Assisted fertilization of mouse oocytes and preliminary results for human oocytes using zona drilling

The zona-drilling procedure was investigated in mouse oocytes prior to a study on human oocytes. The procedure involved the injection of 5-nl volumes of acidic Hepes-buffered medium at pH 2.5 using a microinjection instrument. Zona-drilled mouse oocytes had significantly higher rates of fertilization (60/99; 61%) than zona-intact oocytes (6/103; 6%) at an insemination concentration of 1 x 10(4) sperm/ml (P less than 0.001). The procedure did not induce parthenogenetic activation of oocytes and more than 97% of zygotes developed to the blastocyst stage. A similar rate of live progeny was observed when zona-drilled (38.0%) and control embryos (38.5%) were transferred to pseudopregnant recipients. Chromosome analyses were performed on zona-intact, zona-free, and zona-drilled oocytes inseminated with varying concentrations of sperm and analysed at the first cleavage division. Zona-free oocytes had high rates of polyploidy (greater than or equal to 40%) with varying insemination numbers but the zona-drilled oocytes did not reveal significant increases in the rate of polyploidy or aneuploidy when compared to controls. In the human studies, zona-drilled oocytes achieved higher rates of fertilization than zona-intact oocytes, with sperm numbers as low as 1 x 10(4)/ml (6/8; 75%). Polyspermic fertilization was observed in 1/2 and 2/6 of fertilized oocytes inseminated with 1 x 10(5) and 1 x 10(4) sperm/ml, respectively. With the low sperm concentration 2/4 of those which were normally fertilized developed to healthy blastocysts. These studies suggest that the zona-drilling technique as described can be performed without apparent harm to oocytes and generate normal embryos.(ABSTRACT TRUNCATED AT 250 WORDS)