Numerical and experimental estimating zona pellucida hardness under microinjection to assess oocyte quality

The zona pellucida (ZP) is the extracellular coat that surrounds mammalian oocytes. The precise determination of ZP hardness is mainly unknown due to the lack of appropriate measuring systems and modelling methods. In this study, we have used experimental and numerical models to explain the mechanical behavior of a single oocyte cell to improve the assisted reproductive technology (ART) outcomes by assessing oocyte/embryo quality. This paper presents the development of a microinjection model to estimate the ZP hardness and an experimental procedure to obtain the required data for this model. Our results show that the estimated penetration force provides a performance target for the penetration process during intracytoplasmic sperm injection (ICSI), while the estimated corresponding hardness serves as an indicator of the amount of deformation experienced by the oocyte before penetration. Evaluation of these results shows that a routine assessment of ZP hardness under microinjection would allow for the identification of certain oocyte pools for which further manipulation is recommended in order to increase injection, hatching and finally ART outcomes.