In apoptotic cells, mitochondria have been shown to undergo morphological and structural
changes caused by intra-membrane biochemical events. These subcellular changes are dynamic
and progress over time via distinct morphological stages observable using electron microscopy
(EM). To investigate whether changes in the morphology of the mitochondrial matrix could be
detected and monitored by non-invasive light scattering methods, we performed simulations of
light scattering by synthetic three-dimensional mitochondrial matrix models using the finite
difference time domain (FDTD) technique. Our models consisted of spherical or ellipsoidal
particles (matrix region) contained within a spherical or ellipsoidal mitochondrion. Within the
mitochondrion model, the small particles' refractive index was taken to be 1.4, while the
refractive index of the surrounding volume was taken to be 1.35, and equal to that of the
cytoplasm surrounding the mitochondrion. Depending on the matrix volume ratio, particles
within the mitochondrion can be either overlapping or non-overlapping. Our results suggest that
measurable changes in light scattering by mitochondria can be detected in central dark-field
microscopy. By analyzing the angular dependence of light scattered within the numerical
aperture of a standard 63X objective (~67° solid angle), the simulations suggest that matrix
regions on the order of 100nm can be detected. Light scattering by mitochondria could be
altered both by the shape of these features, and by the matrix volume fraction within the
mitochondrion. Our data suggest that optical scatter microscopy could be used to guide EM
studies by rapidly assessing relative changes in mitochondrial morphology in living cells, and
defining important time points to be further analyzed by EM.
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