The effect of calcium on the first meiotic division of the mammalian oocyte.

In the mammalian ovary, fully grown oocytes are arrested at the diplotene stage of the first meiotic division. While oocytes in vivo resume meiosis only in response to a preovulatory surge of gonadotropic hormones, oocytes isolated from the ovaries and cultured in vitro will spontaneously resume meiosis. Both in vivo and in vitro, meiotic maturation proceeds through the extrusion of the first polar body, where it is again arrested until fertilization. We have used the spontaneous, in vitro maturation of the mouse oocyte to examine the role of calcium in germinal vesicle breakdown (GVBD) and polar body extrusion. In calcium-free medium or in the presence of concentrations of lanthanum greater than 0.5 mM, the oocytes degenerate rapidly. However, there does not appear to be any effect upon maturation that can be distinguished from a general toxicity of these media. In contrast, two treatments that are known to inhibit transmembrane calcium movements, verapamil and tetracaine each, individually, inhibit polar body formation. They have no effect on GVBD. In addition, oocytes cultured in media containing a higher calcium concentration than control media (greater than 10 mM versus 1.71 mM) show a significantly higher percentage of polar body formation. We conclude that, in these culture conditions, extracellular calcium is not required for GVBD but is required for the completion of the first meiotic division of a mammalian oocyte.