Visualization of Sperm Accessory Structures in the Mammalian Spermatids, Spermatozoa, and Zygotes by

Mammalian fertilization is traditionally viewed as a process during which the fertilizing spermatozoon penetrates the egg vestments to plant its chromosomes into the fertile environment of oocyte cytoplasm. Much less attention has been paid to the events that occur between gamete fusion and first embryonic cleavage. The interaction of the mammalian spermatozoa with the oocyte during and after gamete fusion is a complex and meticulously orchestrated cascade of events that can come to a halt at any given stage, resulting in pathological conditions. Researching these “late” events of fertilization and pro-nuclear development is a major challenge, which will be undoubtedly rewarded by new knowledge and, in the long term, by the improved treatments of infertility in humans and improved reproductive performance in farm animals. The fate of various sperm accessory structures came under scrutiny recently, as it became obvious that in addition to the sperm-borne chromosomes, other structures of the fertilizing spermatozoon make important contributions to the mammalian zygote (1 ). Yet other sperm accessory structures are degraded in an orderly fashion as to not interfere with normal embryo development. These include the sperm proximal centriole, perinuclear theca, sperm mitochondria, and axonemal fibrous sheath and outer dense fibers.