The interior of a living cell is a busy place. Molecular motors move materials along prescribed biopolymer tracks. This sort of active transport is required to rapidly move products over large distances within the cell, where passive diffusion is too slow. Just as understanding the flow of traffic is essential for probing the economy of a major city, exploring the intracellular traffic patterns of cells is fundamental to elucidating their activity. We examine intracellular traffic patterns using a new application of spatial light interference microscopy (SLIM). We used this quantitative phase imaging method to measure the dispersion relation, i.e. decay rate vs. spatial mode, associated with mass transport in live cells. This approach applies equally well to both discrete and continuous mass distributions without the need for particle tracking. From the quadratic experimental curve specific to diffusion, we extracted the diffusion coefficient as the only fitting parameter. The linear portion of the dispersion relation reveals the deterministic component of the intracellular transport. Our data show a universal behavior where the intracellular transport is diffusive at small scales and deterministic at large scales. Measurements by our method and particle tracking show that, on average, the mass transport in the nucleus is slower than in the cytoplasm. We further applied this method to studying transport in neurons. By modifying a traditional phase contrast microscope, we are able to use SLIM to map the changes in index of refraction across the neuron and its extended processes. What we found was that in dendrites and axons, the transport is mostly active, i.e., diffusion is subdominant.
Ru Wang's major is in mechanical engineering but in the lab she is developing methods for understanding the mechanics of cells. In one project, she uses microscopy to study the deformation of red blood cells (an important issue for understanding cellular oxygen transport), while another is aimed at developing a low-coherence optical spectroscopy method for extremely fast detection of the microrheology of complex fluids, including biological structures.
-Taken from ECE Media Center
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University of Illinois Urbana-Champaign, Urbana, IL