OBJECTIVE: To study the chromosome constitution and in vitro development of embryos derived from monopronucleated (1PN) zygotes after intracytoplasmic sperm injection (ICSI). DESIGN: Prospective study. SETTING: Private assisted reproduction center. PATIENT(S): Fifty-four 1PN 2PB zygotes obtained from 48 ICSI cycles. INTERVENTION(S): Pronuclear diameter measured 18 ± 2 hours after ICSI and embryonic in vitro development assessed until the blastocyst stage or arrest; embryos disaggregated and all cells analyzed for chromosomes 13, 18, 21, X, and Y by fluorescent in situ hybridization (FISH). MAIN OUTCOME MEASURE(S): Pronuclear size, in vitro embryo development, and cytogenetic characteristics. RESULT(S): All the embryos were chromosomally abnormal (7.4% diploid mosaic, 16.7% haploid mosaics, 5.5% aneuploid mosaic, and 70.4% chaotic mosaic), and 35.2% showed a Y chromosome. No difference in the pronuclear size was observed according to the type of abnormality. Only 7.4% of the embryos resulted in blastocysts with good morphologic features. CONCLUSION(S): Despite showing only one pronucleus, most of the zygotes resulted from fertilization. Embryos derived from 1PN ICSI zygotes are chromosomally abnormal. Neither the pronuclear size nor the in vitro developmental ability of these embryos can be predictive of their chromosomal constitution. According to our results, embryos that develop from 1PN ICSI zygotes should be discarded for any reproductive purpose.
OBJECTIVE: To study the chromosome constitution and in vitro development of embryos derived from monopronucleated (1PN) zygotes after intracytoplasmic sperm injection (ICSI). DESIGN: Prospective study. SETTING: Private assisted reproduction center. PATIENT(S): Fifty-four 1PN 2PB zygotes obtained from 48 ICSI cycles. INTERVENTION(S): Pronuclear diameter measured 18 ± 2 hours after ICSI and embryonic in vitro development assessed until the blastocyst stage or arrest; embryos disaggregated and all cells analyzed for chromosomes 13, 18, 21, X, and Y by fluorescent in situ hybridization (FISH). MAIN OUTCOME MEASURE(S): Pronuclear size, in vitro embryo development, and cytogenetic characteristics. RESULT(S): All the embryos were chromosomally abnormal (7.4% diploid mosaic, 16.7% haploid mosaics, 5.5% aneuploid mosaic, and 70.4% chaotic mosaic), and 35.2% showed a Y chromosome. No difference in the pronuclear size was observed according to the type of abnormality. Only 7.4% of the embryos resulted in blastocysts with good morphologic features. CONCLUSION(S): Despite showing only one pronucleus, most of the zygotes resulted from fertilization. Embryos derived from 1PN ICSI zygotes are chromosomally abnormal. Neither the pronuclear size nor the in vitro developmental ability of these embryos can be predictive of their chromosomal constitution. According to our results, embryos that develop from 1PN ICSI zygotes should be discarded for any reproductive purpose.