Control of cell volume in oocytes and eggs from Xenopus laevis

The mechanism by which Xenopus laevis oocytes and eggs survive in hypotonic media was investigated by measuring cell volume (Vc) and intracellular pressure (P(ic)) during osmotic swelling. After 2 hr in 10 mOsm buffer, eggs remained intact, while 60.2 +/- 7.0% of oocytes had burst. Water uptake was proportional to buffer osmolarity in oocytes, but not in eggs. We did not see a regulatory volume decrease in the cells, nor was there any evidence for osmolyte efflux. Oocyte P(ic) increased from 0.27 cm H2O (range 0.14-0.61 cm H2O) to 5.92 cm H2O (range 2.52-7.18 cm H2O, P < 0.001) after 70 min in 20 mOsm buffer. Egg P(ic) in 20 mOsm buffer (0.04 cm H2O, range -0.07 to 0.014 cm H2O) was not significantly different from the value in isotonic (200 mOsm) buffer (0.14 cm H2O, range 0-0.16 cm H2O). The data indicate that Vc in oocytes and eggs is not regulated by either plasma membrane ion transport or the mechanical properties of the vitelline envelope. While oocytes appear to behave as passive osmometers, we speculate that the ability of eggs to maintain volume is determined, in part, by the cytoplasm, in particular by its gel-like nature.