Statistical dynamics of spatial-order formation by communicating cells

Communicating cells can coordinate their gene expressions to form spatial patterns. 'Secrete-and-sense cells' secrete and sense the same molecule to do so and are ubiquitous. Here we address why and how these cells, from disordered beginnings, can form spatial order through a statistical mechanics-type framework for cellular communication. Classifying cellular lattices by 'macrostate' variables - 'spatial order paramete' and average gene-expression level - reveals a conceptual picture: cellular lattices act as particles rolling down on 'pseudo-energy landscapes' shaped by a 'Hamiltonian' for cellular communication. Particles rolling down represent cells' spatial order increasing. Particles trapped on the landscapes represent metastable spatial configurations. The gradient of the Hamiltonian and a 'trapping probability' determine the particle's equation of motion. This framework is extendable to more complex forms of cellular communication

Notions of Equity in (International) Environmental Law: Inter-generational Equity

Equity has a long history. In the first chapter of this thesis, notions of equity in conventional international law will be discussed. It must be more helpful to understand equity based upon the history of the term since the issues relating to equity were raised in quite a few cases in the past. In the second chapter, by discussing environmental equity in the United States, the only remaining superpower and the largest economy in the world, the thesis tries to see the future of equity in international environmental law. Environmental equity issues in the United States are good sources for the discussion of the notions of equity in international environmental law when there are few countries where the notions of equity in environmental law are actively discussed. The discussion of environmental equity in the United States is divided into two separate parts. The first part of the discussion is focused on environmental equity movement that is far from being close to addressing intergenerational equity. But provided that this movement could be developed into the protection of the interests of future generations, it is not unnecessary to review the equity issues in the United States. The second part of the discussion, which appears in the third chapter focuses on supporting intergenerational equity. In the third chapter, based upon international legal instruments that incorporate the notions of intergenerational equity to solve the problems of intertemporal inequities in the distribution of resources, the author discusses the emerging issue of intergenerational equity

Topological defects in flat nanomagnets: the magnetostatic limit

We discuss elementary topological defects in soft magnetic nanoparticles in the thin-film geometry. In the limit dominated by magnetostatic forces the low-energy defects are vortices (winding number n = +1), cross ties (n = -1), and edge defects with n = -1/2. We obtain topological constraints on the possible composition of domain walls. The simplest domain wall in this regime is composed of two -1/2 edge defects and a vortex, in accordance with observations and numerics.Comment: 3 pages, eps figures. Proceedings of MMM 0

Who Is (Communicatively More) Responsible Behind the Wheel? Applying the Theory of Communicative Responsibility to TAM in the Context of Using Navigation Technology

By examining how perceived usefulness and ease of use relate to the user’s perception (i.e., communicative responsibility), the communicative behavior of the navigation system (i.e., the landmarks used to give directions), and the context of driving (i.e., familiarity of the driving location), this study applies the theory of communicative responsibility to the technology acceptance model to better understand why users are more likely to adopt certain navigation technologies while driving. We hypothesized that users’ perceived symmetry in communicative responsibility independently and interactively (with communicative behavior of the navigation system and the driving situation) affects perceived ease of use and usefulness of the navigation system. Consequently, the perceived ease of use and usefulness may affect the user’s intention to use the navigation system. This study found that usefulness was a significant predictor of behavioral intention. While driving in a less familiar location, the drivers perceived the navigation system to be more useful. When the navigation system provided location-specific landmarks, such as the name of a local store, drivers who attributed more communicative responsibility to the system were likely to find it useful

Phenomenological models in biological physics : cell growth and pluripotency maintenance

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2010.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references.A persistent challenge in quantitatively modeling a biological system is that the system often involves many components and just as dizzying number of interactions among those components. To further complicate matters, the parameters that characterize those interactions and components, like the rates of chemical reactions and concentrations of molecules inside the cell, have evaded detection by the conventional experimental tools. How does one model a system whose crucial parameters are unknown? And even if we know all the parameters inside the cell, there is an increasing uneasiness among many researchers that just writing down an equation for every interaction and components of the system is not practical. Crucially, it is not clear that such an extensive many-parameter model would always enhance our understanding of the complex biological system. A phenomenological model that involves just a few essential, easily measurable parameters that capture the essence of the complex biological system may provide insights that a many-parameter, large scale model may not provide. In this thesis, we describe our attempts at obtaining such a model for two complex biological systems: 1.) Cell growth as a result of glucose metabolism, and 2.) in vitro maintenance of the embryonic stem cell's pluripotency by a complex transcriptional network.by Hyun Youk.Ph.D

Global community effect: large-scale cooperation yields collective survival of differentiating embryonic stem cells [preprint]

“Community effect” conventionally describes differentiation occurring only when enough cells help their local (micrometers-scale) neighbors differentiate. Although new community effects are being uncovered for myriad differentiations, macroscopic-scale community effects - fates of millions of cells all entangled across centimeters - remain elusive. We found that differentiating mouse Embryonic Stem (ES) cells that are scattered as individuals over many centimeters form one macroscopic entity via long-range communications. The macroscopic population avoids extinction only if its centimeter-scale density is above a threshold value. Single-cell-level measurements, transcriptomics, and mathematical modeling revealed that this “global community effect” occurs because differentiating ES-cell populations secrete, accumulate, and sense survival-promoting factors, including FGF4, that diffuse over many millimeters and activate Yap1-induced survival mechanisms. Only above-threshold-density populations accumulate above-threshold-concentrations of factors required to survive. We thus uncovered a previously overlooked, large-scale cooperation that underlies ES-cell differentiation. Tuning such large-scale cooperation may enable constructions of macroscopic, synthetic multicellular structures

Dormancy-to-death transition in yeast spores occurs due to gradual loss of gene-expressing ability

Dormancy is colloquially considered as extending lifespan by being still. Starved yeasts form dormant spores that wake-up (germinate) when nutrients reappear but cannot germinate (die) after some time. What sets their lifespans and how they age are open questions because what processes occur-and by how much-within each dormant spore remains unclear. With single-cell-level measurements, we discovered how dormant yeast spores age and die: spores have a quantifiable gene-expressing ability during dormancy that decreases over days to months until it vanishes, causing death. Specifically, each spore has a different probability of germinating that decreases because its ability to-without nutrients-express genes decreases, as revealed by a synthetic circuit that forces GFP expression during dormancy. Decreasing amounts of molecules required for gene expression-including RNA polymerases-decreases gene-expressing ability which then decreases chances of germinating. Spores gradually lose these molecules because they are produced too slowly compared with their degradations, causing gene-expressing ability to eventually vanish and, thus, death. Our work provides a systems-level view of dormancy-to-death transition