In the course of writing a paper on women and STEM, I came across articles in the Journal of Sex Research, as one does. [The “related papers” button on PubMed is one of the best ways ever to let a whole day get away from you.] Given that I have just moved to a new area and may dip toes into the dating pool, and I’m a scientist, of course I had to investigate the latest research on dating, sex, and loooooove.
Human ovum (egg). The zona pellucida is a thick clear girdle surrounded by the cells of the corona radiata (radiant crown). Via Wikimedia Commons.
It was September of 2006. Due to certain events taking place on a certain evening after a certain bottle (or two) of wine, my body was transformed into a human incubator. While I will not describe the events leading up to that very moment, I will dissect the way in which we propagate our species through a magnificent process called fertilization.
During the fertilization play, there are two stars: the sperm cell and the egg cell. The sperm cell hails from a male and is the end product of a series of developmental stages occurring in the testes. The egg cell (or ovum), which is produced by a female, is the largest cell in the human body and becomes a fertilizable entity as a result of the ovulatory process. But to truly understand what is happening at the moment of fertilization, it is important to know more about the cells from which all human life is derived.
Act I: Of sperm and eggs
A sperm cell is described as having a “head” section and a “tail” section. The head, which is shaped like a flattened oval, contains most of the cellular components, including DNA. The head also contains an important structure called an acrosome, which is basically a sac containing enzymes that will help the sperm fuse with an egg (more about the acrosome below). The role of the tail portion of sperm is to act as a propeller, allowing these cells to “swim.” At the top of the tail, near where it meets the head, are a ton of tiny structures called mitochondria. These kidney-shaped components are the powerhouses of all cells, and they generate the energy required for the sperm tail to move the sperm toward its target: the egg.
The egg is a spherical cell containing the usual components, including DNA and mitochondria. However, it differs from other human cells thanks to the presence of a protective shell called the zona pellucida. The egg cell also contains millions of tiny sacs, termed cortical granules, that serve a similar function to the acrosome in sperm cells (more on the granules below).
Act II: A sperm cell’s journey to the center of the universefemale reproductive system
Given the cyclical nature of the female menstrual cycle, the window for fertilization during each cycle is finite. However, the precise number of days per month a women is fertile remains unclear. On the low end, the window of opportunity lasts for an estimated two days, based on the survival time of the sperm and egg. On the high end, the World Health Organization estimates a fertility window of 10 days. Somewhere in the middle lies a study published in the New England Journal of Medicine, which suggests that six is the magic number of days.
Assuming the fertility window is open, getting pregnant depends on a sperm cell making it to where the egg is located. Achieving that goal is not an easy feat. To help overcome the odds, we have evolved a number of biological tactics. For instance, the volume of a typical male human ejaculate is about a half-teaspoon or more and is estimated to contain about 300 million sperm cells. To become fully active, sperm cells require modification. The acidic environment of the vagina helps with that modification, allowing sperm to gain what is called hyperactive motility, in which its whip-like tail motors it along toward the egg.
Once active, sperm cells begin their long journey through the female reproductive system. To help guide the way, the cells around the female egg emit a chemical substance that attracts sperm cells. The orientation toward these chemicals is called chemotaxis and helps the sperm cells swim in the right direction (after all, they don’t have eyes). Furthermore, sperm get a little extra boost by the contraction of the muscles lining the female reproductive tract, which aid in pushing the little guys along. But, despite all of these efforts, sperm cell death rates are quite high, and only about 200 sperm cells actually make it to the oviduct (also called the fallopian tube), where the egg awaits.
Act III: Egg marks the spot
With the target in sight, the sperm cells make a beeline for the egg. However, for successful fertilization, only a single sperm cell can fuse with the egg. If an egg fuses with more than one sperm, the outcome can be anything from a failure of fertilization to the development of an embryo and fetus, known as a partial hydatidiform mole, that has a complete extra set of chromosomes and will not survive. Luckily, the egg has ways to help ensure only one sperm fuses with it.
When it reaches the egg, the sperm cell attaches to the surface of the zona pellucida, a protective shell for the egg. For the sperm to fuse with the egg, it must first break through this shell. Enter the sperm cell’s acrosome, which acts as an enzymatic drill. This “drilling,” in combination with the propeller movement of the sperm’s tail, helps to create a hole so that the sperm cell can access the juicy bits of the egg.
This breach of the zona pellucida and fusion of the sperm and egg sets off a rapid cascade of events to block other sperm cells from penetrating the egg’s protective shell. The first response is a shift in the charge of the egg’s cell membrane from negative to positive. This change in charge creates a sort of electrical force field, repelling other sperm cells.
Though this response is lightning fast, it is a temporary measure. A more permanent solution involves the cortical granuleswithin the egg. These tiny sacs release their contents, causing the zona pellucida to harden like the setting of concrete. In effect, the egg–sperm fusion induces the egg to construct a virtually impenetrable wall. Left outside in the cold, the other, unsuccessful sperm cells die within 48 hours.
Now that the sperm–egg fusion has gone down, the egg start the maturation required for embryo-fetal development. The fertilized egg, now called a zygote, begins its journey into the womb and immediately begins round after round of cell division, over a few weeks resulting in a multicellular organism with a heart, lungs, brain, blood, bones, muscles, and hair. It’s an amazing phenomenon that I’m honored to have experienced (although I didn’t know I was until several weeks later).
