There are a variety of surface ligands which have been identified as mediators of sperm attachment to the zona pellucida and to the egg membrane. Theoretically, an absence or abnormality of these sites could interfere with fertilization and, in the future, these defects will be tested. A any more severe abnormality, identified by electron microscopy, is the complete absence of the acrosome that gives the sperm a round-headed appearance and leaves them unable to achieve fertilization. A qualitative assessment of sperm activity can be obtained by a color change produced in the organic dye resazurin by metabolically active sperm.