Phenotype and maturation rate of oocytes of chimaeric mice produced from two strains that differ in oocyte quality

The oocytes of females from the inbred mouse strains KE (albino) and CBA (agouti) differ in the following characteristics: the appearance of the cytoplasm is clear in KE but granular in CBA; the cumulus oophorus dispersion with hyaluronidase is quick in KE but slow in CBA; dissolution of the zona pellucida with a proteolytic enzyme is slow in KE but quick in CBA; and the maturation stage at the time of ovulation is metaphase I in KE while in CBA the first polar body has been extruded. Aggregation chimaeras were produced to investigate whether these differences are intrinsic or extrinsic to developing oocytes. Among 13 fertile chimaeric females, two produced only CBA oocytes, four produced both CBA and KE oocytes and were germline chimaeras, and seven produced only KE oocytes, as recognized by progeny testing. The larger number of females producing KE oocytes is a result of strong selection against CBA germ cells. All chimaeric females had cumuli composed of both KE and CBA cells, as recognized by the time of dispersion and glucose phosphate isomerase electrophoresis, but the presence of non-homologous cumulus cells did not change the character of the developing oocytes significantly. The conclusion from this study is that the rate of meiotic maturation, sensitivity of oocyte investments to enzymes, and deposition of granules in ooplasm are determined largely autonomously by genes acting in the germ cells.