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For sperm to successfully fertilize an oocyte, it needs to pass through certain steps prior to, during and after initial recognition of the zona pellucida (ZP). During capacitation, the surface of the sperm head becomes remodelled, priming it to bind to the ZP and subsequently to undergo the ZP-induced acrosome reaction. During capacitation, sperm ZP-binding proteins are ordered in functional protein complexes that only emerge at the apical tip of the sperm head plasma membrane; this is also functionally the exclusive sperm surface area involved in primary ZP binding. After primary ZP binding, the same area is probably involved in the induction of the acrosome reaction. A combination of biochemical and proteomic membrane protein techniques have enabled us to dissect and highly purify the apical sperm plasma membrane area from control and capacitated sperm cells. The actual ZP-binding proteins identified predominantly belonged to the sperm membrane-associated family members of spermadhesins (AQN-3) and were present in the aggregating lipid ordered membrane microdomains (lipid rafts) that emerged during in vitro capacitation in the apical ridge area of the sperm head plasma membrane. This clustering of these rafts was dependent on the presence of bicarbonate (involved in protein kinase A activation) and on the presence of albumin (involved in cholesterol removal). Remarkably, cholesterol removal was restricted to the non-raft membrane fraction of the sperm plasma membrane, but did not cause any depletion of cholesterol in the raft membrane fraction. Interestingly, sperm SNARE proteins (both VAMP from the outer acrosomal membrane, as well syntaxin from the apical sperm head plasma membrane) shared lateral redistribution properties, along with the ZP-binding protein complex and raft marker proteins. All of these were recovered after capacitation in detergent-resistant membrane preparations from sperm thought to represent membrane lipid rafts. We inferred that the capacitation-dependent formation of an aggregated lipid ordered apical ridge surface area in the sperm head plasma membrane was not only relevant for ZP-binding, but also for the ZP-induced acrosome reaction.

The sperm cell has a characteristic polarized morphology and its surface is also highly differentiated into different membrane domains. Junctional protein ring structures seal the surface of the mid-piece from the head and the tail respectively and probably prevent random diffusion of membrane molecules over the protein rings. Despite the absence of such lateral diffusion-preventing structures, the sperm head surface is also highly heterogeneous. Furthermore, lipid and membrane protein ordering is subjected to changes when sperm become capacitated. The forces that maintain the lateral polarity of membrane molecules over the sperm surface, as well as those that cause their dynamic redistribution, are only poorly understood. Nevertheless, it is known that each of the sperm head surface regions has specific roles to allow sperm to fertilize the oocyte: a specific region is devoted to zona pellucida binding, a larger area of the sperm head surface is involved in the acrosome reaction (intracellular fusion), while yet another region is involved in egg plasma membrane binding and fertilization fusion (intercellular membrane fusion). All three events occur in the area of the sperm head where the plasma membrane covers the acrosome. Recently, lipid ordered microdomains (lipid rafts) were discovered in membranes of many biological specimens including sperm. In this review, we cover the latest insights about sperm lipid raft research and discuss how sperm lipid raft dynamics may relate to sperm-zona binding and the zona-induced acrosome reaction.

This research aims firstly to understand, in cellular and molecular terms, how a mature human spermatozoon is prepared for fertilization, and secondly, to identify what factors are involved in the initial signalling interactions between the egg and spermatozoon. In order to achieve these objectives, a combination of approaches is being used, including single-cell imaging, patch clamping and proteomics. Single-cell imaging reveals hidden complexity and heterogeneity in signalling responses in spermatozoa. Characterization of cell physiology at the single-cell level must be a future aim, including the study of ion channel expression and function by patch clamping. Proteomic experiments are aimed at identifying defects in protein expression in specific subgroups of men, e.g. those with globozoospermia. A better understanding of prefertilization events will allow the development of non-assisted reproductive therapy, drug-based treatments for male infertility.

Advances in many areas of reproductive technology have been rapid and, in many respects, have outstripped our knowledge of the fundamental processes of human and animal sperm-egg interactions at fertilization. This is particularly true of human fertilization, where the availability of eggs for research purposes is severely restricted. As a consequence of this, most of the significant advances in our understanding of mammalian fertilization have resulted from studies on animals, particularly the mouse. This review summarizes our current knowledge of the molecular aspects of mammalian fertilization from the point of view of the fertilizing spermatozoon. Particular reference is made to those advances in our knowledge of human fertilization mechanisms. Further understanding of the molecular basis of human fertilization is of paramount importance for the development of new methods of contraception and also for the rational diagnosis and treatment of certain forms of infertility.