Mechanism of fertilization after intracytoplasmic sperm injection

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Recently a substantial progress in understanding of the mechanism of fertilization during ICSI has been made. The following sequence of events leading to the zygote formation has been identified.

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Immobilization of the spermatozoon with a needle induces damage of the sperm plasma membrane so that the sperm nucleus decondensing factor (SNDF) of the oocyte can reach and decondense the sperm nucleus. However, polyvinylpyrrolidone (PVP), which is present in the drop with the sperm, stabilizes the sperm plasma membrane or changes the chemical properties of other molecules in its vicinity thereby preventing sperm and egg interaction. As the PVP becomes diluted following sperm placement into ooplasm, SNDF access the sperm chromatin inducing initial, activation independent, sperm nucleus swelling that ruptures the sperm plasma membrane. This enables a sperm-associated oocyte activating factor (SA OAF), which remains non-identified, to leave spermatozoon and to induce oocyte activation through mobilization of intracellular calcium for oscillations. SAOAF release take place within 30 minutes after injection. Between 2 and 3 hrs after injection resumption of meiosis can be observed at cytogenetic level as an early anaphase of the second meiotic division. Two to three hours later the late anaphase stage is reached, and sperm and oocyte chromosome become indistinguishable from each other. A second polar body is being extruded at this time. Shortly after the male and female chromatin undergo further decondensation to develop into a male and female pronucleus.

Investigation of mechanism of fertilization after ICSI has also contributed to understanding the mechanism of fertilization in general.