An in vitro model for cell-cell interactions

Summary Heterotypic cell-cell interactions appear to be involved in the control of development and function in a wide variety of tissues. In the vasculature, endothelial cells and mural cells (smooth muscle cells or pericytes) make frequent contacts, suggesting a role for intercellular interactions in the regulation of vascular growth and function. We have previously grown endothelial cells and mural cells together in mixed cultures and found that heterocellular contact led to endothelial growth inhibition. However, this mixed culture system does not lend itself to the examination of the effects of contact on the phenotype of the individual cell types. We have therefore developed a co-culture system in which cells can be co-cultured across a porous membrane, permitting intercellular contact while maintaining pure cell populations. Co-culture of endothelial cells and smooth muscle cells across membranes with pore sizes of 0.02, 0.4, 0.6, and 0.8μm maintained the two cell types as homogeneous populations, whereas smooth muscle cells migrated across the membrane through pores of 2.0μm. Vascular cell co-culture across membranes with 0.8-μm pores resulted the inhibition of endothelial cell proliferation and the generation of conditioned media which inhibited endothelial cell growth. The arrangement of the cells in this co-culture system mimics thein vivo orientation of vascular cells in which mural cells are separated from the abluminal surface of the endothelium by a fenestrated internal elastic lamina or basement membrane. Because this co-culture system maintains separable populations of cells in contact or close proximity allowing for biochemical and molecular analyses of pure populations, it should prove useful for the study of cell-cell interactions in a variety of systems.     

Diel Nitrogen Fixation by Cyanobacterial Surface Blooms in Sanctuary Lake, Pennsylvania

Diel nitrogen fixation studies were conducted with assemblages of cyanobacteria sampled from surface blooms on Sanctuary Lake, Pa. The studies were conducted between July and September of 1982 to 1985 by using the acetylene reduction technique. Assemblages with the lowest cell concentrations (0.9 × 10⁹ to 1.0 × 10⁹ cells per liter) exhibited nitrogen fixation activity throughout the day, with maximum fixation rates occurring in mid to late afternoon; fixation proceeded throughout the night at rates equivalent to 23 to 28% of the afternoon maximum. In studies conducted with the highest cell concentrations (3.7 × 10⁹ to 6.7 × 10⁹ cells per liter), fixation rates reached maximum values in mid to late morning. The rates declined rapidly throughout the midday period and subsequently ceased from late afternoon until sunrise on the following day. The afternoon decline and cessation of fixation exhibited by high cell concentrations correlated with photosynthetically induced low total CO₂ and supersaturating O₂ concentrations. The midday decline could be prevented and partially reversed by experimentally lowering O₂ and increasing total CO₂ concentrations. Under experimental conditions which simultaneously prevented supersaturating O₂ concentrations and maintained high total CO₂ availability, nitrogen fixation continued throughout the solar day, with maximum rates occurring at midday. These observations indicate that temporal changes in photosynthetic activity may affect diel fluctuations in nitrogen fixation.     

Overgrowth in a marine epifaumal community: Competitive hierarchies and competitive networks

Summary The frequencies with which organisms of a species overgrew or were overgrown by organisms of other species in a marine epifaunal community were estimated. The ranking of the ability of the major taxonomic groups to overgrow others was basically hierarchical:ascidianssponges>bryozoans>barnacles, polychaetes, tubicolous amphipods, hydroids. In contrast, the ranking of the competitive ability of species in the community did not form a simple linear hierarchy and there was no single competitively dominant species (measured in terms of overgrowth). There were often no significant differences in the ability of species to overgrow each other within the three major taxonomic groups of sponges, ascidians and bryozoans. Such results were common also between the species of large sponges and ascidiams which dominated substrata immersed for periods longer than two years.A lack of a significant difference in the competitive ability of species was usually the result of (a) frequent formation of delay/ties or standoffs and (b) changes in the outcome of interactions due to change in the relative size of interacting colonies. In many two-species interactions the species which had the larger colony in a given encounter had a greater probability of winning.When the range of colony sizes of two species was similar there was often no significant difference between the competitive ability of each species. Such cases without a clearcut winner often represented a backloop in an otherwise hierarchical sequence of competitive ability, i.e. Species A beats Species B, Species B beats Species C, no significant differences in competitive ability between Species C and A. No examples of competitive networks of the form Species A beats Species B, Species B beats Species C, Species C beats Species A were found. Backloops in otherwise hierarchical sequences (no significant differences in competitive ability) occurred most frequently between species within the same major taxonomic groups and were the result of a very even balance in the generalised competitive mechanism of overgrowth.It seems probable that backloops in hierarchical sequences are more commonly due to the absence of clear competitive dominance in interactions between species (reversals in the outcome of overgrowth interactions and standoffs), rather than to direct backloops formed by a specialised or to a generalised competitive mechanism. Network-like arrangements of competitive ability formed by the type of processes described here are likely to contribute significantly to the high levels of species diversity observed in many marine epifaunal communities subject to low levels of physical disturbance.     

