Unsolved Problems in the Locomotion of Mammalian Sperm
Infertility is a significant health problem. On the other hand, unintended pregnancies also remain a major concern for women worldwide. Improved methods could be developed for diagnosing and treating infertility, as well as for contraception, if more were known about how sperm move through the female reproductive tract. Such information would also benefit dairy production, because fertility of cattle has been declining. Four major questions remain about sperm movement: (1) How do sperm pass through the cervix? (2) How do sperm pass through the uterotubal junction? (3) How are sperm stored and released in the oviduct? (4) Are sperm guided by chemotaxis to the egg? One important aspect of these unknowns is how physical features of the female tract affect the movement of sperm. Expertise in microfluidics and the modeling of movement at low Reynolds numbers would help biologists immensely in addressing these questions.
Key wordsSperm spermatozoa cervix uterus oviduct fallopian tube fertilization
Infertility is a significant human health problem. According to a 2002 survey by the US CDC, 7% of 2.1 million married couples in which the woman was of reproductive age reported that they had not used contraception for 12 months and yet the woman had not become pregnant (http://www.cdc.gov/ART/). It is estimated that roughly half of the cases of infertility are due to a male factor, such as abnormal sperm motility . The developments of in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) have done much to alleviate infertility problems; however, these procedures are associated with health risks. When a large cohort of siblings in which one was conceived naturally and the other by IVF/ICSI were compared, the IVF/ICSI babies were more likely to have been born prematurely and with low birth weight .
In the US dairy industry, 58% of farms use artificial insemination to impregnate cows ; however, it produces the undesired outcome of producing more male dairy calves than does natural mating . Technology has been developed to produce “sexed semen”, which produces about 90% female calves . The high cost of sexed semen has pushed the industry to use fewer sperm per insemination, despite the fact that the lower dose reduces fertility [31, 32]. Fertility has also been declining significantly over the past few decades, even without the use of sexed semen. While some of this is blamed on difficulties with controlling hormonal cycles of cows, some of the problems could be overcome if more information were known about sperm movement through the female tract. This could be used to optimize the process of artificial insemination.
Thus, there is much to be done to improve the outcome of reproductive technologies for human couples and for domestic cattle. These technologies are also being adapted by zoos and conservationists in order to save endangered species.
Contraception also remains a major human health problem. In the US, about half of pregnancies are unintended . According to the Guttmacher Institute, the contraceptive method most used in the US is the birth control pill, which is currently used by almost 11 million women (http://www.guttmacher.org). The use of the combined ethinylestradiol/progestagen pill significantly increases the risk of venous thromboembolism and myocardial infarction , and impacts the development of osteoporosis by reducing bone mass acquisition in young women . There is still an urgent need to develop contraceptives that are highly effective, do not produce harmful side effects, are reversible and easy to use, and also reduce risk of contracting sexually transmitted diseases. Greater understanding of the mechanisms of sperm movement through the female tract can contribute to the improvement of contraceptive technology.
Information about how sperm interact with the female reproductive tract could inspire development of new methods to diagnose and treat infertility, as well as development of new safe and effective contraceptives. In this review, unsolved problems of the four main events of sperm interaction with the mammalian female reproductive tract will be considered.
2 Problem 1: How Do Sperm Pass Through the Cervix?
One of the major functions of the cervix is to guard the female reproductive tract against invasion by infectious microbes. At the same time, it must allow the passage of sperm and also serve as the birth canal. Some species solve the dilemma of getting sperm through the cervix by bypassing it altogether. In the pig, the penis is shaped like a corkscrew and the cervix contains complementary furrows. During copulation, the penis screws into the cervix to inseminate semen directly into the uterus [10, 11].
In humans and other primates, as well as domestic cattle, however, the male deposits semen in the vagina at the entrance to the cervix and the sperm must swim through the cervix reach the uterus.
While it is not yet technically possible to observe sperm directly as they pass through the cervix, the evidence strongly indicates that the sperm do it by swimming rather than being pushed or pulled by the female tract. However, there is also some evidence that the female provides preferential pathways specifically designed for sperm that guide them through the cervix. Nevertheless, although the existence of preferential pathways for sperm was proposed more than 20 years ago, the hypothesis is yet to be tested, due to lack of the technology to do it.
The preliminary evidence for a preferential pathway arose from a careful study of the gross and microscopic anatomy of the bovine cervix, which was made by Mullins and Saacke in 1989 . They produced graphic reconstructions of tracings of serial cross sections of bovine cervixes and used these to follow the course of folds in the interior surface of the cervical canal. They found large primary folds that branched into smaller secondary folds and ran the length of the cervix. Small (micro) grooves, on the order of 10 μm in width, were seen in the surfaces of the folds. Many of the micro grooves that cut into the tissue surfaces at the base of the folds could be traced long distances through the cervix, whereas grooves near the apices of the folds were much shorter.
Sperm may also be guided through the cervix by the microarchitecture of the cervical mucus itself, which is comprised of long, flexible, linear molecules. It has been proposed that these long molecules are aligned as they are secreted into the micro grooves and their alignment guides sperm. Human and bovine sperm orient themselves along the long axis of threads of bovine cervical mucus [3, 35] and human sperm swim through cervical mucus in a straighter path than they do in seminal plasma or medium .
The proposal that preferential pathways guide sperm through the cervix is intriguing, because it could account for the ability of large numbers of sperm to travel through the cervix while infectious microbes are discouraged from doing so. The size, shape, and mechanics of sperm movement are quite different from those of the common infectious organisms, such as bacteria, viruses, and flagellate protozoa.
