Despite being motile once inside the female reproductive tract, spermatozoa are not yet capable of fertilizing an oocyte at this stage. Spermatozoa require a species-dependant amount of time to travel from the site of deposition, in the vagina, to the site of fertilization, in the fallopian tubes [16], while simultaneously acquiring fertilizing ability by undergoing a series of complex changes along the way, collectively referred to as capacitation [16].

The process of capacitation has been defined as a “functional maturation of the spermatozoon” [17]. Capacitation involves considerable alterations to the surface of the plasma membrane, with various molecules being removed, rearranged, or revealed. These changes are facilitated by the removal of cholesterol and glycoproteins, resulting in a more fluid plasma membrane with an increased permeability to Ca^{2 + 16}. The subsequent upsurge in Ca^{2 +} leads to increased intracellular cAMP levels and, consequently, increased motility [3].

The uterus aids in the steps of capacitation by secreting sterol binding albumin, lipoproteins, and glycosidases such as heparin [18]. Sterol binding albumin promotes cholesterol efflux from the sperm plasma membrane [19], while heparin [20] and lipoproteins [21] have been shown to accelerate the initiation of both capacitation and the acrosome reaction in vitro. Extracellular Ca^{2 +} plays an important role as a signalling molecule during capacitation and the acrosome reaction, and is a prerequisite for both processes [16]. The acrosomal area also undergoes changes during capacitation in preparation for a possible acrosome reaction closer to fertilization.

As a result of the increased intracellular Ca^{2 +} levels mentioned before, and by the time the fallopian tubes are reached, the spermatozoa become hyperactivated, indicating the successful completion of the capacitation process.

Hyperactivation is a motility pattern usually occurring in vivo in the fallopian tubes [22], and is characterized by increased amplitude and asymmetrical flagellar beating [23] together with increased amplitude of lateral head displacement (ALH) [24]. Hyperactivation is triggered by a rise in intracellular flagellar Ca^{2 +} and requires an increase in both pH and ATP production [23]. Since, hyperactivation is normally only displayed at the site of fertilization, it may therefore be modulated by chemotactic signals to direct and turn spermatozoa toward the oocyte [23], while the increased sideways displacement of the head increases the spermatozoon’s chances of encountering the egg [22]. Furthermore, hyperactivation enhances the ability of the spermatozoa to traverse the viscous fluids in the fallopian tubes, affording increased flexibility and facilitating the penetration of the spermatozoon through the cumulus complex and zona pellucida [22].

The oocyte is surrounded by various external structures; the outermost is the cumulus oophorus, a gel-like hyaluronic acid that the spermatozoa encounters initially and needs to penetrate first. Inside of the cumulus is the zona pellucida, to which sperm must bind. This acts as the final mechanical barrier for spermatozoa before fertilization. In order for the spermatozoa to fuse with the oolemma, it must bind to the zona pellucida, reorientate, and finally, penetrate the oocyte [25]. The zona pellucida consists predominantly of glycoproteins, among which ZP1, ZP2, ZP3, and ZP4 [26] are highly species-specific and complimentary to glycoproteins on the surface of the head of the spermatozoa. The zona glycoproteins ZP1, ZP3, and ZP4 are primarily responsible for the tight binding of capacitated, acrosome-intact spermatozoa and initiating a cascade of cellular interactions that culminate in fertilization. In vivo, the acrosome reaction is initiated when the spermatozoon comes into contact with or in very close proximity to the oocyte’s cumulus layer. The purpose of the acrosome reaction is the release of the enzymes (hyaluronidase and acrosin [27]) contained inside the acrosomal cap, which is responsible for digestion of the zona pellucida and oolemma in order to allow the spermatozoon to penetrate the oocyte. The enzymes that are released digest the cumulus cells surrounding the oocyte, exposing the oocyte to the acrosin attached to the inner membrane of the sperm. Acrosin digests the zona pellucida and membrane of the oocyte. Progesterone, secreted by the cumulus cells, is an important cofactor for the initiation of this process [27], as is the increase in calcium permeability due to successful capacitation [28].

Upon completion of the acrosome reaction, the inner membrane of the spermatozoa subsequently fuses with the oolemma and the contents of the sperm head enter the oocyte during a process termed penetration [29].

Upon penetration, the oocyte becomes activated. It completes its secondary meiotic division and the two haploid nuclei—paternal and maternal—fuse to form a diploid zygote. In order to prevent polyspermy and minimize the possibility of producing a triploid zygote, several changes to the oocytes’ membranes render them impenetrable shortly after the first sperm enters the egg through a process called the zona reaction [29].

