Contribution of the mitochondrial compartment to the optical properties of the rat liver: a theoretical and practical approach.

The purpose of this work was to analyze the contribution of the mitochondria to the optical properties, i.e., light absorption and scattering, of the blood-free rat liver. Firstly, a theoretical model of the reduced scattering coefficient of the liver was performed by using the Mie theory, the Rayleigh-Debye-Gans approximation, and the electron microscopy descriptions of the liver ultrastructure. Compared with the hepatocyte volume, the nucleus and the peroxisomes, the mitochondria compartment, accounting for 22% of the liver cell volume, seemed to be the predominant factor for the light scattering of the liver. Second, by using time-resolved spectroscopy and a sample substitution method, we have measured the absorption and reduced scattering coefficients of blood-free perfused rat livers, isolated hepatocyte suspensions, and isolated mitochondria suspensions. A subsequent extrapolation of the isolated mitochondria data to the in vivo mitochondrial content and a comparison with the whole liver measurements lead to the following conclusions: 1) the mitochondria account for about 50% of the liver absorption coefficient at 780 nm (mu a = 0.25 cm-1 extrapolated from isolated mitochondria vs. 0.53 +/- 0.05 cm-1 measured for the liver); and 2) the mitochondrial compartment is the primary factor for the light scattering in the rat liver (mu s' = 15.5 cm-1 extrapolated from the isolated mitochondria versus 15.9 +/- 2.4 cm-1 measured for the liver), demonstrating the relevancy of our preliminary theoretical study.