Estimation of organelle water fractions from frozen-dried cryosections

Local dry mass or water fractions can be measured on frozen-dried cryosections assuming constant section thickness in the hydrated state and no net water movements and no differential shrinkage during freezing and drying. These assumptions have been tested on a model consisting of isolated rat liver mitochondria in an albumin matrix with a concentration similar to the dry mass concentration of the cytoplasm. The dry mass concentrations of mitochondria before freezing as measured by interference microscopy and after freezing and freeze-drying of the sections as measured by X-ray microanalysis and scanning microdensitometry are shown to be equal as long as the ice crystals in the medium are smaller than about 100 nm. It is concluded, therefore, that the above-mentioned assumptions could also hold for the cryopreparation of cells and tissues.