Effect of Ca²⁺/Mg ²⁺-free Culture Condition on Spontaneous First Cleavage of Rat Oocytes

Abstract

The aim of this study was to investigate the effect of Ca²⁺- and Mg²⁺-free culture conditions on induction of spontaneous first cleavage in rat oocytes. Oocytes recovered from superovulated Wistar females 17 h after hCG injection were exposed for 1.5 h to Ca²⁺/Mg²⁺-free, Ca²⁺-free, Mg²⁺-free, or control mKRB medium supplemented with 5 μg/ml cytochalasin-B (CB). The oocytes were then cultured for 4.5 h in control medium containing CB and for 28 h in CB-free control medium. The oocytes exposed to the Ca²⁺/Mg²⁺-free medium (23%) cleaved better than those exposed to Ca²⁺-free (8%), Mg²⁺-free (0%), or control (0%) media. The cleavage rates of oocytes recovered 12, 17 and 22 h after hCG injection and exposed to the Ca²⁺/Mg²⁺-free medium were 4, 17 and 22%, respectively. The efficiency of oocyte activation in 1.25 mM Sr²⁺ treatment for 6 h was not influenced by the culture background of bivalent cations (cleavage rates: both 49% in Ca²⁺/Mg²⁺-free and Ca²⁺-free medium). These results indicate that Ca²⁺/Mg²⁺-free culture condition induced the spontaneous first cleavage of rat oocytes in an age dependent manner, but had no additional effect on Sr²⁺-induced oocyte activation.