I-4: Sperm Preparation: Biomimetic Aspects

With the advent of human assisted reproduction in the year 1978 numerous techniques were developed to isolate spermatozoa capable to fertilize oocytes. While early methodologies only focused on the aim of isolating viable, motile spermatozoa, with further progress of assisted reproductive technology, particularly for ICSI, it rapidly became clear that these two parameters are insufficient for the identification of the most suitable and most functional spermatozoon for fertilization. Conventional sperm preparation techniques, namely swim-up, density gradient centrifugation or glass wool filtration are definitely not efficient enough to isolate sperm populations that minimize or even exclude the risk of the transfer of mutagenic or lethal DNA damages to the progeny. This is because these conventional sperm preparation techniques are physical rather than physiological and are not modeled on the sperm selection processes taking place in the female genital tract. Therefore, reproductive biology/medicine should learn from nature and try to mimic the extremely stringent selection process in the female genital tract, which only allows one male germ cell out of tens of millions to fuse with the oocyte. Sites of natural sperm selection in the female genital tract are the cervix with its mucus, uterus, utero-tubal junction, oviduct, cumulus oophorus and the zona pellucida. Newer and more sophisticated (advanced) strategies of sperm preparation intend to mimic the principles underlying physiologic sperm selection founded on (i) sophisticated morphological assessment by means of ‘motile sperm organelle morphological examination (MSOME), (ii) electrical charges and (iii) molecular binding characteristics of the sperm cell. Whereas separation methods based on electrical charge take advantage of the sperm cells’ zeta potential to either adhere to the test tube surface or movement by electrophoresis, molecular binding techniques use annexin V or hyaluronan as substrate for sperm binding. Techniques to be mentioned in this category are magnetic-activated cell sorting technique (MACS), annexin V-activated glass wool filtration and flow cytometry as well as hyaluronic acid (HA)-bound spermatozoa that are removed by gently rinsing the HA droplet so that the bound spermatozoon can be removed or picked (P) with an ICSI pipette (PICSI) from the dish. Future developments may include Raman microspectrometry, confocal light absorption and scattering spectroscopic (CLASS) microscopy and the use of the birefringence of the sperm head.