Biomedical subjects

A Baas

Publications and source records attributed to A Baas.

7 recordsLinked to original sources

PubMed2006

Bose-Einstein condensation of exciton polaritons.

Phase transitions to quantum condensed phases--such as Bose-Einstein condensation (BEC), superfluidity, and superconductivity--have long fascinated scientists, as they bring pure quantum effects to a macroscopic scale. BEC has, for example, famously been demonstrated in dilute atom gas of rubidium atoms at temperatures below 200 nanokelvin. Much effort has been devoted to finding a solid-state system in which BEC can take place. Promising candidate systems are semiconductor microcavities, in which photons are confined and strongly coupled to electronic excitations, leading to the creation of exciton polaritons. These bosonic quasi-particles are 10(9) times lighter than rubidium atoms, thus theoretically permitting BEC to occur at standard cryogenic temperatures. Here we detail a comprehensive set of experiments giving compelling evidence for BEC of polaritons. Above a critical density, we observe massive occupation of the ground state developing from a polariton gas at thermal equilibrium at 19 K, an increase of temporal coherence, and the build-up of long-range spatial coherence and linear polarization, all of which indicate the spontaneous onset of a macroscopic quantum phase.

Journal Article↗

PubMed2006

Quantum degeneracy of microcavity polaritons.

We investigate experimentally one of the main features of a quantum fluid constituted by exciton polaritons in a semiconductor microcavity, that is, quantum degeneracy of a macroscopic fraction of the particles. We show that resonant pumping allows us to create a macroscopic population of polaritons in one quantum state. Furthermore, we demonstrate that parametric polariton scattering results in the transfer of a macroscopic population of polariton from one single quantum state into another one. Finally, we briefly outline a simple method which provides direct evidence of the first-order spatial coherence of the transferred population.

Journal Article↗

PubMed2001

Parametric polariton amplification in semiconductor microcavities.

We present novel experimental results demonstrating the coherence properties of the nonlinear emission from semiconductor microcavities in the strong coupling regime, recently interpreted by parametric polariton four-wave mixing. We use a geometry corresponding to degenerate four-wave mixing. In addition to the predicted threshold dependence of the emission on the pump power and spectral blueshift, we observe a phase dependence of the amplification which is a signature of a coherent polariton wave mixing process.

Journal Article↗

PubMed1999

Drosophila APC2 is a cytoskeletally-associated protein that regulates wingless signaling in the embryonic epidermis.

The tumor suppressor adenomatous polyposis coli (APC) negatively regulates Wingless (Wg)/Wnt signal transduction by helping target the Wnt effector beta-catenin or its Drosophila homologue Armadillo (Arm) for destruction. In cultured mammalian cells, APC localizes to the cell cortex near the ends of microtubules. Drosophila APC (dAPC) negatively regulates Arm signaling, but only in a limited set of tissues. We describe a second fly APC, dAPC2, which binds Arm and is expressed in a broad spectrum of tissues. dAPC2's subcellular localization revealed colocalization with actin in many but not all cellular contexts, and also suggested a possible interaction with astral microtubules. For example, dAPC2 has a striking asymmetric distribution in neuroblasts, and dAPC2 colocalizes with assembling actin filaments at the base of developing larval denticles. We identified a dAPC2 mutation, revealing that dAPC2 is a negative regulator of Wg signaling in the embryonic epidermis. This allele acts genetically downstream of wg, and upstream of arm, dTCF, and, surprisingly, dishevelled. We discuss the implications of our results for Wg signaling, and suggest a role for dAPC2 as a mediator of Wg effects on the cytoskeleton. We also speculate on more general roles that APCs may play in cytoskeletal dynamics.

Actins↗

PubMed1999

Peptide binding motifs and specificities for HLA-DQ molecules.

HLA-DQ molecules have been associated with susceptibility to a number of autoimmune and other diseases, possibly through the peptide repertoire that can be presented by different allelic products. It is thus of importance to understand which peptides can be bound by different HLA-DQ allelic products. Recently, a model for HLA-DQ has been described and used to derive peptide positional environments for HLA-DQ allelic products. By combining the peptide positional environments with known HLA-DQ peptide binding motifs, a set of predictions of likely anchor motifs for many of the products of HLA-DQ allelic variants are made and presented in a table referred to as a roadmap for HLA-DQ peptide binding specificities.

Antigen Presentation↗

PubMed1999

Expression and function of beta 2 integrin CD11B/CD18 on leukocytes from patients with psoriasis.

The expression of beta 2 integrin CD11b on granulocytes and monocytes from patients with psoriasis vulgaris and pustular psoriasis was examined by flow cytometry. The amount of CD11b expressed on both granulocytes and monocytes was greater in 4 patients with pustular psoriasis than in 16 patients with psoriasis vulgaris. Its expression correlated with the development of pustules on the skin. No difference was seen between healthy blood donors and patients with active psoriasis vulgaris. Three patients with pustular psoriasis were followed during retinoid treatment. Granulocytes and monocytes showed a decrease in CD11b expression after administration of retinoids, in parallel with clearing of the skin. The adherence of granulocytes isolated from psoriasis patients was tested on cultured human umbilical vein endothelium. No significant difference in adherence was observed between control cells and cells from patients with active psoriasis vulgaris. These data indicate that the development of microabscesses in the dermis in psoriasis vulgaris is not related to enhanced beta 2 integrin function. The increased CD11b expression found in patients with pustular psoriasis may, however, serve as a triggering factor for pustule formation in pustular psoriasis.

Aged↗

PubMed1995

One month treatment with the once daily oral beta 2-agonist bambuterol in asthmatic patients.

Bambuterol is a new long-acting oral bronchodilator for once daily use in patients with asthma. It is a prodrug of terbutaline, designed to be slowly metabolized to terbutaline. Results from comparative studies have shown that it has similar clinical efficacy to other oral bronchodilators, but less side-effects. The present study was aimed at verifying the 24 h effect duration of bambuterol, 10 and 20 mg in comparison with placebo during a one month treatment period. The study was conducted as a double-blind, randomized, parallel group placebo-controlled, multicentre trial. It started with a one week run-in period with placebo, when oral bronchodilators were withdrawn. At the end of this reference period, the patients were randomized to one of three treatments: placebo, bambuterol 10 mg, or bambuterol 20 mg, once daily in the evening. The treatment period lasted for 4 weeks. Four hundred and eighty seven patients with a mean age of 45 yrs were included. Mean baseline forced expiratory volume in one second (FEV1) and FEV1% of predicted were 2.05 l and 62%, respectively. Administration of 10 mg bambuterol resulted in a significant 24 h effect duration, expressed as an increase in mean daily morning and evening peak expiratory flow (PEF) (+11 l.min-1, adjusted means) throughout the study, as compared with placebo. Bambuterol, 20 mg, gave a significant 24 h effect duration in both FEV1 and morning and evening PEF as compared with placebo. Furthermore, the adverse events observed during the study were relatively few and mild.(ABSTRACT TRUNCATED AT 250 WORDS)

Administration, Oral↗