what role(s) played by the Equatorial Segment Protein 1 (ESP1) during fertilization. Solution The structure of a spermatozoa consists of three parts. They are, Acrosome is the elongated head region of the sperm containing 2/3rd of the nucleus. The enzymes present in the acrosome are essential for the penetration of a sperm into a secondary oocyte (egg). The acrosomal layer necessary for the fusion and penetration into the oocyte is deficient in mice lacking Equatorial Segment Protein 1 (ESP1). This indicates that ESP1 is necessary for the development of \"fusion-competent sperm.\".