A couple in search of an oocyte donor advertises in a college newspaper seeking an attractive young woman from an athletic family. A cancer patient stores her oocytes before undergoing treatment. Two years later, she has several of them fertilized in a laboratory dish with her partner’s sperm, and has a cleav- age embryo implanted in her uterus. She becomes a mother. A man paralyzed from the waist down has sperm removed and injected into his partner’s oocyte. He, too, becomes a parent when he thought he never would.
Lisa and Jack Nash sought to have a child for a differ- ent reason. Their daughter Molly, born on July 4, 1994, had Fanconi anemia (MIM 227650). This autosomal recessive con- dition would destroy her bone marrow and her immunity. An umbilical cord stem cell transplant from a sibling could likely cure her, but Molly had no siblings. Nor did her parents wish to have another child with a one in four chance of inheriting the disorder, as Mendel’s first law dictates. Technology offered another solution.
In late 1999, researchers at the Reproductive Genetics Institute at Illinois Medical Center mixed Jack’s sperm with Lisa’s oocytes in a laboratory dish. After allowing fifteen of the fertilized ova to develop to the eight-cell stage, research- ers separated and applied DNA probes to one cell from each embryo. A cell that had wild type Fanconi anemia alleles and that matched Molly’s human leukocyte antigen (HLA) type was identified and its seven-celled remainder implanted into Lisa’s uterus. Adam was born in late summer. A month later, physi- cians infused his umbilical cord stem cells into Molly, saving her life ( figure 21.1 ).
Increased knowledge of how the genomes of two indi- viduals come together and interact has spawned several novel ways to have children. Assisted reproductive technolo- gies (ARTs) replace the source of a male or female gamete, aid fertilization, or provide a uterus. These procedures were developed to treat infertility, but are increasingly encompass- ing genetic screening. In the United States the government does
not regulate ARTs, but the American Society for Reproductive Medicine provides voluntary guidelines. The United Kingdom has pioneered ARTs and its Human Fertilisation and Embryol- ogy Authority has served as a model for government regula- tion. A great advantage of the British regulation of reproductive health services and technologies is that databases include suc- cess rates of the different procedures.
Key Concepts
1. Assisted reproductive technologies provide innovative ways to conceive offspring.
2. ARTs are used to avoid conception of a child with a particular genetic condition, or to overcome infertility.
21.2 Infertility and Subfertility
Infertility is the inability to conceive a child after a year of fre- quent intercourse without the use of contraceptives. Some spe- cialists use the term subfertility to distinguish those individuals and couples who can conceive unaided, but for whom this may take longer than usual. On a more personal level, infertility is a seemingly endless monthly cycle of raised hopes and crushing despair. In addition to declining fertility, as a woman ages, the incidence of pregnancy-related problems rises, including chro- mosomal anomalies, fetal deaths, premature births, and low- birthweight babies. For most conditions, the man’s advanced age does not raise the risk of pregnancy complications, although sperm motility declines with age.
Physicians who specialize in infertility treatment can iden- tify a physical cause in 90 percent of cases. Of these, 30 percent of the time the problem is primarily in the male, and 60 percent of the time it is primarily in the female. When a physical prob- lem is not obvious, the cause is usually a mutation or chromo- somal aberration that impairs fertility in the male. The statistics are somewhat unclear, because in 20 percent of the 90 percent, both partners have a medical condition that could contribute to infertility or subfertility. A common combination is a woman with an irregular menstrual cycle and a man with a low sperm count. One in six couples has difficulty in conceiving or giving birth to children.
Male Infertility
Infertility in the male is easier to detect but sometimes harder to treat than female infertility. One in twenty-five men is infertile.
Some men have difficulty fathering a child because they pro- duce fewer than the average 20 to 200 million sperm cells per milliliter of ejaculate, a condition called oligospermia. It has several causes. If a low sperm count is due to a hormonal imbal- ance, administering the appropriate hormones may boost sperm output. Sometimes a man’s immune system produces IgA anti- bodies that cover the sperm and prevent them from binding to
oocytes. Male infertility can also be due to a varicose vein in the scrotum. This enlarged vein produces too much heat near developing sperm, and they cannot mature. Surgery can remove a scrotal varicose vein.
Most cases of male infertility are genetic. About a third of infertile men have small deletions of the Y chromosome that remove the only copies of key genes whose products control spermatogenesis. Other genetic causes of male infertility include mutations in genes that encode androgen receptors or protein fer- tility hormones, or that regulate sperm development or motility.
Reading 21.1 describes the recently identified first type of auto- somal recessive male infertility that is not part of a syndrome.
