Fertilization and early embrology: Atypical maturation of oocytes of strain I/LnJ mice

The maturation of strain I/LnJ oocytes was compared to oocytesof selected inbred strains. The time of germinal vesicle breakdown(GVB) of I/LnJ oocytes was greatly delayed compared to all otherstrains tested. In addition, 5% of the cumulus cell-enclosedoocytes isolated from antral follicles of I/LnJ mice failedto undergo GVB in vitro and 58% of the oocytes that underwentGVB failed to progress beyond metaphase I. Similar defects inthe progression of meiosis occurred when maturation was stimulatedin vivo by the administration of exogenous gonadotrophins. Whenin-vitro matured metaphase II oocytes were selected for in-vitrofertilization, similar percentages of I/LnJ oocytes underwentfertilization and cleavage to the 2-cell stage as oocytes fromanother inbred stain, C57BL/6J, and similar percentages of 2-cellstage oocytes completed the 2-cell stage to blastocyst transitionin vitro. However, unlike C57BL/6J oocytes, a much lower percentageof oocytes that matured in vivo in response to exogenous gonadotrophinsunderwent fertilization and cleavage to the 2-cell stage thanoocytes that underwent maturation in vitro. Likewise, lowerpercentages of 2-cell stage embryos derived from in-vivo maturedI/LnJ oocytes developed to blastocysts than embryos derivedfrom in-vitro matured oocytes. These results show than I/LnJoocytes are atypical in the progression of both nuclear andcytoplasmic maturation. These defects may account for the poorreproductive performance of I/LnJ mice. Thus, I/LnJ mice mightbe a useful model for studying infertility resulting from defectiveoocytes.