Abstract
The morphological assessment of oocytes is important for embryologists to identify and select MII oocytes in IVF/ICSI cycles. Dysmorphism of oocytes decreases viability and the developmental potential of oocytes as well as the clinical pregnancy rate. Several reports have suggested that oocytes with a dark zona pellucida (DZP) correlate with the outcome of IVF treatment. However, the effect of DZP on oocyte quality, fertilization, implantation, and pregnancy outcome were not investigated in detail. In this study, a retrospective analysis was performed in 268 infertile patients with fallopian tube obstruction and/or male factor infertility. In 204 of these patients, all oocytes were surrounded by a normal zona pellucida (NZP, control group), whereas 46 patients were found to have part of their retrieved oocytes enclosed by NZP and the other by DZP (Group A). In addition, all oocytes enclosed by DZP were retrieved from 18 patients (Group B). No differences were detected between the control and group A. Compared to the control group, the rates of fertilization, good quality embryos, implantation and clinical pregnancy were significantly decreased in group B. Furthermore, mitochondria in oocytes with a DZP in both of the two study groups (A and B) were severely damaged with several ultrastructural alterations, which were associated with an increased density of the zona pellucida and vacuolization. Briefly, oocytes with a DZP affected the clinical outcome in IVF/ICSI cycles and appeared to contain more ultrastructural alterations. Thus, DZP could be used as a potential selective marker for embryologists during daily laboratory work.
Highlights
A fundamental and important part of daily in-vitro fertilization (IVF) laboratory work is the morphological assessment of oocytes and embryos, which guides the identification and selection of MII oocytes with the potential to develop into good quality embryos with high implantation potential [1,2]
The rates were significantly lower in Group B compared to those in the control group of normal fertilization (2PN) in IVF and ICSI cycles (59.4% vs. 76.3 and 67.7% vs. 82.3%) (Table 2), good quality embryos (51.7% vs. 66.0%), implantation (18.6 vs. 34.5%) and clinical pregnancy (25.0% vs. 53.3%) (Figure 1)
In several previous studies on oocytes, which were surrounded by a dark zona pellucida (DZP), no negative effects were found on the outcomes [23,27]
Summary
A fundamental and important part of daily in-vitro fertilization (IVF) laboratory work is the morphological assessment of oocytes and embryos, which guides the identification and selection of MII oocytes with the potential to develop into good quality embryos with high implantation potential [1,2]. The density, structure or interaction of the ZP glycoproteins are correlated with the quality of the follicles and could be used to predict implantation and miscarriage rates [15]. Taken together, these findings are correlated with the expression of specific genes in the cumulus-oophorous- complex (COC) [2], which in infertile mice with defective ZP, are dependent upon the lack of genes for ZP2 and ZP3 [16,17]. Variation in the thickness of the zona pellucida has been found to correlate with fertilization [18,19], embryo quality and implantation rates [20,21]
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