Cytoplasmic and Nuclear Maturation of Rabbit Oocytes in Vitro

Rabbit oocytes from preovulatory follicles resume meiosis up to metaphase II in normal delay when cultured in TC 199 supplemented with rabbit and calf sera and chick embryo extract. They are fertilizable, but such oocytes are unable to assume transformation of the sperm head into a normal nucleus, the male pronucleus. Rabbit preovulatory follicle can be maintained in healthy conditions for at least 24 hours, if gas pressure is increased to 5 to 10 bars/C M2during culture (air + 0.5 p. 100 C0 2), and in the presence of gonadotropins. In such follicles, oocytes remain in the dictyate stage when crude horse pituitary extract is added to the culture medium. However, if subphysiological doses of ovine LH or FSH (or better, FSH and LH) are present in the culture medium, meiosis resumes and complete cytoplasmic maturation occurs in all oocytes, as proved by normal fertilization and growth of the male pronucleus. Thus, two steps may be considered in complete oocyte maturation : i. The resumption of meiosis by rupture of the inhibitory effect of the granulosa layer. This is not basically dependent on gonadotropins. 2. On the contrary, passage of the male pronucleus growth factor, or its precursor, inside the oocyte is gonadotropicfollicular dependent. These processes are both the necessary prerequisites to normal fertilization, and they can be activated in a pure in vitro system.