Our study conducted a retrospective analysis of the embryological characteristics and clinical outcomes in women presenting with indented ZP oocytes during IVF/ICSI treatment. We observed that oocytes with indented ZP were associated with significantly lower rates of normal fertilization, embryo cleavage, clinical pregnancy, and live birth (P < 0.05).
The ZP is crucial during fertilization as it is the initial binding site for spermatozoa [8]. Dysmorphology of the ZP, including irregularities in composition, color, thickness, or shape, may interfere with the recognition and binding of sperm to the oocyte [2, 22, 23, 24]. Specifically, oocytes encased by an indented ZP exhibited a notably lower normal fertilization rate (46.8%, 168/359) in IVF cycles, suggesting that fertilization impairment may be a primary factor contributing to infertility in these patients [11]. A considerable proportion of IVF cycles (58.3%, 28/48) necessitated rescue ICSI following fertilization failure. This subgroup, which underwent delayed ICSI insemination, achieved a 48.8% (124/254) normal fertilization rate, significantly lower than that of the traditional ICSI insemination group (74.1%, 206/278) (P = 0.001). Subsequent embryo development in the rescue ICSI group was significantly compromised, exhibiting impaired cleavage and reduced embryo utilization (Table 4). Furthermore, rescue ICSI in indented ZP group exhibited inferior outcomes compared to normal ZP group. Despite rescue ICSI being effective in preventing fertilization failure and achieving positive clinical outcomes for oocytes with indented ZP, this delayed intervention still falls short of the optimal insemination window, adversely affecting embryo utilization (P = 0.001). ICSI remains the preferred method to maximize the number of viable embryos for improved clinical outcomes. Regrettably, the presence of dysmorphic oocytes cannot be confirmed prior to commencing IVF treatment. Previous studies have indicated that this ZP dysmorphology could be anticipated by abnormal OCCC with sparse granular and cumulus cells [11]. Hence, early expert assessment before insemination and timely ICSI injection are imperative in future IVF treatments.
Our findings also revealed that women with recurring oocyte dysmorphism underwent more treatment cycles due to repeated clinical failures. The IZP group exhibited a higher rate of oocyte immaturity in conventional ICSI cycles compared with controls (P < 0.001), corroborating earlier findings [10, 11]. The immaturity of oocytes may be partly due to the constriction of the PVS, necessitating ICSI insemination to enhance oocyte utilization. The exact cause of metaphase I (MI) arrest remains unclear; however, potential contributing factors include genetic defects, absence of meiotic recombination, abnormal microfilament activity, and failure to synthesize key cell cycle regulators [25]. These factors could adversely affect nuclear maturation and cytoskeleton integrity. Sousa et al. proposed that the observed reduction in oolemma resistance and decreased ooplasmic viscosity in oocytes with indented ZP might result from compromised microfilament function [10]. This hypothesis warrants further investigation.
Clinical outcomes of embryo transfers with IZP have been variable in prior reports, and the impact of different fertilization methods remains ambiguous. In our study, 32 patients in the IZP group underwent fresh embryo transfer, resulting in 14 pregnancies and 11 live births. Notably, one case achieved pregnancy from IVF cycles. The rates of clinical pregnancy, implantation, and live birth in the IZP group were significantly lower than those in the NZP groups, diverging from previous reports. [26, 27]. Additionally, 31 patients underwent FET, with no significant differences in clinical outcomes between the case and control groups. This finding may be attributed to the superior quality of frozen embryos and adequate patient conditioning before FET cycles. It has been suggested that facilitating the hatching process may enhance pregnancy outcomes in embryos with ZP dysmorphology [28]. In our cohort, 8 cases underwent assisted hatching, with 3 resulting in pregnancy. However, due to the limited sample size, no significant improvement in clinical outcomes was observed with assisted hatching. Further research with a larger sample size is necessary.
Indented ZP can arise from extrinsic factors, such as stimulation regimens, or intrinsic factors, such as age and genetic background. [29, 30]. Our study subjects, who consistently produced dysmorphic oocytes through various stimulation protocols, suggest a stronger association with intrinsic factors. The age distribution in the case group was similar to that of the control group, implying that the type of infertility—predominantly primary infertility (P = 0.001)—may be influential, prompting earlier entry into IVF treatments [26]. Current data lead us to speculate that dysmorphic oocytes result from abnormal synthesis of ZP glycoproteins. There is no consensus on the impact of mutations in genes encoding human ZP glycoproteins on ZP dysmorphology. Evidence from knockout mice studies suggests that indented ZP may be linked to abnormal ZP1 synthesis [31]. While sequence variations in genes encoding human ZP proteins, particularly ZP2 and ZP3, have been associated with common anomalies in ZP dysmorphology, these mutations may also represent polymorphisms [8]. Therefore, further investigation into the relationship between ZP proteins and human ZP dysmorphology is warranted.
In conclusion, our study sheds light on the embryological characteristics and clinical outcomes associated with indented ZP oocytes. Patients with IZP experienced suboptimal embryological and clinical outcomes. Thus, for patients with ZP dysmorphology, direct ICSI insemination is recommended to assist infertile women in achieving pregnancy. Compared to fresh embryo transfer, FET clinical outcomes did not differ between groups. Additional research is needed to explore whether the cryopreservation process influences ZP heterogeneity.