In Vitro Development of Mouse Oocytes1

Several aspects of the early in vitro development of the laboratory mouse were examined: oocyte nuclear maturation, fertilization, fertilization of oocytes matured in vitro, and fetal development of oocytes matured and fertilized in vitro. Initially, optimum conditions for both oocyte nuclear maturation and fertilization were established. Oocyte nuclear maturation was greatest (91.7%) when oocytes were cultured with cumulus cells. The reduction in maturation when oocytes were cultured without cumulus cells (to 59.2%) was significant (p < 0.02) only when serum was not present in the medium. Fertilization in vitro of oviducal oocytes, measured by the number of two-cell ova formed, was greatest (90.1%) when the medium contained 30 mg/ml protein and the oocytes were fertilized with adhering cumulus cells. The removal of the cumulus cells from the oocytes prior to fertilization reduced (p < 0.001) the number of two-cell ova formed (90.1% to 46.3%). Using the optimum conditions which produced 91.7% nuclear maturation and 90.1% fertilization, 37.5% of oocytes matured in vitro were fertilized in vitro. When serum was omitted from the maturation medium a significant drop (p < 0.01) to 22.5% fertilization occurred. When oocytes matured and fertilized in vitro were transferred into foster mothers, 3.2% developed into 15-day-old fetuses, thus demonstrating that normal oocyte maturation processes can occur in vitro.