The molecular interaction of the sperm with the zona pellucida (ZP) is an important event that precedes fertilization, and also relates to various sperm functions, such as capacitation completion and ligand-induced acrosome reaction. In clinical practice, defective sperm-ZP interactions are a relatively common cause of fertilization disorders. Sperm-ZP binding tests help in detecting subtle abnormalities in the sperm, which cannot be identified by semen analyses. A recent prospective study provided further credence to the effectiveness of such tests in selecting competent sperm for improved ICSI outcome. The findings of the study have been published in the recent issue of the journal, Reproductive BioMedicine Online.
Daniela Paes de Almeida Ferreira Braga and coworkers from the Assisted Fertilization Center Av. Brigadeiro Luis Antonio, Brazil, conducted the study to assess the ability of the sperm-ZP tests in choosing sperms with high fertilization potential and high rate of successful embryo development. Conventionally selected sperm were injected into 50% of the metaphase II (MII) oocytes in each patient (controls; n=194), while the remaining eggs were inserted with ZP-bound sperm (ZP binding group; n=194). The two groups were compared with respect to the rates of fertilization, high-quality embryo, and embryo transfer.
The study findings are listed in the table below.
| Parameters Tested | Control Group | ZP binding Group |
| Fertilization rate (%) | 76.8 | 77.3 |
| % of high quality embryo | 70.0 | 83.3 |
| Rate of embryo transfer (%) | 43.6 | 54.6 |
Although no significant variation was found between the two groups in terms of fertilization rate, a greater percentage of high quality embryos were noted in the ZP binding group compared to the controls. Also, upon performing random embryo selection without noting the experimental group origin, more number of embryos from the ZP binding group were selected for transfer. The results thus indicated the potential of sperm–ZP binding tests in improving ICSI outcome.
In another recent study by Black et al (Fertility and Sterility, 2009), an improvement in implantation and clinical pregnancy rates with ICSI using ZP-bound sperm was reported on comparing the outcomes with the traditional ICSI method. The findings encourage the need for further assessment in larger cohorts to confirm the effectiveness of ZP-bound sperm.
According to an earlier review by Aitken (International Journal of Andrology, 2006), a successful conception could be achieved if the concentration of morphologically normal, motile sperm exceeded a defined threshold level. Hence, conventionally, semen evaluation using factors such as sperm number, motility, and morphology, was used to diagnose male infertility. Research indicating that fertility is determined by functional competence and not just by the number of sperm, led to the development of several in vitro tests that analyze sperm function. These tests helped in predicting the fertilization outcome of sperm both in vitro and in vivo by monitoring various aspects, such as movement, penetration of cervical mucus, capacitation, recognition of ZP, ZP sperm interactions, acrosome reaction, and sperm-oocyte fusion.
Although several sperm function tests are available, relatively few are routinely used for clinical evaluation. With more evidence suggesting the usefulness of sperm-ZP binding tests in enhancing the ICSI outcome, researchers highlight the need for initiating international collaborations that can help in arriving at a consensus on protocols and clinical thresholds for the clinically relevant molecular and functional tests.
References
1. de Almeida Ferreira Braga DP, Iaconelli A Jr, de Cássia Sávio Figueira R, Madaschi C, Semião Francisco L, Borges E Jr. Outcome of ICSI using zona pellucida-bound spermatozoa and conventionally selected spermatozoa. Reprod Biomed Online. 2009 Dec;19(6):802–7. [Epub ahead of print]
2. Black M, Liu DY, Bourne H, Baker HW. Comparison of outcomes of conventional intracytoplasmic sperm injection and intracytoplasmic sperm injection using sperm bound to the zona pellucida of immature oocytes. Fertil Steril. 2009 Oct 28. [Epub ahead of print]
3. Aitken RJ. Sperm function tests and fertility. Int J Androl. 2006 Feb;29(1):69-75; discussion 105-8.
4. Lewis SE. Is sperm evaluation useful in predicting human fertility? Reproduction. 2007 Jul;134(1):31-40.
