Configuration of maternal and paternal chromatin and pertaining microtubules in human oocytes failing to fertilize after intracytoplasmic sperm injection

The microtubules and chromosomes of 180 human oocytes failing to fertilize after intracytoplasmic sperm injection were observed in order to establish how sperm chromatin and sperm astral microtubule configuration is related t o the phases of oocyte cell cycle, and to find the defects in those structu res causing fertilization arrest. As many as 125 (69%) oocytes were arreste d at metaphase II. In one-fourth of them, damages of the second meiotic spi ndle were noted. In their cytoplasm intact sperm were found in 38 (30%) cas es, a swollen sperm head in 36 (29%) and prematurely condensed sperm chromo somes (G1-PCC)-a result of active mitosis promoting factor (MPF)-in 51 (41% ) cases. G1-PCC were mostly (73%) surrounded by the bipolar paternal spindl e instead of astral microtubules. A male pronucleus was never presented in metaphase II oocytes. In 19 (11%) oocytes, arrested at anaphase II, no inta ct sperm were found. As many as 9 (47%) oocytes contained sperm in G1-PCC f orm, which proves that anaphase II oocytes mostly retain active MPF, despit e oocyte activation. As many as 78% of 36 monopronucleate oocytes contained sperm, with delay in the process of sperm nucleus decondensation. Sperm in G1-PCC form and a bipolar paternal spindle were never found in monopronucl eate oocytes. From this we conclude that sperm that does not activate the o ocyte may continue decondensing the chromatin, but the oocyte prevents male pronucleus formation before the female one, mostly by causing PCC in the s perm and by duplicating the sperm centrosome. (C) 2000 Wiley-Liss, Inc.