Electro-rotation of mouse oocytes: single-cell measurements of zona-intact and zona-free cells and of the isolated zona pellucida.

Passive electrical properties of oocytes and of zonae pellucidae, and the mechanical coupling between them, can be elucidated by means of rotating-field-induced rotation. In low-conductivity media (25-100 microS/cm) rotation of mouse oocytes (with or without their zonae) requires fields in the 1-100 kHz frequency range. However, an isolated zona shows weak rotation in the opposite direction to that of a cell, and in response to much higher field frequencies (approx. 1 MHz). In zona-intact mouse oocytes, the rotation of cell and zona are not rigidly coupled: thus rotation of the cell can still be induced when the zona is held stationary. However, rotation of freely suspended zona-intact cells is much slower than that of zona-free cells and requires an optimum field frequency that is approximately 1.5 kHz higher. These observations show that the electrical properties of the oocyte that are measured by rotation are altered by the presence of the zona pellucida, even though no such influence has been detected using micro-electrodes. The data are consistent with the zona acting as a porous shell with a conductivity of 40 microS/cm (preliminary estimate made at a single medium conductivity of 26 microS/cm). Measurements on cells from which the zonae had been removed gave values for the membrane capacity and resistivity of 1.2-1.3 microF/cm2 and 400 omega.cm2, respectively. These values may reflect the presence of plasmalemma microvilli. The results strongly suggest that the technique may be useful for studies of cell maturation and for in vitro fertilization, because the cells may be further cultured after measurement.