Sperm-egg interaction at fertilization: glycans as recognition signals.

This article first examines the events occurring in male and female genital tracts, which prepare human sperm to encounter the egg. Central is a glycoprotein, gp20, homologous to the leukocyte antigen CD52. This protein is secreted in the epididymal cells, inserted in the sperm plasma membrane and exposed in the equatorial region of the head at the end of the capacitation process. The mechanisms and molecules of the first interaction event between gametes in the mollusk bivalve Unio elongatulus and the current state of our knowledge of the same interaction in other species is then considered. The egg of Unio is very peculiar because it is highly polarized. Similar to other well-known egg models, the ligand for recognition is located on the egg coat which is a sort of fibrous network made up of very few glycoproteins, while the receptor is on the sperm surface. The difference is that in this egg, the ligand molecules are not uniformly distributed but are restricted to an area of the egg coat at the vegetal pole, the crater area. The role of carbohydrates in ligand function and of a specific type of oligosaccharide chain in particular, is discussed in the wider context of glycans acting as recognition signals.