Mammalian spermatozoa have complex structures. The structure-function relationship of sperm has been studied from various viewpoints. Accumulated evidence has shown that the sperm components undergo sequential changes from the beginning of spermatogenesis to the time of fertilization/embryogenesis. Structural analyses have been performed using various new techniques of light and electron microscopy as well as immunohistochemistry and immunocytochemistry in combination with specific probes such as antibodies against sperm components. Recently developed gene-manipulation techniques have...