Maturation of mammalian spermatozoa: modifications of the acrosome and plasma membrane leading to fertilization

The mammalian spermatozoon is a terminally differentiated cell. Its surface is covered by a continuous plasma membrane that is divided into distinct domains in which functional molecules are distributed. The acrosome is an internal organelle located at the anterior head and contains hydrolytic enzymes. The anterior acrosome participates in the acrosome reaction, which is an indispensable event during fertilization. These surface domains and the acrosome are formed during spermiogenesis, during which associated molecules are transported and organized. Many of the molecules thus arranged are functionally immature but gradually become mature during epididymal maturation. Some of them are further altered and redistributed during the fertilization process and play various roles. Here, the sequential changes in the acrosome and plasma membrane during spermatozoan maturation are reviewed, particular attention being paid to molecules derived from the testis.