Sperm‐egg interactions during fertilization in invertebrates

This article examines comparatively sperm‐egg interactions in different invertebrate groups with the aim of outlining common events and mechanisms, also in the light of our knowledge of these interactions in mammals. The moment when the gametes recognize each other and interact is now acknowledged as crucial for successful fertilization. Despite their apparent variability in different groups and species, a similar plan or organization of the structures involved in the interaction process can be recognized in all types of gametes so far studied. ,The oocyte is generally enclosed by an extracellular coat and another acellular or cellular envelope while the apical region of the sperm cell is characterized by an acrosomal apparatus which undergoes a series of changes on interaction with the oocyte. These changes enable the sperm to cross the external layer and fuse with the oocyte. A review of recent literature shows that the acrosome reaction is an event mediated by a ligand‐receptor interaction, the ligand molecules being part of the extracellular coat of the oocytes. In the bivalve Unto, the protein components of the egg coat have been isolated and the ligand identified, while in eggs of other invertebrates only protein complexes containing the ligand have been characterized. The nature of the receptors on the plasma membrane of the spermatozoa is discussed, as well as the key role of carbohydrates in the adhesive properties of the ligand molecules.