Implementation of intracytoplasmic sperm injection (ICSI) in human in-vitro fertilization (IVF) has highlighted the need for information about the risk of nuclear spindle damage caused by this procedure. For this purpose we studied the final products of oocyte meiosis at the first cleavage division of multipronuclear zygotes arising after ICSI, and compared the results with abnormally fertilized oocytes after conventional in-vitro insemination. Of 37 successfully analysed tripronuclear zygotes, 18 had three individual metaphases. Abnormal complements of 11 zygotes in this group indicated that non-disjunction occurred predominantly at the second meiotic division of the oocytes. Nine of the 37 tripronuclear zygotes exhibited two individual metaphases. Seven were abnormal and there were some indications that non-disjunction took place during oocyte meiosis. Of the 37 tripronuclear zygotes, 10 had a single metaphase and three showed an aneuploid number of chromosomes. The overall rate of aneuploidy among tripronuclear microinjected zygotes was 56.7%. In addition, seven zygotes with more than three pronuclei arising after ICSI displayed severely depleted chromosome complements. The incidence of non-disjunction in oocytes fertilized by conventional in-vitro insemination was significantly lower (20.0%, P < 0.01), since only four zygotes had an aneuploid number of chromosomes. Our findings suggest that ICSI might interfere with regular chromosome segregation at the second meiotic division of the oocyte
