The Natural Sperm Selection in Intracytoplasmic Sperm Injection (ICSI)

The quality of the selected sperm used for intracytoplasmic sperm injection (ICSI) plays a detrimental role in embryonic quality and development. In in vitro fertilization (IVF) and during the sperm journey through the female reproductive tract in vivo, sperm interact with the zona pellucida (ZP) of the oocyte, which is the last stage of sperm selection before entering the oocyte. The ZP is selective with regard to binding and can bind to normally functioning sperm, especially those with a normal acrosomal region. According to Liu et al., only 14 % of the motile spermatozoa in fertile men can bind to the ZP. Only those spermatozoa with relatively normal size and shape of the acrosomal region and those with no or low Deoxyribonucleic acid (DNA) fragmentation can bind to and the ZP and fuse with the plasma membrane of the oocyte (oolemma) and thus are capable of fertilizing the oocyte. Human sperm vary in size, morphology, DNA integrity, motility, membrane composition, etc., and this can be observed even in the same ejaculate. It is a given fact that thorough sperm selection procedures befall spermatozoa in the female genital tract to filter superior sperm and allow only a small subpopulation of spermatozoa with superior quality to reach the site of fertilization where another sperm selection occurs (i.e. ZP interaction).

The sperm traits that make in vitro fertilization effective are still debated. ICSI is now the standard practice for most Assisted Reproduction technologies (ART) centers worldwide and accounts for approximately 70% of all in vitro fertilization. The routine selection of spermatozoa for ICSI depends on an embryologist subjectively selecting sperm based on their motility and morphology. It is done after an analysis of the seminal fluid, which is a poor predictive tool of male fertility and does not express the fertilization capacity of the sperm. It had assumed that mimicking the natural sperm selection may improve the quality of selected spermatozoa and hence, the clinical outcomes of ICSI. Ideally, a sperm selection method that reduces the number of spermatozoa to a subpopulation with potentially the highest quality can improve fertilization and embryo quality and development and subsequent clinical outcomes of ICSI.

Over the years, several sperm selection techniques have been developed for ICSI. However, these techniques were designed to select sperm based on a single sperm parameter (i.e. motility, density, sedimentation, nuclear integrity, etc.) and ignoring other sperm parameters related to the capability to fertilize the oocyte. Sperm selection techniques such as swim-up, microfluidics, and density gradient centrifugation yield a population of highly motile sperm but fail to mimic the rigorous natural sperm selection that considers other sperm parameters. Moreover, most of these methods require centrifugation which may negatively affect the paternal DNA and reduces the quality of sperm by increasing reactive oxygen species. Following these techniques, an embryologist has to subjectively select sperm based on their motility and morphology, which does not guarantee DNA integrity and is potentially time-consuming.

One of the developed sperm selection methods to relatively duplicate the natural selection is the hyaluronic acid (HA) binding-based Physiological intracytoplasmic sperm injection (PICSI)® dishes. The cumulus oophorus layer surrounding the oocyte consists mainly of HA. However, there is conflicting data on the results using PICSI dishes. Moreover, sperm-ZP binding comprises parameters other than HA that are not featured in PICSI dishes.

Sperm-oocyte interaction is a multi-step process involving physical and molecular interactions. It involves a complex and complementary receptor/ligand-based process between the surface proteins expressed on the ZP and the sperm. Research has unveiled several ZP protein candidates postulated to play a role in binding sperm. The main protein of these is the zona pellucida glycoprotein 3 (ZP3), whose O-linked oligosaccharide chains bind to an acrosome-intact sperm and induce an acrosomal reaction.