SPERM TRANSPORT IN THE FEMALE GENITAL TRACT

The duration of copulation varies among different domestic animals (see Chapter 3). It takes less than a minute for ruminants to copulate, somewhat longer in horses, several minutes in pigs, and in dogs five to 30 minutes may be necessary. In several mammalian species (cows, sheep, rabbit, dog, cat and primates) the semen is deposited into the cranial vagina. In others (pigs, horses and camelids) the semen is either ejaculated directly into the cervix (pig) or, via the processus urethralis, into both the cervix and uterus.

In ruminants, the ejaculate is small in volume (in general only 3 to 4 ml) but contains an enormous concentration of spermatozoa. It was assumed for a long time that, after being deposited in the uterus by artificial insemination, most spermatozoa ascend towards the oviducts. However, recent studies have clearly demonstrated that a high proportion of these spermatozoa are lost by retrograde transport; over 60% of spermatozoa deposited in the uterus are lost to the exterior within 12 hours after artificial insemination. This loss may be much higher if spermatozoa are deposited into the cervix and this can result in fertility being compromised.

In the boar, the volume of the ejaculate is large (200 to 400 ml) with a comparatively low concentration of spermatozoa. Due to its large volume, most of the ejaculate flows from the cervix into the uterus. The boar ejaculates a series of seminal fractions with different characteristics. The first fraction contains few spermatozoa and consists mainly of secretions from the accessory sex glands. The second fraction is rich in spermatozoa. Most of the final fraction originates from the bulbourethral glands, forming a coagulum that reduces retrograde sperm loss.

The stallion ejaculates in a series of ‘jets’, of which the first usually contains the fraction rich in spermatozoa. The seminal plasma of the later jets is highly viscous and, as in the pig, may serve to minimize retrograde sperm loss from the female’s genital tract.

In the dog, the first of the three fractions of the ejaculate originates from the prostate, the only accessory sex gland in this species. The volume of this clear and acellular so-called presperm fraction ranges from 0.5 to 5 ml, depending on the breed. The second fraction, opalescent in colour, is rich in spermatozoa. Its volume varies between 1 and 4 ml and it contains between 300 million and two billion spermatozoa. The last fraction is also contributed by the prostate. Its volume may vary over a broad range, from 1 to even 80 ml, depending on the breed. Ejaculated in surges, this last fraction of prostatic fluid may force the sperm-rich fraction cranially into the uterus. In the cat the ejaculate volume is small (0.2 to 0.3 ml) and it is unclear whether it comprises multiple fractions.

Retrograde loss of spermatozoa from the female genital tract depends on several factors. The more important are volume and physical nature of the ejaculate and the site of its deposition within the female genital tract. As already mentioned, in some species, like the pig, proteins from the seminal plasma form a conspicuous vaginal plug that prevents spermatozoa from being lost to the exterior. In some laboratory rodents the solid vaginal plug that is formed after copulation can be seen from outside and is used to determine the time of mating (see Chapter 20).

Transport of spermatozoa to the ampulla of the oviduct is primarily the result of elevated tone and motility of the tunica muscularis of the female genital tract. It can be divided into two phases, a rapid and a sustained transport phase. Within a few minutes after mating, spermatozoa have already reached the oviduct. Although male gametes are close to the oocyte after a very short period, these spermatozoa are not viable and do not play a role in fertilization; it is the sustained phase of sperm transport that is important for successful fertilization. In the sustained phase, spermatozoa are transported to the oviducts from presumed reservoirs in the uterotubal junction or the cervix over an extended time period, delivering them in a more uniform manner.

A major barrier for spermatozoal transport is the cervix uteri that can also serve as a reservoir of spermatozoa in several species. In ruminants, and to a lesser degree in the mare, the cervix possesses a convoluted system of folds and grooves. As in other domestic species, the epithelium of the ruminant cervix produces a highly viscous mucus that prohibits the penetration of spermatozoa through the cervical canal during most times of the oestrous cycle. It is only during oestrus that the mucus changes its viscosity, when a low viscosity mucus rich in sialomucins is produced in the basal regions of the cervical crypts. A second type of mucus, containing mainly sulphomucins and much more viscous, is secreted by the apical portions of the epithelium covering the tips of the cervical folds. These two different types of secretion create two different compartments within the cervical canal, one basal with low viscosity and one more central with high viscosity. The low viscosity environment in the basal regions of the folds offers a ‘privileged pathway’ through which spermatozoa can quite easily move towards the uterus. The ability of spermatozoa to use this privileged pathway depends on their ability to swim through the crypts of the cervix; non-motile spermatozoa cannot progress and are eliminated. Consequently, the cervix acts as a filter for removing non-viable spermatozoa.

CAPACITATION

Spermatozoa are not able to fertilize the oocyte immediately after arrival in the female genital tract; to acquire fertility they must reside there for a certain period of time (Fig. 5-2). The changes that occur during this period constitute capacitation of spermatozoa. The site where capacitation takes place varies to some extent among species. In species in which spermatozoa are delivered into the mid-cervix (sow), or into the caudal cervix and immediately enter the corpus uteri, capacitation probably begins in the uterus and ends in the isthmus of the oviduct. In species with intravaginal semen deposition, capacitation is probably initiated during the passage of the spermatozoa through the cervix. Not all spermatozoa are capacitated at the same time and, as the process usually extends over several hours, individual spermatozoa can show a different degree of capacitation depending on their location within the female genital tract.

