Effects of divalent cations on in vitro maturation of bovine oocytes.

Lowering the external concentrations of both Mg+2 and Ca+2 caused failure of meiotic resumption in vitro of bovine, oocyte-cumulus complexes. Lowering of external Ca+2 levels singly had no effect on either meiotic resumption or completion of the first meiotic division. Lowering of external Mg+2 concentrations alone, although having no effect on meiotic resumption in vitro when Ca+2 was present, did interfere with the completion of the first meiotic division. The result was arrest of oocyte maturation between germinal vesicle breakdown and formation of the first metaphase plate.