Water, potassium, and sodium during amphibian oocyte development

Water, K+, and Na+ were measured in Rana pipiens oocytes during growth and prematurational development using low-temperature microdissection. Whole oocytes were analyzed during previtellogenic and vitellogenic growth. Ooplasm and germinal vesicle (nucleus) were analyzed at the onset and conclusion of vitellogenic growth. In previtellogenic oocytes (less than 40 micrograms), water, K+, and Na+ concentrations resembled those in somatic cells and were independent of cell size. With the onset of yolk deposition, water and K+ concentrations progressively decreased and Na+ progressively increased. These changes were restricted to ooplasm, the site of yolk deposition. In full-grown oocytes, vegetal ooplasm, with greater yolk density than animal ooplasm, contained less water and K+ and more Na+ than animal ooplasm. Collectively, the data indicate that yolk is poorer in water and K+ and richer in Na+ than yolk-free ooplasm (cytoplasm) or nucleoplasm. Yolk concentrations were estimated to be approximately 32%, water, approximately 69 meq K+/liter H2O, and approximately 94 meq Na+/liter H2O. Several nonyolk parameters, such as cation activities and nucleoplasmic binding, also appear to change during oogenesis.