Being independent living cells, spermatozoa must sustain themselves and support their functions and motility for an extended period of time under varying conditions [30], and as they are not directly attached to the female body or linked to the bloodstream, they must survive on very limited resources. Spermatozoa are maintained at a low energy consumption state during epididymal storage, conserving energy and favoring long-term cell survival [3]. Despite being very small cells, spermatozoa require a substantial amount of energy in the form of ATP in order to support cellular processes and functions such as protein phosphorylation and motility [30]. The preferred metabolic pathway for ATP production varies greatly between species.

In humans, immature spermatids seem to favor the substrates lactate and pyruvate, indicating that oxidative phosphorylation is the main energy source during early spermatogenesis. However, as spermatozoa undergo maturation in the epididymis, they expand their energy production ability to furthermore utilize glycolysis as an alternative source [30].

While the acrosome reaction utilizes lactate and pyruvate for ATP production by oxidative phosphorylation, gamete fusion requires glucose to produce NADPH by the pentose pathway. Normal sperm motility appears to be sustained by relatively low ATP levels, but increased ATP levels are required for tyrosine phosphorylation linked to hyperactivation. Thus, each individual process and event requires a different substrate and metabolic pathway [30], although it is important to note that utilization of these two metabolic pathways is not mutually exclusive.

Glycolysis appears to be indispensable to the spermatozoa of all species and has been shown to compensate for a lack of oxidative phosphorylation, thereby leading to the recovery of most sperm functions. Spermatogenic glycolytic enzymes may be more flexible in the use of substrates, and able to adapt to unexpected conditions in the female reproductive tract [30].

Ultimately, it appears that the metabolic pathway utilized in vivo is greatly influenced by the conditions of the female reproductive tract [31], which, in turn, may be influenced by a variety of factors, not the least of which is the menstrual cycle.

After sperm have been in the epididymis for 18–24 h, they develop the capability of motility, even though several inhibitory proteins in the epididymal fluid still prevent final motility until after ejaculation. Smooth muscle contractions of the female reproductive tract, ciliary beats, fluid currents, and flagellar activity of sperm are primary mechanisms of sperm movement and transport [32]. Sperm can live for many weeks in the suppressed state in the male reproductive tracts, but the life expectancy of ejaculated sperm in the female genital tract is only about 5 days [3].

Normal motile, fertile spermatozoa are capable of flagellated movement at velocities of up to104 mm/min. The motility of sperm is significantly enhanced in a neutral and slightly alkaline medium, as it exists in the ejaculated seminal plasma, but it is greatly suppressed in even a slightly acidic medium. The activity of spermatozoa accelerates markedly with increased temperature, but so does the rate of metabolism, leading to a reduction in the lifespan of the spermatozoa.

Activated motility as seen in freshly ejaculated spermatozoa refers to the low amplitude symmetric waves propagating along the length of the flagellum and resulting in linear propulsion of the sperm cell [33]. Hyperactivated motility, as seen at the site of fertilization occurs when the flagellar movement becomes asymmetrical with higher amplitude, resulting in highly curved trajectories [34].

The passage of sperm through the female reproductive tract is regulated to maximize the chance of fertilization and ensure that morphologically and functionally normal spermatozoa will be the ones to succeed. A single spermatozoon is about 60 μm long, and needs to swim a distance of approximately 20 cm to reach the oocyte. Millions of spermatozoa are produced by the male as a first attempt to increase the chance of successful fertilization, as many spermatozoa die along the way. Spermatozoa must survive the journey independently and without the benefit of reparative mechanisms available as is the case with somatic cells [35], while being subjected to various physical and chemical stressors (refer to Fig. 6.1). In humans, it has been observed that sperm are still viable and fertile in the female up to 5 days after intercourse.

The remainder of this chapter will be dedicated to the obstacles spermatozoa face in the female reproductive tract, and, more importantly, the mechanisms that exist to aid in the survival of the spermatozoa in this hostile environment.

During intercourse, semen deposition is usually confined to the anterior vaginal region, near the cervical os. Spermatozoa deposited in the vagina face two major complications: gravity and a sudden acidic environment. Upon ejaculation, the semen immediately coagulates in the vagina and liquefies after 30–60 min. This initial clotting of the semen inside the vagina not only prevents sperm loss and leaking due to gravity, but it furthermore provides the spermatozoa with a pH buffered and substrate rich milieu in the interim. During the sequential liquefaction process mucus molecules present in the seminal plasma arrange themselves to form channels that help guide the motile spermatozoa out of the plasma coagulum and toward the opening of the cervix. This process serves as the first selection step toward a viable sperm fraction, as immature/immotile sperm will not be able to migrate efficiently from the seminal plasma [35].

Being exposed to the external environment, the vagina is susceptible to infections, and is therefore well equipped with antimicrobial defenses, which, in addition to the acidic pH, includes high incidence of immune cells [35]. However, due to the prostaglandins’ immune-suppressing effects and pH buffering of the seminal plasma, spermatozoa can survive in the vagina for 24–48 h before they are removed by the female immune cells [3].