ATYPICAL ACTIVATION AND FERTILIZATION PATTERNS IN HUMANS

Abnormalities of fertilization resulting in suppression of second pola r body formation, activation without sperm fusion and multiple sperm f usion occur frequently following assisted (intracytoplasmic sperm inje ction; ICSI) and IVF in the human. Their genetic status was investigat ed by multiprobe fluorescence in-situ hybridization (FISH) or polymera se chain reaction (PCR) on sibling blastomeres. IVF embryos with singl e pronuclei at the zygote stage were usually diploid and fertilized. E mbryos derived from IVF with single pronuclei can be safely replaced b ack to the patient. These embryos develop following formation of a sin gle or aggregated pronucleus, a process found to occur in sea urchins. Single pronucleate ICSI zygotes are usually activated but not fertili zed. The parental status of individual pronuclei was investigated in d ispermic embryos. It was found that the distal pronucleus was usually male in origin and that the sex ratio was restored in enucleated zygot es, however, dispermic embryos become mosaic. Genetic heterogeneity wa s not restored in enucleated dispermic embryos: none of them became tr uly diploid. Mosaicism, on the other hand, was not common among digyni c ICSI embryos and any such mosaicism originated at the third cleavage division, a pattern which is similar in mosaic monospermic embryos. M ost of the digynic embryos were triploid, indicating that the first di vision was bipolar in origin. Most digynic embryos from which a female pronucleus was removed became diploid and their genetic condition was considered normal. From this work it is concluded that the sperm cent riole is active in the human oocyte, rendering most monospermic embryo s, including those that are digynic, non-mosaic. Removal of a single m ale pronucleus will not revert dispermic embryos to a normal status be cause of the activity of extra sperm centrioles. Transfer of enucleate d dispermic embryos or their use as models for embryonic development s hould be reconsidered.