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 (<40 μg), 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 ∼32% water, ∼69 meq K⁺/liter H₂O, and ∼94 meq Na⁺/liter H₂O. Several nonyolk parameters, such as cation activities and nucleoplasmic binding, also appear to change during oogenesis.