Maturation of mouse oocytes in vitro is influenced by alkalization during their isolation

Dictyate oocytes were isolated from the ovaries of 22-day-old mice and cultured for 24 h. Isolation and classification of the oocytes were done at a stable pH of 7.45, during gradual increase of pH from 7.45 to 8.3, or at pH 8.3. In cumulus-enclosed oocytes alkalization beyond pH 7.45 resulted in an increased rate of degeneration and decreased rates of germinal vesicle breakdown and polar body formation. In the naked oocytes, only the rate of degeneration was affected. The results stress the importance of a stable pH during the period of isolation, and of a careful classification of the oocytes before culture.