Sperm with extra chromosomes are ten times more likely to occur in men who had vasectomies reversed. Exactly how this happens isn’t known, but it may be related to blocking the male reproductive system to keep developing sperm out of semen. If sperm with an abnormal number of chromosomes fertilize oocytes, the imbalance may end development so early that repeated pregnancy losses appear to be infertility.
For many men with low sperm counts, fatherhood is just a matter of time: They are subfertile, not infertile. If an ejacu- late contains at least 60 million sperm cells, fertilization is likely eventually. To speed conception, a man with a low sperm count can donate several semen samples over a period of weeks at a fertility clinic. The samples are kept in cold storage, then pooled.
Some of the seminal fluid is withdrawn to leave a sperm cell con- centrate, which is then placed in the woman’s body. It isn’t very romantic, but it is highly effective at achieving pregnancy. Men who actually want a very low sperm count—those who have just had a vasectomy for birth control—can use an at-home test kit to monitor their sperm counts. Fewer than 250,000 sperm cells per milliliter of seminal fluid makes pregnancy highly unlikely.
Sperm quality is more important than quantity. Sperm cells that are unable to move—a common problem—or are shaped abnormally, cannot reach an oocyte. However, the genetic pack- age of an immobile or abnormally shaped sperm cell can be injected into an oocyte, although this does not always lead to fertilization. If the cause of male infertility is hormonal, replac- ing the absent hormones can sometimes make sperm move. Even sperm that look and move normally may be unable to fertilize an oocyte.
Faulty apoptosis (programmed cell death) can also cause male infertility. Apoptosis normally kills abnormally shaped sperm. Men with high percentages of abnormally shaped sperm often have cell surface molecules that indicate impaired apoptosis.
Female Infertility
Abnormalities in any part of the female reproductive system can cause infertility. ( figure 21.2 ). Many women with sub- fertility or infertility have irregular menstrual cycles, making it difficult to pinpoint when conception is most likely. In an average menstrual cycle of 28 days, ovulation usually occurs around the 14th day after menstruation begins. This is when a woman is most likely to conceive.
For a woman with regular menstrual cycles who is under 30 years old and not using birth control, pregnancy typically happens within 3 or 4 months. A woman with irregular men- strual periods can tell when she is most fertile by using an ovulation predictor test, which detects a peak in the level of luteinizing hormone that precedes ovulation by a few hours.
Another way to detect the onset of ovulation is to record body temperature each morning using a digital thermometer with subdivisions of hundredths of a degree Fahrenheit, which can indicate the 0.4 to 0.6 rise in temperature when ovulation starts.
Sperm can survive in a woman’s body for up to 5 days, but the oocyte is only viable for 24 to 48 hours after ovulation.
The hormonal imbalance that usually underlies irregu- lar ovulation has various causes. These include a tumor in the ovary or in the pituitary gland in the brain that controls the reproductive system, an underactive thyroid gland, or use of steroid-based drugs such as cortisone. Sometimes a woman produces too much prolactin, the hormone that promotes milk production and suppresses ovulation in new mothers. If prolac- tin is abundant in a nonpregnant woman, she will not ovulate.
Fertility drugs can stimulate ovulation, but they can also cause women to “superovulate,” producing more than one oocyte each month. A commonly used drug, clomiphene, raises the chance of having twins from 1 to 2 percent to 4 to 6 percent.
If a woman’s ovaries are completely inactive or absent (due to a birth defect or surgery), she can become pregnant only if she uses a donor oocyte. Some cases of female infertility are due to
“reduced ovarian reserve”—too few oocytes. This is typically discovered when the ovaries do not respond to fertility drugs.
Signs of reduced ovarian reserve are an ovary with too few folli- cles (observed on an ultrasound scan) or elevated levels of follicle- stimulating hormone on the third day of the menstrual cycle.
The uterine tubes are also a common site of female infer- tility because fertilization usually occurs in open tubes. Block- age can prevent sperm from reaching the oocyte, or entrap a fertilized ovum, keeping it from descending into the uterus. If an embryo begins developing in a blocked tube and is not removed and continues to enlarge, the tube can burst and the woman can die. Such a “tubal pregnancy” is called an ectopic pregnancy.
Uterine tubes can also be blocked due to a birth defect or, more likely, from an infection such as pelvic inflammatory dis- ease. A woman may not know she has blocked uterine tubes until she has difficulty conceiving and medical tests uncover the prob- lem. Surgery can sometimes open blocked uterine tubes.
