A flow cytometric procedure was used to follow the effect of bicarbonate, a key inducer of sperm capacitation in vitro, on the transbilayer behavior of C6NBD-phospholipids in the plasma membrane of living acrosome-intact boar

To carry out its fertilizing role, the mammalian spermatozoon must express several key functions amongst which is the exocytosis of its acrosome (the so-called ‘acrosome reaction’) as a response to contact with the extracellular egg coat (the zona pellucida). As ejaculated, the mature spermatozoon is incapable of this response, and indeed of the expression of other fertilizing functions: a priming process is first required, termed ‘capacitation’. In vivo, capacitation takes place in the female reproductive tract, but it can be induced in vitro by incubation of the spermatozoon in special media that have been developed empirically to support in vitro fertilization. The component of these media particularly responsible for inducing capacitation appears to be bicarbonate/CO2. (For recent reviews, see Yanagimachi, 1994; Harrison, 1996.) Studies over the years have identified a range of changes in sperm that take place during incubation under capacitating conditions. The large majority appear to constitute or reflect changes in the architecture of the sperm plasma membrane and/or cell surface. This is hardly surprising, given the importance of membrane fusion events in fertilization, viz. the sperm’ exocytotic acrosome reaction and actual fusion between the sperm and the egg. It seems clear that capacitation is a process that somehow renders the sperm plasma membrane less stable, i.e. more fusible (see Harrison, 1996). Changes in plasma membrane architecture and function are generally considered to concern principally the protein and glycoprotein membrane components. However, recently, increasing attention has been paid to the lipid components. In all mammalian cell types studied, including spermatozoa (Müller et al., 1994; Nolan et al., 1995; Gadella et al., 1999), the two leaflets of the plasma membrane bilayer differ in phospholipid composition. The aminophospholipids phosphatidylserine (PS) and phosphatidylethanolamine (PE) are concentrated in the inner leaflet and the choline phospholipids sphingomyelin (SM) and phosphatidylcholine (PC) in the outer leaflet. In somatic cell types, it appears that 2407 Development 127, 2407-2420 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 DEV3163