Ionic requirements for induction of maturation (meiosis) in full-grown and medium-sized Xenopus laevis oocytes.

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Abstract
Full-grown (1.3 mm in diameter) and medium-sized (0.8-1.0 mm in diameter) oocytes were compared. Provided the medium is K+-free, valinomycin induces maturation in full-grown, but not in medium-sized, oocytes. Increasing the CaCl2 (20 mM) or the MgSO4 (40 mM) content of the medium induces maturation in full-grown, but not in medium-sized, oocytes; the latter undergo germinal vesicle breakdown after treatment with either progesterone or ionophore A23187 if there is an excess of Ca2+ or Mg2+ in the medium. Maturation is possible in a Na+-free medium, but amiloride inhibits germinal vesicle breakdown when NaCl is present in the medium. Maturation is apparently controlled by changes in the balance between the various ions rather than by Ca2+ alone.