Correlation of spinule dynamics and plasticity of the horizontal cell spectral response in cyprinid fish retina: quantitative analysis

Summary A negative feedback interaction between luminosity type horizonatal cells (HCs) and green-sensitive cones generates the long-wavelength-sensitive depolarizing response in biphasic chromaticity type HCs. This interaction is suppressed in the dark and is potentiated by light adaptation of the retina. HCs are morphologically plastic; during light adaptation, their dendritic terminals within cone pedicles extend, giving rise to spinules. This paper examines whether there is a quantitative correlation between the time course of light-dependent formation of the spinules and enhancement of the feedback interaction. The strength of the feedback interaction in isolated retinac of the roach was determined as the neutral wavelength at which reversal of spectral response polarity occurred in biphasic HCs. A good correlation was found between the neutral wavelength and the spinule/ribbon ratios of retinae. Biphasic HCs were intracellularly stained with horseradish peroxidase and the correlative ultrastructure of the contacted pedicles was examined. Neutral wavelength was found to be correlated with the spinule number, weighted according to the number of synaptic contacts mediating feed-forward transmission. The latter was estimated from the total number of labelled C_b/H2 HC processes (central and lateral) at synaptic triads. A model in which spinules mediate the negative feedback interaction of HCs in the retina of cyprinid fish is presented.     

Isolation and identification of granule-associated proteins relevant for poly(3-hydroxyalkanoic acid) biosynthesis in Chromatium vinosum D

Abstract Poly(3-hydroxybutyric acid) granules, which harbored only four major granule-associated proteins as revealed by SDS polyacrylamide gel electrophoresis, were isolated from crude cellular extracts of Chromatium vinosum D by centrifugation in a linear sucrose gradient. N-Terminal amino acid sequence determination identified two proteins of M _r 41 000 and M _{r 40 000} as the phaE _Cv and phaC _Cv translational products, respectively, of C. vinosum D. In a previous study it was shown that both proteins are required for the expression opf poly(3-hydroxyalkanoic acid) synthase activity. The N-terminus of the third protein ( M _r 17 000) exhibited no homology to other proteins. Lysozyme, which was during purification of the granules, exhibited a strong affinity to PHB granules and was identified as the fourth protein enriched with the granules.     

haemolysin of Escherichia coli: Comparison of pore-forming properties between chromosome and plasmid-encoded haemolysins

Abstract Lipid bilayer experiments were performed with chromosome-encoded haemolysin of Escherichia coli . The addition of the toxin to the aqueous phase bathing lipid bilayer membranes of asolectin resulted in the formation of transient ion-permeable channels with two states at small transmembrane voltages. One is prestate (single-channel conductance 40 pS in 0.15 M KCl) of the open state, which had a single-channel conductance of 420 pS in 0.15 M KCl and a mean lifetime of 30 s. Membranes formed of pure lipids were rather inactive targets for this haemolysin. Experiments with different salts suggested that the haemolysin channel was highly cation-selective at neutral pH. The mobility sequence of the cations in the channel was similar if not identical to their mobility sequence in the aqueous phase. The single-channel data were consistent with a wide, water-filled channel with an estimated minimal diameter of about 1 nm. The pore-forming properties of chromosome-encoded haemolysin were compared with those of plasmid-encoded haemolysin. Both toxins share common features, oligomerize probably to form pores in lipid bilayer membranes. Both types of haemolysin channels have similar properties but different lifetimes.     

A morphometric comparison of dissimilar early development in sibling species of Platynereis (Annelida,Polychaeta)

Summary Early development of Platynereis massiliensis was studied in serial sections of fixed embryos and in living or fixed embryos whose nuclei had been made visible with a fluorescent label. The unfertilized egg is an ellipsoid with three axes of differing length. The longest axis corresponds to the dorsoventral axis of the developing embryo. Egg volume is ten times that in the sibling species, P. dumerilii, mainly due to increased yolk content. The timing and spatial pattern of cleavage were observed from first cleavage to the 62-cell stage. Volumes of the blastomeres, their nuclei, their yolk-free cytoplasm and their yolk were determined from serial sections up to the 29-cell stage. In the P. massiliensis embryo, cell cycles are on average 3.7 times longer than in P. dumerilii; volume proportions among the blastomeres also differ and the macromeres containing the bulk of yolk are particularly large, but otherwise the cleavage patterns, differential segregation of yolk and yolk-free cytoplasm, and the histogenetic fates of the blastomeres are the same as in P. dumerilii. This equivalence of cell lineage and of cytoplasmic segregation mechanisms in both species, maintained in spite of the different appearance of the embryos, suggests functional importance of and selective constraint on these developmental features. The relatively accelerated divisions of the 2d cell line in P. massiliensis may be interpreted as the precocious development of cell lines which give rise to adult structures. Several structures, obviously functional in developing P. dumerilii, have lost their function in P. massiliensis: the egg contains few cortical granules, giving rise to only a moderate egg jelly layer in the zygote; prototroch cells develop cilia, but the heavy embryo is unable to swim; the larva develops three pairs of parapodia but, unlike the corresponding stage in P. dumerilii, is not capable of coordinate locomotion. This loss of motility is related to the brooding habit of the species developing inside the parental tube and is explained as the result of a switch from pelagic to benthic, protected reproduction in P. massiliensis. Offprint requests to: A.W.C. Dorresteijn