Purification of molecularly bridged metal nanoparticle arrays by centrifugation and size exclusion chromatography

Size exclusion chromatography and centrifugation separation protocols were developed and compared for isolating enriched fractions of phenylethynyl-bridged metal nanoparticle dimers and trimers from the monomeric particle starting material. Both methods enabled the isolation of enriched fractions of a desired array without causing significant sample aggregation or replacement of the phenylethynyl bridge. Solutions containing ca. 70% bridged gold dimers were obtained using either method. The further development of methods for separating discrete arrays of covalently bridged nanoparticle homo and hetero structures is expected to help advance our understanding of collective metal particle electronic structure-function relationships.

Microglia and astroglia have a common progenitor cell

Disaggregated neopallial cells from newborn C3H/HeJ mice were cloned in Grenier hybridoma tissue culture dishes, and culture wells that contained only one cell were marked. After 8-10 days of culturing, the cultures were fixed and double immunolabeled for microglia with Mac-1 antibody and for astroglia with antibody to GFAP. Each marked well containing a clone was identified as either a microglia, astroglia, mixed microglia-astroglia, or an unlabeled clone. The effect of LM cell line conditioned medium (LM-CM), which contains colony-stimulating factor-1, on the development of mixed microglia-astroglia clones was determined. Formation of mixed clones was dose dependent (P < 0.0001). We concluded that microglia and astroglia have a common progenitor cell and that the development of mixed clones is LM-CM dependent.

Calcium ion current from an extracellular electrolyte toward a channel opening in an insulating membrane: quantitative model with rotational symmetry

A mathematical model of three-dimensional (3-D) ion transport is formulated in an approximation assuming rotational symmetry. The model consists of three particle-conservation equations for sodium, calcium, and chlorine ions complemented with the Poisson equation. The numerical method of solution is based on the Gummel-Scharfetter semianalytical approach, the program is written in FORTRAN and the system of discrete equations is solved explicitly in the axial direction and by iterations in the radial direction. The present report deals with calcium flux toward a channel opening in an insulating impermeable membrane, assuming depolarization to zero potential. The initial homogeneous concentrations of sodium, calcium, and chlorine ions are 8.729 x 10(19), 6.02 x 10(17), and 8.849 x 10(19) (cm-3), respectively, corresponding to molar concentrations of 145-mM NaCl and 1-mM of CaCl2; the calcium concentration in the circle representing the channel entry is set at 0.1 microM, corresponding approximately to the concentration of free calcium ions in the cytoplasm. The calculations were carried out up to 3 microseconds. The calcium flux caused a perturbation of quasi-neutrality and the formation of a space charge, which reached the maximum value (i.e., maximum in absolute value) of -0.2 Ccm-3 at the channel entry; the corresponding maximum of the axial component of the electric field was about 1 kV/cm. The maximum value of the calcium current was 0.362 pA, decreasing to 0.283 pA at 3 microseconds. A review of several experimental studies of calcium currents yielded the average current values for higher and lower conductance channels (mainly L- and T-type) 0.76 and 0.42 pA, respectively. This implies that, at Ca++ concentrations of 1 mM or lower the calcium ion current may be limited by the ion influx from an extracellular medium.

Electric Potential and Concentration of Ion Species in the Proximity of a Cell Membrane: ab initio Calculations

The mathematical model of ion transport in the field of electric forces and diffusion gradients is based on particle conservation equations and the Poisson equation which governs the distribution of the electric potential. Designed for evaluating the currents through passive ion channels in cellular membranes, the model is formulated in a two-dimensional approximation assuming rotational symmetry. Three ionic species are considered in the present study: sodium, calcium and chlorine. The program developed for the numerical solution is based on a semi-analytical approximation suggested by Gummel & Scharfetter. The present contribution reports calculations of the steady-state distributions of ionic species induced by an electric field due to a negative potential drop across the cellular membrane, which represent initial conditions for future calculations of the time development of the ion flux through the calcium channel. The numerical calculations were performed from the moment of potential application up to the time 6.2 μs, when a new steady state is established. At this stage, the maximum values of the sodium and calcium concentrations at the membrane surface are about 22% and 47% above the initial values of 145 mM and 1 mM, respectively, while the chlorine concentration is approximately 18% below. The electric field at the membrane surface is about 6.7x10(4) and decreases exponentially with distance. The length of the boundary layer is about 40 A. Since the model is based on fundamental principles, it can be used for the quantitative solution of any problem possessing rotational symmetry where a continuum approach is applicable and some elementary conditions are fulfilled, such as equilibrium (or at least quasi-equilibrium) of the particle energy distributions with respect to time and electric field, and the assumption that the transport coefficients are defined as functions of the field and their numerical values are known. Copyright 1997 Academic Press Limited

