Activation of in vitro matured mouse oocytes arrested at first or second meiotic metaphase.

Some mammalian oocytes fail to complete maturation in vitro and arrest development at the first metaphase stage. The response of such blocked oocytes to sperm penetration was investigated. Ovarian mouse oocytes from two inbred strains, CBA/Kw and KE, were cultured in vitro for 20 h. Both oocytes arrested at the first metaphase (MI oocytes) and second metaphase (MII oocytes) were then inseminated. The majority of MII and MI oocytes reinitiated meiosis in response to sperm penetration, although those from the CBA strain did with higher frequency. Moreover, a high proportion of unpenetrated oocytes from CBA, but not the KE strain, resumed meiosis (33% for MII and 48% for MI oocytes, respectively). Parthenogenetic activation of MI-arrested oocytes was demonstrated in (CBAxKE)F1 mice; ovarian oocytes matured in vitro and then treated by electric shock were activated with a similar total frequency of 52.4% for MI and 47.8% for MII oocytes. The rate of activation increased equivalently for both MI and MII oocytes as the length of maturation prolonged. This demonstrates that mouse oocytes arrested at MI during their maturation in vitro continue cytoplasmic maturation and become capable of undergoing activation in a way similar to those maturing to MII. Additionally, in MII oocytes cultured for an equal time in vitro the rate of activation increased with the time lapse after first polar body (PB1) extrusion. This indicates that after PB1 extrusion, the oocyte requires some resting time before it may be activated, perhaps to restore the proper balance between elements of the cell cycle controlling the mechanism involved in first meiotic division.