Paz Martínez¹ and Antoni Morros²

¹ Instituto de Biología Fundamental. Unidad de Inmunología, Universitat Autònoma de Barcelona, 08193 Bellaterra (Barcelona) Spain.

² Unitat de Biofísica. Departament de Bioquímica i de Biologia Molecular,Universitat Autònoma de Barcelona, 08193 Bellaterra (Barcelona) Spain

Received 05/06/96; Accepted 06/18/96; On-line 07/01/96

The highly differentiated and polarized human spermatozoa need to fulfill various requirements before they fertilize the oocyte: 1) Protection and stability for their travelling through the female reproductive tract. 2) Hyperactivated motility to swim across the various regions of the female genital conducts. 3) High fusogenicity of sperm membranes at the right time and position for acrosome reaction to occur. Then, spermatozoa have full fertilizing ability to penetrate the oocyte and fuse with its membrane. The preparative events that render the spermatozoa competent for fertilization are collectivelly known as capacitation. Capacitation is a process that takes place in vivo after the spermatozoa have been present in the female tract for a period of time or in vitro under certain conditions and media. This has been widely reviewed by several authors (1-6).

There are several structural and functional aspects of sperm membranes that may contribute to the flexibility of this type of cell to become well adapted to the different environments that it will encounter. It should be emphasized that drastic changes occur in the plasma membranes of sperm from their epididymal maturation to their capacitation in the female reproductive tract. Major substances responsible for these functional changes are lipids. Sperm membranes have a very distinctive lipid composition: highly unsaturated plasmalogens that may contribute to the formation of non-diffusible membrane regions or domains capable of regionalizing both lipids and proteins. The high amount of non-bilayer forming lipids in the sperm plasma membrane seems to induce unstable membrane domains that have been described as playing a major role in membrane fusions during the acrosome reaction (7-10). The plasma membrane of the sperm head has high amounts of cholesterol. Cholesterol is known to regulate the fluidity of membrane lipid bilayers and to play an important role in capacitation. As spermatozoa pass through the female genital tract, cholesterol is removed from sperm membranes and is picked up by albumin and high density lipoproteins (11-12).

During the transit of sperm through the female reproductive tract, which contains high concentrations of glycosaminoglycans, coating proteins previously bound to sperm surface are trapped by heparin-like glycosaminoglycans: removal of proteins occurs in a time-dependent manner and this is considered to constitute the first step towards capacitation. The role of these external coating-proteins is to protect sperm on its long path to the oocyte and to prevent early development of acrosome reaction (13-18).