Excess tissue growing in the uterine lining may make it inhospitable to an embryo. This tissue can include benign tumors called fibroids or areas of thickened lining from a condition called endometriosis. The tissue can grow outside of the uterus too, in the abdominal cavity. In response to the hormonal cues to menstruate, the excess lining bleeds, caus- ing cramps. Endometriosis can hamper conception, but curi- ously, if a woman with endometriosis conceives, the cramps and bleeding usually disappear after the birth.
Secretions in the vagina and cervix may be hostile to sperm. Cervical mucus that is thick or sticky due to infection can entrap sperm, keeping them from moving far enough to
In fewer than a tenth of a percent of men who are infertile, sperm cells lack the tip, called the acrosome, where the enzymes that break through the layers surrounding an oocyte collect. This condition is called “globozoospermia” (figure 1). An Ashkenazi Jewish family led researchers to a gene that, when mutant, causes an autosomal recessive form of male infertility due to round-headed sperm.
The family went to a center for reproductive medicine in Brussels, the Netherlands. Of the six sons, three were infertile (figure 2).
Three daughters were fertile. The affected sons’ sperm were obviously misshapen, and the mode of inheritance obviously recessive, since the parents were fertile.
Researchers suspected that consanguinity was involved, because a shared ancestor increases risk of inheriting a very rare autosomal recessive condition if the mutation is in the family. But the family denied knowing a relative who had married a relative.
Reasoning that perhaps DNA could reveal what the family did not know, researchers scanned the genomes of all six sons for regions of homozygosity. Recall from chapter 7 that these areas in a genome
reflect inheritance from shared ancestors, indicating consanguinity.
A region of homozygosity in this case was defined as 25 consecutive SNPs that were homozygous. The genomes of all six sons were riddled with these regions, suggesting that at some point, cousin married cousin or an aunt/uncle wed a nephew/niece. One region of homozygosity was seen in all three infertile brothers, but was heterozygous in two of the three fertile brothers. The remaining brother was homozygous wild type for the region.
Next, the researchers delved into the part of the long arm of chromosome 3 where the telltale region of homozygosity lay. It houses fifty genes, only one of which is expressed specifically in the testes.
This gene is called “spermatogenesis-associated 16”, or SPATA16 (MIM 102530). It has eleven exons, and the mutation in the Ashkenazi family is a single base change, from G to A, at the 848th position in the gene, near the end of exon 4. The mutation likely upsets the splicing out of introns.
The wild type protein product of the SPATA16 gene is transported from the Golgi apparatus into vesicles that take it to the acrosome as it telescopes out of the front end of a sperm cell. By attaching the gene for the jellyfish’s green fluorescent protein (see figure 19.1) to the wild type SPATA16 gene in cells growing in culture, researchers visualized the protein being transported to the forming acrosome in immature sperm.
Reading 21.1
The Case of the Round-Headed Sperm—and a Review of This Book
Figure 1 gu gure re 1 1 1 A misshapen sperm cannot fertilize an oocyte.A misshapen sperm cannot fertilize an oocyte.
Figure 2 In a family with autosomal recessive globozoospermia, three of six sons are infertile.
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encounter an oocyte. Vaginal secretions may be so acidic or alkaline that they weaken or kill sperm. Douching daily with an acidic solution such as acetic acid (vinegar) or an alkaline solution, such as bicarbonate, can alter the pH of the vagina so that in some cases it is more receptive to sperm cells. Too little mucus can prevent conception too; this is treated with low daily doses of oral estrogen. Sometimes mucus in a woman’s body harbors antibodies that attack sperm. Infertility may also result if the oocyte fails to release sperm-attracting biochemicals.
One reason the incidence of female infertility increases with age is that older women are more likely to produce oocytes with an abnormal chromosome number, which often causes spontaneous abortion because defects are too severe for devel- opment to proceed for long. The cause is usually misaligned spindle fibers when meiosis resumes, causing aneuploidy (extra
or missing chromosomes). Perhaps the longer exposure of older oocytes to harmful chemicals, viruses, and radiation contrib- utes to the risk of meiotic errors. Losing very early embryos may appear to be infertility because the bleeding accompany- ing the aborted embryo resembles a heavy menstrual flow.
Infertility Tests
A number of medical tests can identify causes of infertility.
The man is checked first, because it is easier, less costly, and less painful to obtain sperm than oocytes.
Sperm are checked for number (sperm count), motility, and morphology (shape). An ejaculate containing up to 40 per- cent unusual forms is still considered normal, but many more than this can impair fertility. A urologist performs sperm tests.