Atypical maturation of oocytes of strain I/LnJ mice

The maturation of strain I/LnJ oocytes was compared to oocytes of selected inbred strains. The time of germinal vesicle breakdown (GVB) of I/LnJ oocytes was greatly delayed compared to all other strains tested. In addition, 5% of the cumulus cell-enclosed oocytes isolated from antral follicles of I/LnJ mice failed to undergo GVB in vitro and 58% of the oocytes that underwent GVB failed to progress beyond metaphase I. Similar defects in the progression of meiosis occurred when maturation was stimulated in vivo by the administration of exogenous gonadotrophins. When in-vitro matured metaphase II oocytes were selected for in-vitro fertilization, similar percentages of I/LnJ oocytes underwent fertilization and cleavage to the 2-cell stage as oocytes from another inbred stain, C57BL/6J, and similar percentages of 2-cell stage oocytes completed the 2-cell stage to blastocyst transition in vitro. However, unlike C57BL/6J oocytes, a much lower percentage of oocytes that matured in vivo in response to exogenous gonadotrophins underwent fertilization and cleavage to the 2-cell stage than oocytes that underwent maturation in vitro. Likewise, lower percentages of 2-cell stage embryos derived from in-vivo matured I/LnJ oocytes developed to blastocysts than embryos derived from in-vitro matured oocytes. These results show than I/LnJ oocytes are atypical in the progression of both nuclear and cytoplasmic maturation. These defects may account for the poor reproductive performance of I/LnJ mice. Thus, I/LnJ mice might be a useful model for studying infertility resulting from defective oocytes.