1 Department of Obstetrics and Gynaecology, Göteborg University, SU/Sahlgrenska, 13 45 Gothenburg, Sweden
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Abstract |
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Key words: digyny/oocyte/oocyte fusion/triploid
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Introduction |
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Case report |
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The fluorescence visualization showed that the oocyte had two polar bodies and two metaphase spindles (Figure 1). The first set of MII/PB-structures was oriented perpendicular to the viewer (Figures 1 and 3
, upper set) while the second set was on top of the oocyte facing towards the viewer. The sibling GV oocyte arrested at the GV stage and degenerated quickly. Nine of the ten MII oocytes were fertilized, seven showing 2PN and two showing 3PN at 20 h post-insemination. The embryos developed slowly and resulted in a day 3 transfer where only three embryos of 46 cells were available. Two embryos were transferred, not resulting in a pregnancy.
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Discussion |
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If we hypothesize that both observed sets of MII/PB-structures have a normal chromosomal set-up, the chromosomal number had to already be doubled when meiosis was commenced during oogenesis. This means that the first meiosis was then normal, including two separate metaphase I spindles, each extruding its first PB and resulting in two normal separate sets of MII/PB-structures. The question then ariseshow can an oocyte with a double set of chromosomes be created?
One mechanism might be the fusion of two primary oocytes (follicles) creating a single oocyte containing two germinal vesicles. After the migration of primordial germ cells to the ovary in early embryogenesis, it may be that two primary oocytes become surrounded by a common sheet of follicle cells, thus making the fusion of two oogonia possible. In our laboratory, two GV oocytes with a common zona pellucida have been observed (Figure 2) supporting the theory that one of the mechanisms behind a `double' oocyte could be fusion of two oocytes inside a single follicle. It is possible that the result of this oogonial fusion depends on where in the developmental progression the follicle/oocyte is, i.e. an oocyte of normal size will arise from fusion of two small, immature oocytes, at an early stage, while enlarged oocytes would develop from fusion at later developmental stages. Enlarged oocytes have been reported in human IVF and their chromosomal content cytologically determined as diploid (Rosenbusch et al., 2001
). Such giant oocytes have also been observed in the hamster (Funaki and Mikamo, 1980
). These giant oocytes occur infrequently and upon fertilization often produce digynic triploid zygotes. In contrast, the oocyte reported here was of normal size and had two separate sets of MII/PB, something that was not reported in the other studies.
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An additional explanation for this observation that cannot be excluded is that the oocyte had gone through a normal meiosis and was actually haploid. In such cases the metaphase plates observed would be haploid with unreplicated chromosomes and hence the oocyte would be in a state of meiotic arrest. Since the images were obtained by DAPI-staining of the chromosomes it is very difficult to determine whether the DNA content of the spindle is normal or not.
To conclude, our finding shows that human MII oocytes can have two sets of PB/MII structures. Assuming that the chromosomal content of both PB/MII sets is diploid, it may offer an additional explanation for the origin of triploid zygotes in humans.
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Notes |
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References |
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Submitted on December 3, 2001; resubmitted on February 2, 2002; accepted on March 25, 2002.