Sperm Capacitation

Trish Berger

Department of Animal Science, University of California Davis, Davis, California, U.S.A. Abstract

Sperm capacitation is a membrane destabilization process that allows membrane fusion to occur prior to and during mammalian fertilization. Removal of seminal plasma coating proteins from sperm is characteristic of capacitation in ejaculated sperm; hence ejaculated sperm typically capacitate more slowly than epididymal sperm. Increased exposure to bicarbonate leading to elevated cyclic AMP, activation of protein kinase A, disruption of the asymmetrical distribution of membrane phospholipids at the anterior sperm head, subsequent loss of membrane cholesterol, increased membrane raft forma tion and increased membrane protein tyrosine phosphorylation are characteristic of capacitation.


Capacitation refers to the multistep process that transforms an ejaculated sperm into a sperm capable of interacting with an oocyte to achieve fertilization in vivo. It is also used to describe less physiological processes occurring in vitro, including those utilizing epididymal sperm that also lead to a spermatozoan capable of fertilizing an oocyte. Sperm characteristics that appear typical of the multistep process such as membrane tyrosine phos-phorylation may be used to assess apparent progress through the process. Currently the only widely accepted marker (other than fertilization) that a spermatozoan has completed capacitation is for the sperm to undergo the acrosome reaction while still being a viable sperm.


Some diversity of opinion exists on the end point of capacitation. Capacitation might be considered to be the events that lead to a sperm undergoing an acrosome reaction in response to a physiological inducer such as zona pellucida or progesterone but not including the acrosome reaction itself; in this case capacitation may be fully reversible. Others consider the acrosome reaction to be the final step in capacitation. An alteration of the equatorial segment of the plasma membrane occurs, which allows the sperm to interact with the oocyte plasma membrane; this change is believed to be a result of the acrosome reaction, although the precise timing relative to membrane vesiculation during the acrosome reaction is currently unknown. Hence, a strict interpretation of capacitation might include events that follow the acrosome reaction, which allow a sperm to complete fertilization. Here, capacitation will refer to the changes that sperm must undergo, which allow them to respond to a physiological inducer of the acrosome reaction. Hence, in the life of a sperm, capacitation follows maturation in the epididymis and precedes the acrosome reaction and fertilization.


Over 50 years ago, Austin[1] and Chang[2] independently reported the need for changes in the sperm during its residence in the female reproductive tract, before the sperm had the capacity to fertilize oocytes. Although scientists have been able to achieve in vitro fertilization (and in vitro capacitation) for a number of mammalian species for several decades, many details are still missing in our molecular understanding of capacitation. Early observations indicated that exposure to the uterine environment followed by exposure to the oviducal environment led to a more rapid capac-itation than exposure to either alone. The time period required for the capacitation varies among species. A six-hour interval is required for in vivo capacitation of ejaculated rabbit sperm, but ejaculated porcine sperm may require only two hours in vivo. Epididymal spermatozoa capacitates faster than ejaculated sperm from the same species.[3] Removal of seminal plasma-coating proteins, reduction of plasma membrane cholesterol, an alteration of plasma membrane proteins and charge, redistribution of specific proteins to different membrane domains, and alteration in sperm motion termed "hyperactivation'' were among the earlier noted characteristics of capacitation. Exposure to seminal plasma appears to have two effects: (i) spermatozoa are coated with seminal plasma proteins that are removed during capacitation and (ii) the seminal plasma provides an environment in at least some species that prevents removal of cholesterol from the sperm plasma membrane.[4] Reexposure to seminal plasma decapacitates partially capacitated sperm.