Axonal Transport and Metabolism of Glycoproteins in Rat Sciatic Nerve

The distribution of 5'-nucleotidase activity, dopaminergic [3H]spiperone binding sites, and [3H]quinuclidinyl benzilate (QNB) binding sites in different subcellular fractions of bovine caudate nucleus has been studied. Each activity was enriched in a microsomal (P3) preparation from that tissue. The microsomal preparation was further fractionated by different techniques. First, the P3 fraction, or a sonicated P3 fraction, was fractionated on a discontinuous sucrose density gradient. Second, the P3 fraction, or a digitonin pretreated P3 fraction, was fractionated on a continuous sucrose density gradient. The results obtained demonstrate that 5'-nucleotidase activity does not cofractionate with radioligand binding activity, although no difference between the distributions of [3H]spiperone binding and [3H]QNB binding were seen. It is concluded that the two radioligand binding activities are located on nonglial membranes.     

Intraspecific and intergeneric mobilization of non-conjugative resistance plasmids by a 24.5 megadalton conjugative plasmid of Neisseria gonorrhoeae

pLE2451, a 24.5 megadalton conjugative plasmid from Neisseria gonorrhoeae, was capable of efficiently mobilizing gonococcal beta-lactamase plasmids between gonococci and from gonococci to Haemophilus influenzae and restriction-deficient Escherichia coli. Donor strains of N. gonorrhoeae carrying pLE2451 were also found to be capable of mobilizing a variety of non-conjugative plasmids originally derived from enteric bacteria or Haemophilus species when such plasmids were resident in E. coli. Nevertheless, pLE2451 was not detected physically in E. coli or H. influenzae transconjugants. This suggests that the plasmid is unstable in these hosts but survives transiently to provide transfer functions for mobilization. The proficiency of pLE2451 in promoting intraspecific and intergeneric mobilization was not paralleled by pUB701, pRI234 and pFR16017, a series of conjugative plasmids derived originally from Haemophilus species. These plasmids were incapable of mobilizing even Haemophilus beta-lactamase plasmids, such as RSF0885, between Haemophilus species.     

Pathway of plasmid transformation in Pneumococcus: open circular and linear molecules are active.

We have extended the analysis of plasmid transformation in Streptococcus pneumoniae by finding that monomeric and dimeric open circular and linear forms of pMV158 were active in transformation. Their efficiencies were at least 35-fold lower than those of the corresponding closed circular forms. The evidence came largely from analysis of S1 nuclease-digested plasmid deoxyribonucleic acid by combinations of dye-buoyancy, gel electrophoresis, and sedimentation velocity methods. As with closed circular forms, monomer open circular forms gave second-order kinetics and dimer forms gave first-order kinetics. Unique linear products of digestion by either of two restriction enzymes were inactive, but a mixture of the two digests was active, as was the mixture of linear monomer deoxyribonucleic acids produced by S1 nuclease. Absolute efficiencies of transformation were low even for closed circular donors. All of the results, including the low efficiencies, were consistent with the interpretation that plasmid replicons were assembled in the recipient cell by pairing of fragments of single strands that had entered the cell separately from duplex donors that had been cut on the cell surface.     

Silver staining of the lampbrush chromosomes ofTriturus cristatus carnifex

Lampbrush chromosomes fromTriturus cristatus carnifex were stained using the ammoniacal silver staining (AgAS) technique. Many of the recognized marker structures proved to be silver positive, plus between six and fifteen lateral loop pairs. None of the stained loop pairs corresponded to known sites of the nucleolus organizers, although the extrachromosomal nucleoli were silver positive. The ammoniacal silver staining technique does not demonstrate the specificity for active ribosomal cistrons in lampbrush chromosomes that it does in a wide variety of mammalian mitotic chromosomes.     

Minireview possible roles of dietary fats in carcinogenesis

Epidemiological data have implicated dietary fat as an important factor in the aetiology of various cancers in humans. This suggestion is supported by the results of experiments with animals which have shown that increased amounts of dietary fat, in particular polyunsaturated fat, increase the incidence of some spontaneous and induced tumours. The enhancement of carcinogenesis by dietary fats appears to be exerted at the promotional stage of carcinogenesis. Changes induced by dietary fats in several biological systems involved in carcinogenesis may well indicate the underlying mechanisms responsible for the results of these experiments. These include the effects of dietary fats on the metabolism of chemical carcinogens, the structure and function of membranes, immunocompetence, DNA repair potential and endocrine function.