OVIDUCT PROTEINS IN FERTILIZATION AND EARLY EMBRYO DEVELOPMENT

The oviduct controls the environment in which the gametes are transpor ted and fuse, and in which embryonic development begins. The ultrastru ctural topography of the ampulla and isthmus is similar, consisting of ciliated and secretory cells, but a different array of proteins is se creted by each segment along with various serum components. Amino acid s are selectively secreted by the oviduct; these amino acids probably interact with the gametes or embryo to facilitate the processes of fer tilization and development. An oviduct-specific glycoprotein is synthe sized by the ampulla of sheep and cattle in response to oestrogen and secreted mainly from day -1 to day 3 of the ovarian cycle. This oestru s-associated glycoprotein (EGP) has a variable molecular mass of 80-97 kDa and a pi value ranging from 4.7 to 5.5. The bovine (b) and ovine (o) EGP genes are 95.5% identical and consist of 1560 base pairs encod ing 519 amino acids containing one N-linked and several O-linked glyco sylation sites. The terminal glycosides are N-acetylglucosamine and ga lactose-N-acetylgalactosamine for bEGP, and fucose, galactose and sial ic acid residues are also identified for oEGP. EGP binds to zona pellu cida and blastomere membranes, but evidence for EGP binding to sperm m embranes is equivocal. After in vitro fertilization the proportion of sheep oocytes cleaving was increased in the presence of oEGP, but when single-cell embryos were cultured with oEGP, these cleavage rates wer e reduced. In addition, consistent positive effects of oEGP were obser ved on blastocyst formation. Elaboration of the mechanism of synthesis of EGP, its action and its role in fertilization and embryo developme nt is important for our understanding of the events of early pregnancy .