Power laws in microrheology experiments on living cells: comparative analysis and modelling

We compare and synthesize the results of two microrheological experiments on the cytoskeleton of single cells. In the first one, the creep function J(t) of a cell stretched between two glass plates is measured after applying a constant force step. In the second one, a micrometric bead specifically bound to transmembrane receptors is driven by an oscillating optical trap, and the viscoelastic coefficient $G_e(\omega)$ is retrieved. Both $J(t)$ and $G_e(\omega)$ exhibit power law behavior: $J(t)= A(t/t_0)^\alpha$ and $\bar G_e(\omega)\bar = G_0 (\omega/\omega_0)^\alpha$, with the same exponent $\alpha\approx 0.2$. This power law behavior is very robust ; $\alpha$ is distributed over a narrow range, and shows almost no dependance on the cell type, on the nature of the protein complex which transmits the mechanical stress, nor on the typical length scale of the experiment. On the contrary, the prefactors $A_0$ and $G_0$appear very sensitive to these parameters. Whereas the exponents $\alpha$ are normally distributed over the cell population, the prefactors $A_0$ and $G_0$ follow a log-normal repartition. These results are compared with other data published in the litterature. We propose a global interpretation, based on a semi-phenomenological model, which involves a broad distribution of relaxation times in the system. The model predicts the power law behavior and the statistical repartition of the mechanical parameters, as experimentally observed for the cells. Moreover, it leads to an estimate of the largest response time in the cytoskeletal network: $\tau_m \approx 1000$ s.Comment: 47 pages, 14 figures // v2: PDF file is now Acrobat Reader 4 (and up) compatible // v3: Minor typos corrected - The presentation of the model have been substantially rewritten (p. 17-18), in order to give more details - Enhanced description of protocols // v4: Minor corrections in the text : the immersion angles are estimated and not measured // v5: Minor typos corrected. Two references were clarifie

Physical and mechanical regulation of macrophage phenotype and function

Macrophages are tissue-resident immune cells that play a critical role in maintaining homeostasis and fighting infection. In addition, these cells are involved in the progression of many pathologies including cancer and atheroscloerosis. In response to a variety of microenvironmental stimuli, macrophages can be polarized to achieve a spectrum of functional phenotypes. This review will discuss some emerging evidence in support of macrophage phenotypic regulation by physical and mechanical cues. As alterations in the physical microenvironment often underlie pathophysiological states, an understanding of their effects on macrophage phenotype and function may help provide mechanistic insights into disease pathogenesis

Allium genetic resources

An overview of the developments in Allium genetic resources during the past 25 years is presented in this chapter. A first important development has been the introduction and further development of web-based genebanking information systems (e.g. GENESYS, PLANTSEARCH), which facilitated the exchange of data to a large extent between Allium collection holders worldwide. These information systems made it possible to obtain an overview of the Allium genetic resources managed worldwide and identify the gaps in collections which still need to be filled, especially in the face of the ongoing genetic erosion. A second important area of progress has been the development of new methods for the maintenance of Allium germplasm, especially cryopreservation. This method has made it possible to maintain Allium accessions in a cheap and effective way. The method is especially important for the conservation of vegetatively maintained germplasm. Other developments in Allium genebanking are the improvement of the health status of the germplasm kept in the collections and the continuing characterization and evaluation of germplasm, which stimulates the utilization of the Allium genetic resources held in genebanks. Significant changes could also be observed with respect to acquisition and exchange of plant genetic resources due to many and complex new regulations on the legal and organizational levels due to the adoption of the CBD and IT-PGRFA by many countries. It makes the handling of the plant accessions safer and more consistent but also more circumstantial. Finally, we need to underline that in an increasingly changing world with all the threats of genetic erosion and extinction due to disappearance of traditional cultivation methods, devastation of our environment and climatic change, the conservation of genetic resources is of prime importance for agriculture. Especially for breeders, a highly diverse genepool of a crop plant is an invaluable treasure. The importance to keep this treasure will no doubt become even more important in the future