Understanding how sperm swim through the cervix could inspire development of new contraceptives for humans and improve the success rate of artificial insemination in dairy cattle.
3 Problem 2: How Do Sperm Pass Through the Uterotubal Junction?
The diameter of junction between the uterus and oviduct is much smaller than that of the cervix. A scanning electron microscope study of the linings of bovine uterotubal junctions that had been cut open revealed that the inner surface is comprised of folds in the pattern of cul de sacs. Such an arrangement would seem to steer sperm into dead ends. However, if one considers how these folds would be positioned in the intact junction, one could imagine that they form funnels directing sperm through the center of the junction into the oviduct.
Even more curious than the enigma of the shape of the folds is the observation that mouse sperm require certain proteins on their surface in order to pass into the oviduct. Mice are used extensively as model species for the study of mammalian reproduction, because they can be genetically manipulated in order to examine the functions of various genes. In this case, a strain was developed in which the gene that produces the protein ADAM3 was inactivated. ADAM3 is a protein that is expressed on the front surface of the mouse sperm head . Male mice lacking an active ADAM3 gene are infertile, because their sperm cannot pass into the oviduct nor penetrate the proteinaceous shell around the egg [23, 38]. The question is: how does ADAM3 enable sperm to pass through the uterotubal junction?
Unfortunately, this approach is limited because one can only watch sperm for a few minutes, because the oviduct is disconnected from its blood supply. Consequently, the sperm seen in the uterotubal junction cannot be followed through to the oviduct to see exactly how they move through it. We propose that ADAM3-induced sporadic sticking of sperm to the walls enables sperm to advance, but we do not know how this works. Development of mathematical and microfluidics models may help to elucidate the process.
4 Problem 3: How Are Sperm Stored and Released in the Oviduct?
As the time approaches that the egg will be released from the ovary into the distal end of the oviduct, sperm are gradually released from the storage reservoir. The gradual release assists in preventing more than one sperm from reaching an egg simultaneously and fertilizing it, which would completely disrupt subsequent embryonic development [12, 25].
There is some evidence that sperm are released from the reservoir by shedding the proteins that bind them to the oviductal epithelium  and are further assisted by hyperactivation of flagellar movement. Hyperactivated sperm produce high-amplitude bends on one side of the flagellum. These deep bends appear to rip the sperm off of the epithelium . In transilluminated oviducts, sperm are seen to detach and reattach several times and are thus thought to gradually work their way out of the reservoir .
Three different binding proteins have been identified on bull sperm and four different receptor proteins have been identified in the bovine oviduct (Fig. 4) [8, 17]. Each type of binding protein on sperm can act alone to enable sperm to adhere to oviductal epithelium. Thus, the release and subsequent advances of sperm may depend on various combinations of interactions between the three sperm proteins and the four oviductal receptors, plus appropriate triggering of hyperactivation. As with the other problems of sperm movement, understanding the release of sperm requires elucidation of the physical aspects of sperm interactions with the oviduct and begs for the attention of biophysicists.
5 Problem 4: Are Sperm Guided by Chemotaxis to the Egg?
It has been well established that sperm of various species of marine invertebrates respond to chemotactic signals from eggs via Ca2 + -mediated changes in the degree of flagellar beat asymmetry .
In human sperm, odorant receptors have been detected at the base of the flagellum and the sperm have been reported to respond chemotactically to the floral odorant bourgeonal (the odor of lilies of the valley) . The problem is that bourgeonal is a floral product and the human homologue has yet to be identified, despite several years of searching.
Progesterone has also been implicated in human sperm chemotaxis [13, 36, 37]. Progesterone is secreted by the cumulus cells around the oocyte  and could therefore direct sperm toward the site of fertilization. The level of progesterone that acts as chemoattractant for human sperm is 0.01–10 nM . A rise in sperm Ca2 + levels has been implicated in a flagellar response of human sperm to progesterone, particularly when sperm are exposed to a progesterone gradient  Nevertheless, only low percentages of sperm have been reported to orient into gradients of progesterone and extensive statistical analysis has been required in the attempt to demonstrate chemotaxis [9, 36]. Whereas, the response of sea urchin sperm to specific chemoattractants from the egg can be seen convincingly in videos, the response of human sperm to progesterone is so low that it cannot be directly observed in videos.
Despite the lack of strong data in support of the existence of sperm chemotaxis in humans and other mammals, it is generally thought that chemotaxis must exist, because the egg is such a small target for sperm within the oviduct (Fig. 3) and is only viable for a short time.
Because it has not been possible as yet to follow sperm movement continuously from the storage reservoir in the lower oviduct to the egg in the upper oviduct or to duplicate the physical environment of the oviduct in vitro, biologists have been frustrated in their attempts to determine whether and how sperm are guided to the site of fertilization. The talents of biophysicists and bioengineers are badly needed to attack this problem.
- 3.Chretien FC (2003) Involvement of the glycoproteic meshwork of cervical mucus in the mechanism of sperm orientation. Acta Obstet Gynecol Scand 82:449–461Google Scholar
- 11.Hunter RHF (1982) Reproduction of farm animals. Longman, London, p 149Google Scholar
- 22.NAHMS (2009) Dairy 2007, Part IV: reference of Dairy cattle health and management practices in the United States, 2007. #N494.0209. Centers for Epidemiology and Animal Health, USDA:APHIS:VS, Fort CollinsGoogle Scholar
- 27.Pacey AA, Hill CJ, Scudamore IW, Warren MA, Barratt CL, Cooke ID (1995) The interaction in vitro of human spermatozoa with epithelial cells from the human uterine (fallopian) tube. Hum Reprod 10:360–366Google Scholar