The time between sperm deposition and the availability of oocytes to fertilize can vary dramatically in species with prolonged estrus such as pigs, horses, and dogs. This is in contrast to induced ovulators, such as rabbits in which the interval between sperm deposition and ovulation is relatively constant. Capacitation is ultimately a membrane destabilization process, preparing the sperm for two significant membrane fusion events. The first membrane fusion event is the acro-some reaction, an ordered vesiculation of plasma membrane and the outer acrosomal membrane. The second membrane fusion event is fertilization, the fusion between the equatorial segment of the sperm head plasma membrane and the oocyte plasma membrane. Viability of a fully capacitated sperm (destabilized membrane) is believed to be very short; a fully capacitated sperm either fertilizes an oocyte relatively rapidly or dies. Hence, species with varying intervals of residence of sperm in the reproductive tract before fertilization have the challenge of timing the delivery of capacitated sperm to the availability of recently ovulated oocytes. The two obvious solutions are (i) a heterogeneous population of sperm that capacitates at varying rates and (ii) a storage site. The pig is an example of how these two solutions work together. The capacitation process includes the passage of sperm to the uterotubal junction owing to myometrial contractions and through the uterotubal junction, leading to the removal of sperm from the seminal plasma environment, "storage'' in the lower isthmus of the oviduct, and a gradual release of the sperm from this storage site and transport to the ampullary-isthmic junction, the site of fertilization. Ovulation may trigger a more effective release from the lower isthmus.[5,6] In species where sperm is deposited in the vagina and migrates through the cervical mucus (e.g., cattle), the initial removal of sperm from the seminal plasma environment is achieved by passage through the cervix.

Within the oviduct, sperm are exposed to an elevated bicarbonate concentration, a crucial early event in capacitation. Bicarbonate is also required in in vitro capacitation systems[7] and provided either as a component of the medium or is the result of solutions reaching an equilibrium in a CO2 incubator. Bicarbonate exposure leads to the activation of adenylate cyclase, elevated cAMP, and the activation of protein kinase A. As a result, the asymmetrical distribution of phosphatidylserine and phosphatidyl-ethanolamine in the inner leaflet of the lipid bilayer collapses, and these two phospholipids appear in the outer leaflet at the anterior head of the sperm.[8] The redistribution of these two phospholipids is presumably a result of scramblase activity. Collapse of asymmetry leads to a net loss of cholesterol from the plasma membrane. Albumin in the milieu serves as an acceptor for this cholesterol. At least in the boar, cholesterol is lost from the fluid bilayer, but capacitation appears to be accompanied by increased raft formation, with cholesterol maintained in the membrane rafts. Rafts may act to bring together protein assemblies for subsequent signaling, which eventually leads to the acrosome reaction.

Capacitation is characteristically accompanied by membrane protein tyrosine phosphorylation, by hyper-activated sperm motility, and by changes in chlortetra-cycline membrane labeling. Tyrosine phosphorylation is not an early event in capacitation, but it is widely observed across species and it may be a crucial event.[8] In contrast, hyperactivation can be separated from capacitation in bovine sperm, and chlortetracycline-labeling changes can be separated from capacitation in porcine sperm.[9] Cryopreservation and subsequent thawing have some similar effects on membrane desta-bilization to capacitation, and previously cryopreserved sperm may capacitate more rapidly than ejaculated sperm. However, membrane destabilization following thawing of cryopreserved sperm is not inherently capacitation.[10]

Membrane destabilization induced by bicarbonate and further facilitated by alteration and loss of membrane proteins, loss of fluid membrane cholesterol, and potentially other less widely identified contributions may also be promoted by increases in temperature. Sperm are stored in the epididymis at a temperature several degrees below core body temperature prior to insemination and would be warmed by deposition in the female reproductive tract. Sperm passing from "storage'' in the isthmus to the fertilization site in the ampulla would appear to increase the temperature by approximately one degree.[11,12]


Sperm capacitation is a programmed membrane desta-bilization process that allows subsequent membrane fusion to occur. The process can be successfully mimicked in vitro with bicarbonate and albumin or an albumin substitute as critical components of the medium. Although a molecular understanding of the process is not complete, significant progress toward this understanding has been made in the last decade.


Fertilization, p. 411.


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10. Bravo, M.M.; Aparicio, I.M.; Garcia Herreros, M.; Gil, M.C.; Pena, F.J.; Garcia Marin, L.J. Changes in tyro sine phosphorylation associated with true capacitation and capacitation like state in boar spermatozoa. Mol. Reprod. Dev. 2005, 71 (1), 88 96.

11. Hunter, R.H.; Nichol, R. A preovulatory temperature gradient between the isthmus and ampulla of pig oviducts during the phase of sperm storage. J. Reprod. Fertil. 1986, 77 (2), 599 606.

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