Introduction Molecules involved in gamete recognition

1. Introduction

The penetration of the oocyte zona pellucida is a crucial step during fertilization. The zona pellucida is the extracellular network-like matrix enveloping the egg that protects the growing egg and the preimplantation embryo against physical damages. Sperm that are unable to recognize and bind to the zona pellucida glycoproteins, as well as sperm, upon binding, that are unable to respond with it by undergoing the acrosome reaction fail to fertilize the egg. After entering the Fallopian tube sperm are stored in the isthmic Ž part of the oviduct by binding to the ciliated cell lining the oviductal epithelium Suarez, . 1999 . Sperm are stored under conditions that maintain sperm viability and synchronize sperm transport and the capacitating process. This enables sperm to interact with the oocyte in the appropriate manner. Reaching the site of fertilization sperm recognize the egg by carbohydrate–protein interactions. It is well accepted that the corresponding carbohydrate-binding proteins of the sperm surface bind defined oligosaccharide ligands of the zona pellucida. Upon Ž . binding the signaling cascade leading to the acrosomal exocytosis acrosome reaction is activated. This allows sperm to penetrate the zona pellucida. After penetrating the zona pellucida sperm interact and fuse with egg vitelline membrane and trigger thus the Ž . embryonic development program Yanagimachi, 1994 . The current perspective focuses on the molecules involved in the complex hierarchy of interactions between sperm and egg in pig and other domestic animals.

2. Molecules involved in gamete recognition

Close to the time of ovulation sperm are released from the oviductal epithelium and swim to the site of fertilization to meet the egg. The hyperactivated motility that sperm develop during the capacitating process not only enables the sperm to reach the oocyte but also may be necessary for the collision with the egg assisting the manifestation of Ž . the adhesion between sperm and egg Thaler and Cardullo, 1996 . The adhesion between both gametes is a complex sequence of binding events implicating low and high affinity Ž . binding sites Thaler and Cardullo, 1996 . The fundamental mechanism of gamete recognition appears to be conserved throughout the evolution from marine invertebrates to eutherian mammals and is based on carbohydrate–protein interactions between the sperm and the oocyte envelope. Oligosaccharides that are presented with a certain arrangement within the supramolecular architecture of the egg envelope are recognized by complementary carbohydrate receptors of the sperm, thereby mediating gamete recognition and coordinating sperm functions to warrant fertilization.

3. Zona pellucida