Potential application of the FDTD technique to study mitochondrial apoptosis

Abstract

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.