Oocyte-sperm interactions

The penetration of the zona pellucida is a crucial step during fertilizatio n. Spermatozoa that are unable to recognize and bind to the zona pellucida glycoproteins or respond to the zona pellucida by undergoing the acrosome r eaction fail to fertilize the egg. In most mammalian species, after enterin g the fallopian tube sperm are stored in the isthmic part of the oviduct un der conditions that maintain sperm viability and synchronize both sperm tra nsport and the process of acquisition of fertilizing ability, called capaci tation. Only capacitated sperm are enabled to recognize the oocyte and resp ond to the oocyte signals in an appropriate manner. Close to time of ovulat ion sperm are released from the oviductal epithelium and swim to site of fe rtilization. The oviduct and the oocyte itself appear to coordinate sperm f unction and gamete interaction. The gamete recognition and the next levels of interaction are probably granted by the carbohydrate-protein interaction s. Upon binding the signal cascade leading to acrosomal exocytosis is activ ated, eventually initiated by aggregation of zona pellucida receptor molecu les. These signal transducing mechanisms are primed during the capacitation process. Tyrosine phosphorylation, tightly connected to the cholesterol ef flux from the plasma membrane, and hyperpolarization seem be involved in th is priming by activation of Ca2+ pathways. Further preparational steps of t he acrosome reaction may be mediated by osmosensitive signal transducing me chanisms. The current perspective focuses on the molecules involved in the complex hierarchy of sperm-egg interactions and regulative events priming s perm cell during capacitation for the acrosome reaction. (C) 2000 Elsevier Science B.V. All rights reserved.