Cell cycle time of murine neopallial cells in vitro

Disaggregated glial cells from newborn CD1 mouse neopallia were cultured in low concentration (4.2 x 10(3) cells/cm2) for 72 hr and then either pulse labeled with BrdU by one 2-hr pulse at various times of culturing or continuously labeled for various lengths of time. At the end of incubation, the cells were fixed and immunoreacted with BrdU. All BrdU+ and BrdU- cell nuclei were counted in an area of 4.84 cm2. A three-compartment model for interpretation of the experimental data was developed consisting of active proliferating cells, non-active cells with proliferating potential, and nonproliferating cells. The model is based on assumptions of time invariance of culture conditions, random re-entry of cells into cell cycle and random exit from the proliferating pool. Furthermore, it is assumed that average values are representative for describing the numbers of cells in specific compartments as functions of time. A set of relationships representing the numbers of labeled cells for pulse labeling and continuous labeling assays is derived from these assumptions and the generally accepted representation of cell progress through the cell cycle, i.e., a genetically predetermined sequence of post-mitosis rest phase, S-phase, pre-mitosis rest phase, and mitosis. These relationships are used to evaluate the S-phase time tau s and cell cycle time tau c of proliferating cells. Under our particular conditions, we obtain approximately tau s = 8 hr and tau c = 16 hr, respectively. The applicability of the model and possible distorting factors are discussed.

A possible interpretation of some colony forming assays: 'developmental tree' model

The characteristic form of the probability distributions of in vitro grown colonies is analysed and experimental results are shown to be well approximated by an exponential dependence. It is argued that this type of dependence is very frequent in nature and does not provide sufficient evidence to resolve the dilemma whether the development of haemopoietic lineages proceeds according to a schema of 'progressive and stochastic restriction' or more limiting 'developmental tree' with some random control process involved. Detailed analysis of the experimental conditions and results is needed to provide insight into collective phenomena in cellular proliferation and differentiation. At present, the complexity of the phenomena involved prevents unique interpretation of the experimental distributions. Nevertheless, one particular quantitative model, based on a developmental tree concept, yields a set of the theoretical probability distributions, which are in a very good agreement with the experimental results.

Stochastic versus deterministic in haemopoiesis: what is what?

'Stochastic' models of haemopoiesis are discussed in juxtaposition to 'deterministic' concepts. It is argued that certain characteristic features attributed to stochastic models are conjectures, not supported by the observations quoted to provide an experimental basis: for example, an assumption of extrinsic control and another assumption that such control is inaccessible to external influences up to the very latest stages of cell development. Furthermore, it is argued that quantitative relationships between early progenitors (stem cells) and the number of colonies produced in vitro, cited as evidence supporting the proliferation and differentiation scheme of the stochastic model of random restriction, are also compatible with other models, providing that some sufficiently complex mechanism is involved in the control process. A simple model based on the principle of ordered development (usually denoted 'proliferation tree') is presented as an example and the calculated distributions are shown to have the same form as the experimental ones.

Quantitative model of mononuclear phagocyte lineage proliferation in murine bone marrow

An analysis and a synthesis of published and original experimental evidence have been performed with a view to providing a global qualitative and quantitative description of the proliferation of monocyte precursors in murine bone marrow. Two versions of a mathematical model are proposed. The first consists of two dividing generations of granulocyte-macrophage colony-forming cells (GM-CFC) and two dividing generations of macrophage colony-forming cells (M-CFC), differentiating into promonocytes (PM) and then into monocytes (MC). The second version consists of two generations of GM-CFC and three generations of M-CFC, followed by PM and MC. Both are compatible with experimental data, and further evidence is needed to elaborate a unique representation. The kinetic constants have been taken from published material whenever possible and estimated from available data and model kinetics in the case of unknown quantities. The models are applied to a pulse labeling assay, and calculated labeled subpopulations are compared with experimental values. Agreement is satisfactory for both versions in the case of MC, while a calculated PM labeling index falls more rapidly than the experimental values. However, inconsistency also exists between PM and MC experimental data.