Intercalation and Pattern Regulation in Hydroids

Several topics and concepts in developmental biology, such as induction of secondary axes (as in amphibians), the rules of distal transformation (valid, e.g., for insect and vertebrate limbs), and the rule one piece regenerates, the other one duplicates (valid also for imaginal discs) originally emerged from pioneering studies in hydroids. In addition, one finds phenomena of intercalation, i.e., the insertion of intervening parts when tissues of distant origin are confronted with each other. However, since not only short-range but also long-range interactions contribute to pattern regulation, one observes more regulative flexibility as compared to insect systems.     

Guanylate Cyclase Activity in Dictyostelium discoideum and Its Increase during Cell Development

Following consumption of the food supply, cells of the cellular slime mould Dictyostelium discoideum aggregate and form a multicellular organism. The mechanism for cell aggregation is chemotaxis. The chemotactic signal in D. discoideum is released periodically from aggregation centers and propagated from cell to cell. cAMP mediates cell aggregation by acting as chemotactic attractant and as propagator of the signal. cAMP signals are measured by cell-surface receptors. Recent evidence indicates a role for cGMP during cAMP-mediated cell aggregation in D. discoideum .
During cell differentiation to aggregation competence, cAMP binding sites appear at the cell surface, and the activity of the enzymes adenylate cyclase and phosphodiesterase increases several-fold. In the present work we investigate the synthesis of cGMP in D. discoideum . Conditions for the assay of guanylate cyclase in cell homogenates are described. Guanylate cyclase activity was followed during cell differentiation to aggregation competence and found to increase fourfold. These results indicate that cGMP is involved in cell differentiation of D. discoideum . In contrast to adenylate cyclase, which is activated by cAMP, guanylate cyclase was under our conditions activated neither by cAMP, nor by folic acid.     

Temporal and spatial patterns in the abundance of wintering Red-breasted Mergansers Mergus serrator in an estuary

Tidal, temporal and spatial aspects in the abundance of wintering Red-breasted Mergansers in the Ythan estuary, northeast Scotland, were investigated. The number and location of birds were examined in relation to four variables: tidal cycle and tidal range, and month and time of day. A significant increase in the numbers of this piscivorous bird occurred as the winter progressed. Numbers were highest in early morning and then decreased significantly during the day. Spring tides were favoured over neap tides and a significant increase in numbers was recorded between low tide and high tide. The lower part of the estuary was preferred over the upper part with no detectable shift in the preference occurring in relation to any of the four variables.     

Electron Microscopic Studies of the Development of Kinetes in Theileria annulata Dschunkowsky & Luhs, 1904 (Sporozoa, Piroplasmea)

SYNOPSIS. Gamogony of Theileria annulata Dschunkowsky & Luhs occurs within the intestine of nymphs of the tick Hyalomma anatolicum excavatum Koch. After the 5th day post repletionem (p.r.) of the ticks spherical and ovoid parasites were found within the intestinal cells. These stages were thought to represent fertilized macrogametes. These underwent a transformation process leading ultimately to the differentiation of a motile stage, the kinete , which leaves the intestinal cells on the 14th-17th day p.r. and penetrates the alveoles of the salivary glands. The transformation of the stationary into a motile stage takes place by formation of a growing protrusion (= anlage) into an inner, enlarging vacuole. During this process the limiting membrane of the vacuole serves as the outer membrane of the developing motile stage, whereas the 2 inner membranes of its pellicle are newly formed. The steps of this differentiation in T. annulata are compared to the process of ookinete formation in haemosporina.     

Use of precision cut human liver slices for studying the metabolism and genotoxic potential of xenobiotics by means of the 32P-postlabelling technique: steps towards method validation using testosterone and 2-aminofluorene

In the present study, a new in vitro model combining the short-term incubation of precision-cut human liver slices with DNA-adduct analysis by the ³²P-postlabelling technique is proposed for investigation of the genotoxic potential of xenobiotics. For method validation, the metabolic turnover of testosterone (TES) and the DNA-adduct inducing potential of 2-aminofluorene (2-AF) were used. Precision-cut human liver slices were prepared from a total of 12 human liver samples which were freshly obtained as parts of resectates from liver surgery. The slices were incubated as submersion cultures with TES and 2-AF for up to 6 h in 12-well tissue culture plates at concentrations of 10-50 and 0.06-28 μM, respectively. Slices recovered from the slicing procedure in the 4 °C cold Krebs-Henseleit buffer as indicated by intracellular potassium concentrations which increased for 2 h and then remained stable until the end of the incubation. TES was extensively metabolized by human liver slices with a similar metabolite pattern as observed in vivo. Almost 90% of the metabolites were conjugates. Major phase-I metabolites were androstendione, 6β-OH-androstendione, 6β-OH-TES, and 15β-OHTES. After incubation with 2-AF, substance related DNA-adducts were detected which increased dose-dependently from 12 to 1146 adducts per 10⁹ nucleotides. The adduct pattern consisted of one major adduct spot, A, representing 80-90% of the total adduct level and up to four minor adduct spots, B-E. In summary, the present data demonstrate that precision-cut liver slices are a valuable alternative in vitro system for DNA-adduct determination to screen chemicals for potential genotoxicity in humans.