The first passage problem for diffusion through a cylindrical pore with sticky walls

We calculate the first passage time distribution for diffusion through a cylindrical pore with sticky walls. A particle diffusively explores the interior of the pore through a series of binding and unbinding events with the cylinder wall. Through a diagrammatic expansion we obtain first passage time statistics for the particle's exit from the pore. Connections between the model and nucleocytoplasmic transport in cells are discussed.Comment: v2: 13 pages, 6 figures, substantial revision

Spindle Positioning by Cortical Pulling Forces

SummaryProper spatial control of the cell division plane is essential to any developing organism. In most cell types, the relative size of the two daughter cells is determined by the position of the mitotic spindle within the geometry of the mother cell. We review the underlying mechanisms responsible for positioning of the mitotic spindle, both in cases where the spindle is placed in the center of the cell and in cases where the spindle is placed away from the center of the cell. We discuss the idea that cortical pulling forces are sufficient to provide a general mechanism for spindle positioning within symmetrically and asymmetrically dividing cells

PAR proteins diffuse freely across the anterior–posterior boundary in polarized C. elegans embryos

FRAP reveals that a stable PAR boundary requires balancing diffusive flux of PAR proteins between domains with spatial differences in PAR protein membrane affinities

Pulsatory Patterns in Active Fluids

We show that pulsatory patterns arise in thin active films in which two chemical species regulate active stress. The regulating species diffuse within the film and are advected by self-generated flows resulting from active stress gradients. Spontaneous pulsatory patterns emerge when the following conditions are met: (i) the fast-diffusing species up-regulates and the slow-diffusing species down-regulates active stress, or (ii) the active stress up-regulator turns over faster compared to the active stress down-regulator. Our study, motivated by pulsatory patterns in the actomyosin cortex in cells and tissues, provides a simple generic mechanism for oscillatory patterns in active fluids

Temperature dependence of cell division timing accounts for a shift in the thermal limits of <i>C.elegans</i> and <i>C.briggsae</i>

Cold-blooded animals, which cannot directly control their body temperatures, have adapted to function within specific temperature ranges that vary between species. However, little is known about what sets the limits of the viable temperature range. Here we show that the speed of the first cell division in C.elegans N2 varies with temperature according to the Arrhenius equation. However, it does so only within certain limits. Outside these limits we observe alterations inthe cell cycle. Interestingly, these temperature limits also correspond to the animal's fertile range. In C.briggsae AF16, isolated from a warmer climatic region, both the fertile range and the temperature range over which the speed of cell division follows the Arrhenius equation, are shifted toward higher temperatures. Our findings suggest that the viable range of an organism can be adapted in part to a different thermal range by adjusting the temperature tolerance of cell division.Facultad de Ciencias ExactasInstituto de Física de Líquidos y Sistemas Biológico

Temperature dependence of cell division timing accounts for a shift in the thermal limits of <i>C.elegans</i> and <i>C.briggsae</i>

Cold-blooded animals, which cannot directly control their body temperatures, have adapted to function within specific temperature ranges that vary between species. However, little is known about what sets the limits of the viable temperature range. Here we show that the speed of the first cell division in C.elegans N2 varies with temperature according to the Arrhenius equation. However, it does so only within certain limits. Outside these limits we observe alterations inthe cell cycle. Interestingly, these temperature limits also correspond to the animal's fertile range. In C.briggsae AF16, isolated from a warmer climatic region, both the fertile range and the temperature range over which the speed of cell division follows the Arrhenius equation, are shifted toward higher temperatures. Our findings suggest that the viable range of an organism can be adapted in part to a different thermal range by adjusting the temperature tolerance of cell division.Facultad de Ciencias ExactasInstituto de Física de Líquidos y Sistemas Biológico

Polarization of PAR Proteins by Advective Triggering of a Pattern-Forming System

In the Caenorhabditis elegans zygote, a conserved network of partitioning-defective (PAR) polarity proteins segregates into an anterior and a posterior domain, facilitated by flows of the cortical actomyosin meshwork. The physical mechanisms by which stable asymmetric PAR distributions arise from transient cortical flows remain unclear. We present evidence that PAR polarity arises from coupling of advective transport by the flowing cell cortex to a multistable PAR reaction-diffusion system. By inducing transient PAR segregation, advection serves as a mechanical trigger for the formation of a PAR pattern within an otherwise stably unpolarized system. We suggest that passive advective transport in an active and flowing material may be a general mechanism for mechanochemical pattern formation in developmental systems