The effect of surface microvilli on the water permeability of single toad oocytes.

The water permeability of isolated toad ovarian oocytes was found from their rate of shrinkage in hypertonic Ringer's solution. Apparent membrane permeability coefficients, (k), calculated on the assumption that the cell surface was smooth, rose from about 20 fim s" in small cells 200 /im in diameter, to 35 fim s' in cells 800 fim in diameter and then fell to 5 fim s' in large cells of 2000 fim diameter. The factor (/) by which microvilh extend the oocyte surface area beyond that of a smooth sphere was estimated from an analysis of electron micrographs of the cell surface. The value of / rose from 2 x at 200 fim diameter, to 11 x at 800 fim diameter and then fell to 5 x at 2000 fim diameter. The correlation coefficient between k and / was o 474 (o 005 > P > 0001). Corrected permeability coefficients, (k'), calculated so as to take account of the effects of the microvilh, (k' = kjf), declined with increasing oocyte size, from 5 fim s" in small cells (200 fim) to 1 fim s" in large cells (2000 fim). The correlation between k and / indicates resistance to water flow by the cell membrane, while the decrease in k' as the cell grows suggests that diffusion in the cytoplasm may also be sufficiently slow to affect water flow out of the cell. It may be calculated that a surface membrane permeability coefficient in the range from 2 to 30 fim s" combined with an internal diffusion coefficient in the range from 6 x io~ to io~* cm' s" could account for the effects found.