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

Abstract

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 µm s^-1 in small cells 200 µm in diameter, to 35 µm s^-1 in cells 800 µm in diameter and then fell to 5µm s^-1 in large cells of 2000 µm diameter. The factor (f) by which microvilli 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 f rose from 2 x at 200 µm diameter, to 11 x at 800 µm diameter and then fell to 5 x at 2000 µm diameter. The correlation coefficient between k and f was 0 474 (0 005 > P > 0.001). Corrected permeability coefficients, (k'), calculated so as to take account of the effects of the microvilli, (k' = k/f), declined with increasing oocyte size, from 5 µm s^-1 in small cells (200 µm) to 1 ,µm s^-1 in large cells (2000 µm). The correlation between k and f 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 µm s^-1 combined with an internal diffusion coefficient in the range from 6 x 10^-8 to 10^-6 cm² s^-1 could account for the effects found.