A preliminary concept, deduced from cytogenetic analyses, for explaining different types of multipronuclear oocytes obtained after intracytoplasmic sperm injection.

Oocytes with three or more pronuclei and one or two polar bodies arising after intracytoplasmic sperm injection were analyzed cytogenetically, and the data were used to develop a general model explaining the possible origin of the abnormal pronuclear stages. The mechanisms, either separately or in variable combinations, that induce the appearance of additional pronuclei are: 1) nonextrusion of the second polar body; 2) incomplete chromatid segregation into the extruded second polar body; and 3) dispersal of the oocyte chromatids in the presence of the second polar body formation.

Separation of a pronucleus by premature cytokinesis: a mechanism for immediate diploidization of tripronuclear oocytes?

OBJECTIVE: To describe an oocyte with a peculiar combination of abnormalities in terms of cytoplasmic fragmentation and formation of pronuclei. DESIGN: Case report. SETTING: University-based IVF unit. PATIENT(S): A chromosomally normal couple undergoing the third treatment by assisted reproduction. INTERVENTION(S): Controlled ovarian hyperstimulation, follicular aspiration, testicular sperm extraction, intracytoplasmic sperm injection (ICSI), and oocyte fixation. MAIN OUTCOME MEASURE(S): Assessment of pronuclear formation and cytogenetic analysis of a tripronuclear fragmented oocyte. RESULT(S): The described oocyte revealed two polar body-like structures after follicular aspiration and developed three pronuclei after ICSI, accompanied by extrusion of another polar body-like globule. Moreover, the pronuclear stage had undergone a premature irregular cytokinesis, including one of the pronuclei in the smaller half of the divided cytoplasm. Cytogenetic analysis showed a triploid karyotype (23,X/23,X,ace/23,Y) commonly found in digynic ICSI zygotes. CONCLUSION(S): The chromosomal constitution suggests that the third globule represents another cytoplasmic fragment and no regular second polar body. Separation of a supernumerary pronucleus by premature cytokinesis might be relevant for the discussion on diploidization of triploid zygotes.

Selective microsurgical removal of a pronucleus from tripronuclear human oocytes to restore diploidy: disregarded but valuable?

OBJECTIVE: To review data on the microsurgical removal of a single pronucleus from tripronuclear human oocytes and evaluate the future potential of this technique for obtaining diploid, transferable embryos. DESIGN: Literature review. SETTING: None. PATIENT(S): None. INTERVENTION(S): None. MAIN OUTCOME MEASURE(S): None. RESULT(S): Ten relevant studies were identified. These differ considering the removal technique itself, the application of cytoskeletal relaxants, and the survival rate after epronucleation. Diploidy and heteroparental inheritances could be confirmed in some preimplantation stages derived from epronucleated oocytes. Transfer of "corrected" embryos has been attempted only once, and resulted in a live birth. Noteworthy pitfalls associated with the procedure concern the exact identification of the supernumerary pronucleus, the presence of two centrosomes in dispermic oocytes, and cytogenetically abnormal pronuclear patterns after intracytoplasmic sperm injection. CONCLUSION(S): Patients with exclusively abnormally or few normally fertilized oocytes would profit from epronucleation to assure embryo transfer or increase the number of transferable embryos. Further research appears necessary and promising.

Mechanisms giving rise to triploid zygotes during assisted reproduction.

OBJECTIVE: To review information on the origin of triploid zygotes as gathered from assisted reproduction techniques. DESIGN: Identification of relevant literature by a MEDLINE search and own experience on the basis of cytogenetic studies of abnormally fertilized oocytes. SETTING: None. PATIENT(S): None. INTERVENTION(S): None. MAIN OUTCOME MEASURE(S): None. RESULT(S): Penetration of two haploid spermatozoa or of a single diploid spermatozoon into the oocyte causes diandric triploidy. The first case can be discerned by formation of a total of three pronuclei, whereas the second process will remain undetected, because it involves a female and a single but diploid male pronucleus. Digynic triploidy after intracytoplasmic sperm injection is characterized by nonextrusion of the second polar body and formation of three pronuclei. Digyny can also result from the fertilization of diploid giant oocytes. Depending on how maturation of these gametes proceeds, three or only two pronuclei will be observed. Thus, the size of the pronuclear stage must be considered for a successful identification of the abnormality. Endoreduplication within the female pronucleus is not detectable and may represent another, albeit rare, origin of digynic triploidy. CONCLUSION(S): Routine inspection of the number of pronuclei is not an absolutely reliable tool for excluding the development of triploid embryos. Observations during assisted reproduction may yield valuable information on the origin of triploidy.

Cytogenetic analysis of human oocytes remaining unfertilized after intracytoplasmic sperm injection.

Oocytes remaining unfertilized after intracytoplasmic sperm injection showed 12.0% aneuploidy (nondisjunction + unbalanced predivision), 3.4% structural aberrations, and 8.5% balanced predivision in fully karyotyped cells. However, the frequently observed complete or partial separation of chromatids, most probably caused by abortive activation, might complicate the evaluation of meiosis I-derived aneuploidy and questions the relevance of balanced predivision.