Fertilization cascade

Distinct successive steps have been identified in the process of fertilization (fertilization cascade): (1) Sperms penetrate the corona radiata and, by means of their receptors, bind to the zona pellucida. (2) The acrosome reaction starts, triggered by the glycoprotein ZP3 of the zona pellucida. Following massive inflow of extracellular calcium into the sperm, enzymes such as acrosine and hyaluronidase are released, which allows penetration of the zona pellucida. (3) The outer membrane of the sperm's acrosomal cap dissolves. The inner acrosomal membrane of the spermatozoon then penetrates the zona pellucida. (4) The ensuing fusion of the cell membranes from spermatozoon and oocyte causes a membrane depolarisation. This triggers a calcium wave, which spreads throughout the oocyte cytoplasm. The increase in calcium concentration is a signal for the oocyte to reinitiate and complete meiosis II. The oocyte then becomes activated and embryogenesis begins. (5) The sperm penetrates the oocyte. (6) Thereby, the cell membrane of the sperm is stripped off. Further pulses of intracellular calcium occur every few minutes and trigger the fusion of cortical granules with the oocyte membrane (cortical reaction). (7) The granules release their contents into the space between oocyte and zona pellucida (subzonal space). This causes the zonal reaction: the structure in the receptors of the zona pellucida is modified in such a way that binding and penetration of other sperms is prevented (block of polyspermy).