Fig. 5-2: Bovine oviduct. Spermatozoa can be temporarily bound to the epithelium of the isthmus in the oviduct by binding mechanisms mediated by sugars (for instance fucose). 1: spermatozoon; 2: kinocilia; 3 microvilli.

Courtesy Sinowatz and Rüsse (2007).

Capacitation encompasses a number of complex processes. It has been clearly shown that the plasma membrane of the spermatozoon (particularly of the head) undergoes marked changes during capacitation. Important processes during capacitation are: removal of the glycoprotein coat and seminal plasma proteins (adsorbed during epididymal storage and ejaculation) from the spermatozoal surface; the functional coupling of the signal-transducing pathways that regulate the initiation of the acrosome reaction by the zona pellucida glycoproteins; alterations in the flagellar motility that are necessary to penetrate the zona pellucida; and, finally, development of the capacity to fuse with the plasma membrane of the oocyte. These processes are accompanied by changes in metabolism, the biophysical properties of the plasma membrane, and protein phosphorylation, together with elevation of intracellular calcium levels and pH, and hyperpolarization of membrane potential.

Capacitation can be reversed by returning already capacitated spermatozoa to seminal plasma. Once decapacitated in this way, they require additional capacitation before they can regain their fertility.

Much of our understanding of the capacitation process has been gained from studies in vitro. Several factors cause capacitation in vitro. First, cholesterol efflux from the sperm membrane is mediated by sterol-binding proteins and initiates many aspects of capacitation. The reorganization of the sperm membrane after cholesterol depletion is considered an early step in capacitation. Second, several sperm plasma membrane proteins are tyrosine phosphorylated through a cAMP-dependent mechanism. Spermatozoa express a bicarbonate-sensitive, soluble form of adenyl cyclase that may control these phosphorylation events. Third, elevation of the intracellular pH and bicarbonate levels may lead to a stimulation of cAMP production. By activating cyclic nucleotide-gated channels in the plasma membrane of the sperm flagellum, spermatozoa could switch to the hyperactivated motility pattern that is characteristic of capacitated spermatozoa.

In vivo, synergistic actions of multiple factors are likely to mediate capacitation. It has been shown that sterol-binding proteins, such as high-density lipoproteins, are present in the oviductal fluid and can accelerate cholesterol efflux from spermatozoa. Additionally, progesterone, derived from the follicular fluid and from secretion by the cumulus cells surrounding the ovulated oocyte, may be involved in the regulation of some aspects of the process.

INTERACTIONS BETWEEN SPERMATOZOA AND THE ZONA PELLUCIDA

The zona pellucida (ZP) is an extracellular matrix surrounding the oocyte and the early embryo that exerts several important functions during fertilization and early embryonic development. In most mammalian species it is composed of three glycoproteins (ZPA, equivalent to mouse ZP2; ZPB, equivalent to mouse ZP1; ZPC equivalent to mouse ZP3), products of the gene families ZPA, ZPB and ZPC that have been found to be highly homologous within mammalian species. Most data on the structure and function of the ZP have been obtained from studies in the mouse. New data from pigs and other domestic animals, however, indicate that findings in the mouse model are not always applicable to other species. For example, whereas ZP3 is the primary sperm receptor in the mouse, it is ZPA and ZPC that possess receptor activity in the pig. Also contrary to the mouse (in which the growing oocyte is the only source of zona glycoproteins), these proteins are expressed in both the oocyte and granulosa cells in a stage-specific pattern in domestic animals.

The ZP is involved in several critical stages of fertilization: the adhesion and binding of capacitated spermatozoa to the ZP; the subsequent induction of the acrosome reaction and penetration of the ZP; and the fertilization-induced modifications of the ZP that prevent polyspermy.

Adhesion of spermatozoa to zona pellucida

The first contact between the spermatozoon and the ZP, adhesion, is a loose, non-specific association between the gametes and appears to be a disconcertingly random affair (Fig. 5-3). It is followed by a relatively firm binding, which is species-specific and mediated by complementary receptors on the ZP (sperm receptor) and on the sperm surface. In the mouse initial sperm-zona adhesion is mediated by ZP3, a constituent glycoprotein of the ZP that binds to receptors on the anterior head of acrosome-intact sperm. This adhesion to the ZP is probably based on protein-carbohydrate recognition processes, through the association of O-linked α-galactosyl residues from ZP3 with a cognate receptor on the sperm. Secondary binding is then mediated by ZP2. Other authors, however, consider that sperm-zona binding is a purely protein-based event.

Fig. 5-3: Acrosome reaction. Once bound to the zona pellucida, the spermatozoa undergo the acrosome reaction, during which hydrolyzing enzymes are released from the acrosome of the sperm head. It begins when the plasma membrane of the spermatozoon forms multiple fusion sites with the outer acrosomal membrane resulting in the formation of many small vesicles. (a) Spermatozoon with intact acrosome. (b) Vesiculation of the plasma membrane and outer acrosomal membrane; (c) Penetration of the zona pellucida by the spermatozoon. 1: Fusion of the plasma membrane and the outer acrosomal membrane of the spermatozoon; 2: Zona pellucida.

Courtesy Sinowatz and Rüsse (2007).

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