Different response of maturing oocytes from two inbred strains of mice to sperm penetration

Oocytes from two inbred strains of mice, CBA and KE, were inseminated at prometaphase of the first meiotic division. Pronuclei were formed in 21-36% of inseminated oocytes from the CBA strain. In the KE strain the formation of pronuclei occurred in 2-5% of oocytes and was at the same level as in the non-inseminated control. These results show that in oocytes of the CBA strain maturity of the cytoplasm is acquired earlier than in those of the KE or other so far studied strains of mice. This fact is discussed in the context of different maturation rates of oocytes from different strains. In oocytes which remained non-activated after penetration, transformation of sperm heads into separate chromosomes was observed. An interstrain difference in efficiency of this process was also found.