Apparent diffusion of water, ions, and small molecules in the Xenopus oocyte is consistent with Brownian displacement

The incoherent displacement of water in living tissues is of considerable interest because of the widespread use of diffusion-weighted MRI, for which image contrast is based on the water apparent diffusion coefficient (ADC). It has been hypothesized that the decrease in water ADC associated with brain injury is primarily due to a reduction in the ADC of water in the intracellular space. Xenopus oocytes permit direct measurement of ADC values for intracellular molecules, thereby providing insight into the nature of intracellular motion. In this study, the measured ADC values of small molecules and ions are shown to be primarily size-dependent, indicating that intracellular water motion in the oocyte is mainly Brownian displacement with little or no role for cytoplasmic streaming. Further, intracellular water ADC values show no dependence on diffusion time over a broad range (3.4-100 ms), suggesting that barriers to displacement are finely spaced (< or = 2-3 microm). The water diffusion shows some small anisotropy, suggesting that the cell has structure, giving water displacement a directional preference. The calculated intracellular apparent viscosity, which reflects the combined effects of barriers to motion, intermolecular binding, and fluid phase viscosity was 2.07 +/- 0.09 cP.