RESTRICTION OF SPECIFICITY IN THE PRECURSORS OF BONE MARROW-ASSOCIATED LYMPHOCYTES

Previous work has shown that the immediate precursor of B lymphocytes (PB cell) has many properties that distinguish it from both B lymphoctes and hemopoietic stem cells. Size, density, tissue distribution, and sensitivity to cytotoxic antisera differ for each type of cell. The work described here was designed to study three aspects of the differentiation of PB cells. First, since PB cells probably have immunoglobulin surface receptors, fluorescein-labeled anti-immunoglobulin antiserum was used in an attempt to investigate directly the physical properties of PB cells. The use of this labeled antiserum revealed a population of cells with properties similar to the PB cells defined by the functional assays. Second, the differentiative potential of PB cells was studied by comparing the size of the total population of PB cells, as determined with fluorescein-labeled anti-immunoglobulin antiserum, to the size of the population of PB cells responding in a functional assay with a specific antigen. The cells responding in the functional assay represent only 0.1% of the total population of PB cells. This observation suggests that PB cells are not pluripotent stem cells of the immune system. Finally, the kinetics of the differentiation of PB cells to B lymphocytes was studied. The differentiation to mature lymphocytes involves at least one intermediate stage in which cells larger than mature B cells are active in a functional assay for B cells. These large B cells are present in irradiated mice soon after transplantation of PB cells, but by 20 days the majority of the B cells are typical small lymphocytes

Relationship between ecosystem productivity and photosynthetically-active radiation for northern peatlands

We analyzed the relationship between net ecosystem exchange of carbon dioxide (NEE) and irradiance (as photosynthetic photon flux density or PPFD), using published and unpublished data that have been collected during midgrowing season for carbon balance studies at seven peatlands in North America and Europe. NEE measurements included both eddy-correlation tower and clear, static chamber methods, which gave very similar results. Data were analyzed by site, as aggregated data sets by peatland type (bog, poor fen, rich fen, and all fens) and as a single aggregated data set for all peatlands. In all cases, a fit with a rectangular hyperbola (NEE = α PPFD Pmax/(α PPFD + Pmax) + R) better described the NEE-PPFD relationship than did a linear fit (NEE = β PPFD + R). Poor and rich fens generally had similar NEE-PPFD relationships, while bogs had lower respiration rates (R = −2.0μmol m−2s−1 for bogs and −2.7 μmol m−2s−1 for fens) and lower NEE at moderate and high light levels (Pmax = 5.2 μmol m−2s−1 for bogs and 10.8 μmol m−2s−1 for fens). As a single class, northern peatlands had much smaller ecosystem respiration (R = −2.4 μmol m−2s−1) and NEE rates (α = 0.020 and Pmax = 9.2μmol m−2s−1) than the upland ecosystems (closed canopy forest, grassland, and cropland) summarized by Ruimy et al. [1995]. Despite this low productivity, northern peatland soil carbon pools are generally 5–50 times larger than upland ecosystems because of slow rates of decomposition caused by litter quality and anaerobic, cold soils

Why Do States Develop Multi-tier Emigrant Policies? Evidence from Egypt

Why do states vary their policies towards their citizens abroad, and why are some emigrant groups treated preferentially to others? The literature on the politics of international migration has yet to explore this as a separate field of inquiry, assuming that states adopt a single policy that encourages, sustains or prevents emigration abroad. Yet, in the case of Egypt, the state developed a multi-tiered policy that distinctly favoured specific communities abroad over others. I hypothesise that policy differentiation is based upon the perceived utility of the emigrant group remaining abroad versus the utility of its return. This utility is determined by two factors: the sending state’s domestic political economy priorities and its foreign policy objectives

Measurement of Oxygenated Polycyclic Aromatic Hydrocarbons Associated with a Size-Segregated Urban Aerosol

Size-segregated atmospheric particles were collected in Boston, MA, using a micro-orifice impactor. The samples were analyzed for oxygenated polycyclic aromatic hydrocarbons (OPAH) using gas chromatography/mass spectrometry. Seven PAH ketones (1-acenaphthenone, 9-fluorenone, 11H-benzo[a]fluoren-11-one, 7H-benzo[c]fluoren-7-one, 11H-benzo[b]fluoren-11-one, benzanthrone, and 6H-benzo[cd]pyrene-6-one), four PAH diones (1,4-naphthoquinone, phenanthrenequinone, 5,12-naphthacenequinone, and benzo[a]pyrene-6,12-dione), and one PAH dicarboxylic acid anhydride (naphthalic anhydride) were identified. Seven additional compounds with mass spectra typical of OPAH were tentatively identified. OPAH were generally distributed among aerosol size fractions based on molecular weight. Compounds with molecular weights between 168 and 208 were ap proximately evenly distributed between the fine (aerodynamic diameter, D_p, 2 μm) particles. OPAH with molecular weights of 248 and greater were associated primarily with the fine aerosol fraction. Most OPAH were distributed with particle size in a broad, unimodal hump similar to the the distributions observed for PAH in the same samples. These results suggest that OPAH are initially associated with fine particles after formation by either combustion or gas phase photooxidation and then partition to larger particles by vaporization and sorption. Two OPAH were distributed in bimodal distributions with peaks at D_p ≈ 2 μm and D_p ≈ 2 μm. These bimodal distributions may be indicative of sorption behavior different from PAH and other OPAH