Dispersion-relation Phase Spectroscopy of Intracelluar Transport

Posted on March 26, 2012 by admin

Cells have developed a complex system to govern the internal transport of materials from single molecules to large complexes such as organelles during cell division . While these processes are essential for the maintenance of cellular functions, their physical understanding is incomplete. It is now well documented that intracellular transport is mediated by both thermal diffusion and molecular motors that drive the cellular material out of equilibrium.  Measurements of the molecular motor driven transport have been made in the past via single molecule tracking . However, developing a more global picture of the spatial and temporal distribution of active transport in living cells remains an unsolved problem. Approaching this question experimentally can be reduced fundamentally to the problem of quantifying spatially heterogeneous dynamics at the microscopic scale. In the past, cellular material has been studied successfully by both externally-driven  and passive particle tracking. Recently, bending fluctuations of microtubules have been used to probe the active dynamics in actin networks .Here we introduce another approach – quantitative phase imaging (QPI) – to study the spatial and temporal distribution of active (deterministic) and passive (i.e. diffusive) transport processes in living cells.

Below figures c-h shows the SLIM images of various cells in culture and the  curves associated with the respective regions. These data exhibit a diversity of behavior, from purely diffusive in the microglia culture (Figs. c-d) to purely directed, in the putative dendrite of a neuron (Figs. e-f) .

Quantitative phase image of a culture of glia (a, g), microglia (c) and hippocampal neurons (e). b) Dispersion curve measured for the cell in a. The green and red lines indicate directed motion and diffusion, respectively, with the results of the fit as indicated in the legend. Inset shows the G(qx, qy) map. d, f, h) Dispersion curves, G(q), associated with the white box regions in c),e) and g), respectively. The corresponding fits and resulting D and Dv values are indicated.