14 research outputs found
Quantitative analysis of multi-spectral fundus images
We have developed a new technique for extracting histological parameters from multi-spectral images of the ocular fundus. The new method uses a Monte Carlo simulation of the reflectance of the fundus to model how the spectral reflectance of the tissue varies with differing tissue histology. The model is parameterised by the concentrations of the five main absorbers found in the fundus: retinal haemoglobins, choroidal haemoglobins, choroidal melanin, RPE melanin and macular pigment. These parameters are shown to give rise to distinct variations in the tissue colouration. We use the results of the Monte Carlo simulations to construct an inverse model which maps tissue colouration onto the model parameters. This allows the concentration and distribution of the five main absorbers to be determined from suitable multi-spectral images. We propose the use of "image quotients" to allow this information to be extracted from uncalibrated image data. The filters used to acquire the images are selected to ensure a one-to-one mapping between model parameters and image quotients. To recover five model parameters uniquely, images must be acquired in six distinct spectral bands. Theoretical investigations suggest that retinal haemoglobins and macular pigment can be recovered with RMS errors of less than 10%. We present parametric maps showing the variation of these parameters across the posterior pole of the fundus. The results are in agreement with known tissue histology for normal healthy subjects. We also present an early result which suggests that, with further development, the technique could be used to successfully detect retinal haemorrhages
On the modeling of endocytosis in yeast
The cell membrane deforms during endocytosis to surround extracellular
material and draw it into the cell. Experiments on endocytosis in yeast all
agree that (i) actin polymerizes into a network of filaments exerting active
forces on the membrane to deform it and (ii) the large scale membrane
deformation is tubular in shape. There are three competing proposals, in
contrast, for precisely how the actin filament network organizes itself to
drive the deformation. We use variational approaches and numerical simulations
to address this competition by analyzing a meso-scale model of actin-mediated
endocytosis in yeast. The meso-scale model breaks up the invagination process
into three stages: (i) initiation, where clathrin interacts with the membrane
via adaptor proteins, (ii) elongation, where the membrane is then further
deformed by polymerizing actin filaments, followed by (iii) pinch-off. Our
results suggest that the pinch-off mechanism may be assisted by a pearling-like
instability. We rule out two of the three competing proposals for the
organization of the actin filament network during the elongation stage. These
two proposals could possibly be important in the pinch-off stage, however,
where additional actin polymerization helps break off the vesicle. Implications
and comparisons with earlier modeling of endocytosis in yeast are discussed.Comment: 15 pages, 7 figures, accepted to Biophysical Journa