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Activation of in vitro matured mouse oocytes arrested at first or second meiotic metaphase
1Department of Genetics and Evolution, Jagiellonian University, Kraków, Poland.
Abstract:
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.
Insights
Mouse oocytes arrested in vitro can resume meiosis and activate, similar to normally maturing oocytes. This suggests arrested oocytes retain developmental potential, offering insights into oocyte maturation and activation processes.
Area of Science:
- Reproductive Biology
- Developmental Biology
- Cell Cycle Regulation
Background:
- Mammalian oocytes can arrest during in vitro maturation, failing to reach the second metaphase stage.
- Understanding the developmental potential of these arrested oocytes is crucial for reproductive research.
Purpose of the Study:
- To investigate the response of mouse oocytes arrested at the first metaphase (MI) to sperm penetration and parthenogenetic activation.
- To compare the developmental capacity of MI-arrested oocytes with those that reached the second metaphase (MII).
Main Methods:
- Ovarian mouse oocytes from CBA/Kw and KE strains were cultured in vitro for 20 hours.
- Both MI-arrested and MII oocytes were inseminated or activated using electric shock.
- Meiotic resumption and activation rates were assessed.
Main Results:
- A majority of both MI and MII oocytes resumed meiosis upon sperm penetration, with higher frequency in the CBA strain.
- Unpenetrated oocytes from the CBA strain showed significant spontaneous meiotic resumption.
- Parthenogenetic activation rates were similar for MI and MII oocytes, increasing with maturation time and post-first polar body extrusion resting time.
Conclusions:
- Mouse oocytes arrested at MI during in vitro maturation retain the capacity for cytoplasmic maturation and activation.
- These findings suggest that MI-arrested oocytes possess developmental potential comparable to MII oocytes.
- Oocyte activation requires a resting period after first polar body extrusion to restore cell cycle balance.