, as did restriction fragment length polymorphism (RFLP) analyses with both single-copy glnA and rbcS gene probes and a multicopy psbA gene probe. RFLP analyses with Anabaena sp. strain PCC 7120 nifD excision element probes, including an xisA gene probe, detected homologous sequences in DNA extracted from the free-living isolates. Sequences homologous to these probes were not detected in DNA from the symbiotically associated cyanobacteria. These analyses indicated that the isolates were not identical to the major cyanobacterial symbiont species residing in leaf cavities of Azolla spp. Nevertheless, striking similarities between several free-living isolates were observed. In every instance, the isolate from A. pinnata displayed banding patterns virtually identical to those of free-living cultures previously isolated from Azolla caroliniana and Azolla filiculoides. These results suggest the ubiquitous presence of a culturable minor cyanobacterial symbiont in at least three species of Azolla.     

Glutamate uptake in primary cultures of biliary epithelial cells from normal rat liver

Biliary epithelial cells (BEC) were isolated from normal rat liver with high purity (> 95%) as revealed by morphological criteria as well as staining for gamma-glutamyl transferase and cytokeratin 19. During cultivation for 96 hr flattening of the cells and a loss of microvilli was apparent, while the cytokeratin 19-positive phenotype was maintained. The BEC contained a sodium-dependent as well as a sodium-independent uptake system for glutamate with high capacity. Both activities increased transiently during cultivation peaking after 72 and 48 hr, respectively. After 72 hr, apparent kinetic constants could be calculated for the sodium dependent (K_m = 13.6 mM; V_max = 388 nmoles/min/mg protein) and for the sodium-independent system. (K_m = 10.8 mM; V_max = 132 nmoles/min/mg protein). The transient increase of both transport systems was suppressed by dexamethasone. The sodium-dependence showed a threshold concentration of about 35 mM sodium. Inhibition by kainate was much less potent for BEC than for hepatocytes. These data indicate that BEC contain transport systems for glutamate different from those in hepatocytes and which may be involved in the intrahepatic reabsorbtion of glutamate from bile.Abbreviations BEC biliary epithelial cells - DMEM Dulbecco's Modified Eagle's Medium - GGT gamma-glutamyl transferase - Dex dexamethasone - Glu glutamate - N-Me-AIB N-methyl-aminoisobutyrate - Hep hepatocytes - FBS Fetal bovine serum     

Computer-assisted assignment of 2D1H NMR spectra of proteins: Basic algorithms and application to phoratoxin B

Summary A suite of computer programs (CLAIRE) is described which can be of assistance in the process of assigning 2D¹H NMR spectra of proteins. The programs embody a software implementation of the sequential assignment approach first developed by Wüthrich and co-workers (K. Wüthrich. G. Wider, G. Wagner and W. Braun (1982)J. Mol. Biol. 155, 311). After data-abstraction (peakpicking), the software can be used to detect patterns (spin systems), to find cross peaks between patterns in 2D NOE data sets and to generate assignments that are consistent with all available data and which satisfy a number of constraints imposed by the user. An interactive graphics program calledCONPAT is used to control the entire assignment process as well as to provide the essential feedback from the experimental NMR spectra. The algorithms are described in detail and the approach is demonstrated on a set of spectra from the mistletoe protein phoratoxin B, a homolog of crambin. The results obtained compare well with those reported earlier based entirely on a manual assignment process.     

Two functional endothelin receptors in guinea-pig pulmonary arteries

The contractile effects of endothelins (ET-1, ET-2, ET-3) were investigated in pulmonary vessels and trachea from the guinea-pig using a sensitive in vitro method. ET-1 and ET-2 were potent agonists that concentration-dependently contracted pulmonary vessels. ET-3 was also an agonist but was less potent. In contrast, ET-1, ET-2 and ET-3 showed equal potencies in inducing contractions of tracheal segments. Using a pharmacological desensitization technique, evidence was provided for two types of functional endothelin receptors. Putatively, ET-1 and ET-2 act on the same functional receptor in the pulmonary artery whereas ET-3 acts on another receptor. Intraregional differences of the responses to endothelins were noticed when small intrapulmonar resistance arteries were compared to large proximal arteries. These differences are probably due to variations in the distribution of endothelin receptors rather than to receptor heterogeneity.