Abstracts
Gaudenz Danuser
Head of BioMicroMetrics Group
Laboratory for Biomechanics,
Dpt of Materials
Swiss Federal Institute of Technology, ETHZ | Recycling noise:
what we can learn from thermal fluctuations in the
cytoskeleton |
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When living cells are observed at high spatial resolution and in short
time intervals, the motion of cell components are often dominated by
seemingly random positional fluctuations. What looks like disturbing
thermal noise at first sight is in fact bearer of valuable information
about the dynamic properties of the cell interior. We report two
applications where the analysis of thermal fluctuations shed new light
on the behavior and function of cytoskeleton components. In the first
example we studied the elasticity of microtubules in an in vitro assay
as an effective means to screen antimitotic drugs (Danuser et al., JM
2000). In the second example we mapped out the lamellipodial f-actin
retrograde flow in motile cells by measuring small random fluctuations
in otherwise stationary actin bundles (Danuser & Oldenbourg,
Biophys. J. 2000). In this talk we will review the critical issues in
designing algorithms for tracking nanometric fluctuations in living
cells, and we will present mathematical frameworks which allow us to
derive theoretical bounds for the ultimate tracking sensitivity. Also,
we will briefly comment on the biological implications of our results,
mainly on the mechanical role of f-actin retrograde flow in cell
motility.
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David Fleet
Xerox PARC and Queen's University
| Bayesian Analysis of Image Sequences:
Detection and Tracking of Motion Boundaries |
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Visual motion analysis concerns the estimation and recognition of motion
from image sequences. Its uses include the estimation of image velocity
(e.g., for video compression and scientific applications), the detection
and estimation of scene structure (e.g. locating surface boundaries), and the
detection and tracking of objects (e.g. 3d human motion capture from video).
This talk will address a long-standing problem in motion analysis, namely,
the detection and estimation of motion in the neighborhoods of surface
boundaries. Motion in these regions is discontinuous, and occlusions
cause image structure to appear or disappear from one image to the next.
Although these "motion boundaries" are often viewed as a source of noise
for current motion estimation techniques, we can also view them as a rich
source of information about the location of surface boundaries and the
depth ordering of surfaces at these locations.
We propose a Bayesian framework for representing and estimating image
motion in terms of multiple motion models, including both smooth motion
and local motion discontinuity models. We compute the posterior probability
distribution over models and model parameters, given the image data, using
discrete samples and a particle filter for propagating beliefs through time.
This talk will introduce the problem and describe our Bayesian approach,
including our generative models, the likelihood computation, the particle
filter, and a mixture model prior from which samples are drawn.
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Thomas C. Vogelmann
Department of Botany
University of Wyoming,
Laramie, WY, USA, 82071-3165
| Leaf Tissue Optics and Photosynthesis
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Within a leaf, the theoretical maximum amount of photosynthetic work that a cell layer can conduct is determined by its photosynthetic capacity, the amount of internal CO2, and the amount of light that is absorbed. At first glance it might seem that it should be relatively easy to estimate the amount of light absorbed for photosynthesis by each cell layer. Unfortunately, this is not an easy task because leaf tissues are optically complex and there have been limitations in experimental methods. Thus, it has been difficult to evaluate the significance of contrasting leaf anatomies with respect to photosynthesis. This lecture will provide an overview of leaf optics and some relationships with photosynthesis.
Leaf tissue anatomy determines how much light migrates through the leaf and its path of migration. In most leaves, the upper (adaxial) epidermis acts as a layer of microscopic lenses that concentrates light on underlying cell layers. Using image analysis, maps of focal intensification were produced and they indicate that light is usually focused two to four-times above incident light but focal concentrations of up to fifteen-fold are not uncommon.
Located immediately beneath the epidermis, leaf mesophyll tissues control light propagation two ways (1) absorption and (2) light scattering. By altering pigment content, leaf mesophyll tissues directly control the amount of light that is absorbed. These tissues can also control light absorption indirectly by altering cell shape which controls light scattering and photon pathlength. As an example, palisade cells act as waveguides (minimal scattering) that facilitate the penetration of light whereas spongy mesophyll scatters light intensely thereby increasing pathlength. The complexity of leaf tissue optics make it difficult to calculate light gradients within leaves but measurements with microscopic light sensors made from optical fiber indicate that light gradients are exponential within the visible region of the spectrum.
Light scattering is one of the most difficult physical parameters to quantify within leaves but recently it has become possible to do this by clocking the time that it takes for a photon to move through a leaf and using this time to calculate pathlength. Utilizing 17 fs pulses from a Ti:Sapphire laser, the mean transit time was clocked at 880 - 1010 fs for Hibiscus leaves, 170 mm thick. This gives a pathlength enhancement factor of 1.2 - 1.4, values that compare favorably with estimates made spectrophotometrically. Measurements for other leaves indicate that there is a correlation between mesophyll anatomy and pathlength, indicating that leaves may control cell shape and packing to influence light absorption.
It follows that subtle changes in leaf anatomy can have profound influences on the profile of absorbed quanta within the leaf and recently a technique has been developed to measure absorption profiles utilizing chlorophyll fluorescence as an intrinsic probe for light absorption. Leaves were irradiated on their upper or lower surface and profiles of chlorophyll fluorescence were captured from the leaf cross-sectional view. The shape of the absorption profile is determined by the wavelength of light and by mesophyll anatomy. For example, in spinach and Eucalyptus pauciflora, the proportion of absorbed quanta rose rapidly in the mesophyll to a maximum, then declined more gradually across the remainder of the leaf. In E. pauciflora, 80% of the blue light was absorbed within the initial 140 mm of tissue whereas 80% of green light was absorbed over 350 mm of tissue. Absorption profiles were narrower in spongy mesophyll tissue because of enhanced light scattering. Internal profiles for light absorption
and photosynthesis were different between a bifacial leaf (spinach) and an isolateral one (E. pauciflora), reflecting their anatomical specialization for performance under different light environments. The fundamentals of leaf tissue optics is now being linked to leaf anatomy and photosynthesis providing a new window on plant adaptation to their light environment.
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