The Afterword: A note on genetics
A normal human cell that is not a sperm or an egg will contain 23 pairs of chromosomes, for a total of 46 chromosomes. Any deviation from this number of chromosomes will lead to developmental misfires that in most cases results in a non-viable embryo. However, in some instances, a deviation from 46 chromosomes allows for fetal development and birth. The most well-known example is Trisomy 21(having three copies of the 21st chromosome per cell instead of two), also called Down’s Syndrome.
The egg and sperm cells are unlike any other cell in our body. They’re special enough to have a special name, gametes, and they each contain one set of chromosomes, or 23 chromosomes. Because they have half the typical number per cell, when the egg and sperm cell fuse, the resulting zygote contains the typical chromosome number of 46. Now you know how we get half of our genes from our father (who made the sperm cell) and half from our mother (who made the egg cell). Did I just put in your head an image of your parents having sex? It’s the birds and the bees, folks—it applies to everyone!
All text and art except as otherwise noted: Jeanne Garbarino, Double X Science Editor
World Health Organization. “A prospective multicentre trial of the ovulation method of natural family planning. III. Characteristics of the menstrual cycle and of the fertile phase,” Fertil Steril(1983);40:773-778
Allen J. Wilcox, et al. “Timing of Sexual Intercourse in Relation to Ovulation — Effects on the Probability of Conception, Survival of the Pregnancy, and Sex of the Baby,” New England Journal of Medicine, (1995); 333:1517-1521
Poland ML, Moghisse KS, Giblin PT, Ager JW,Olson JM. “Variation of semen measures within normal men,” Fertil Steril (1985);44:396-400
Alberts B, Johnson A, Lewis J, et al. “Fertilization,” Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002.
That turkey on your table probably was genetically distinct from this wild turkey. Via Wikimedia Commons.
by Emily Willingham, DXS managing editor
The turkey doesn’t get a lot of respect. We mock its gobble, eat it for major holidays, and use its name as an epithet. Yet, Benjamin Franklin at least privately thought the bird was noble enough to be nominated as the national bird—clearly, the bald eagle won out there—and the lowly gobbler has a long and illustrious secret history that researchers are just now beginning to uncover.
Domestication of animals and plants do not make for gripping headlines, but such achievements represent major turning points in the evolution of human culture. With domestication came villages, then cities. With domestication, people had time to turn to less mission-critical activities, such as art. And the process of domestication itself makes for a fascinating study in species interactions. Think what it must have required to take wild turkeys or donkeys or pigs and assiduously, over the years, produce the relatively docile creatures we see today.
The turkey we belittle so offhandedly was so highly prized among indigenous Americans that it may not even have been used as food at first. But it did eventually become an important protein source for indigenous peoples, in addition to providing feathers and bones for ornamentation and other uses. Strangely enough, the turkey we eat today—the one that gets pardoned at the White House every Thanksgiving and serves as an icon of a U.S. holiday—isn’t even descended from the native turkeys of the Americas.
How did that happen? It all may have begun about 800 B.C.E. when indigenous Americans might have first domesticated the animal. Some research suggests that the event happened twice, a pattern of repeats that already has emerged with Old World domestications of sheep and pigs. One turkey breed achieved importance in South Mexico, where the native peoples domesticated the wild South Mexican turkey.
But the turkey also proved to be an important feature of native culture in what is now the southwestern U.S. Evidence from bones and fossilized excrement—coprolites—from archaeological digs in the area show that the turkey’s importance goes back at least 2000 years there, as well.
In addition, analysis of the mitochondrial DNA—which passes only from mother to offspring and accumulates mutations at a slow, predictable rate—yielded two unexpected finds. First, these birds did not make their way by trade from South Mexico to the American Southwest, as did other domesticated products like maize and beans. Instead, the indigenous peoples of the Southwest did their own domesticating, taking as their starting bird the Rio Grande/Eastern wild turkey. A bottleneck in the genetic history, indicating a severe reduction in numbers and genetic diversity, shows that the native peoples of the Southwest strongly selected for a specific breed of their domesticated bird. And then they propagated it for at least 1000 years.
You might think that the turkeys we eat today in the United States would be descendents of these Southwestern birds. They’re not. When Europeans showed up in the Americas, they grabbed a few of the birds and took them back to Europe, where they bred new strains. Meanwhile, in the American Southwest, the domesticated turkey faded away, replaced by other food stocks like sheep and chickens and wiped out by disease.
Today’s turkeys that appear on Thanksgiving tables, new research indicates, show a considerable lack of diversity in their genome sequence compared to DNA from South Mexican turkeys that were collected in 1899, even less than other domesticated food animals like pigs. So where did they come from? Genetic analysis indicates that they originated in the wild turkeys of South Mexico, but having been bred anew in Europe, they’re really a form of inbred European export with the name of a country (Turkey) that has nothing to do with their origins. They are, according to the new research, genetically distinct from their wild ancestors.
Benjamin Franklin was a wise man, but he couldn’t have known that he was proposing a European breed to represent his newborn nation. That said, the turkey still stands as one of the few native domesticated animals of the Americas as most of our other familiar domesticates originated somewhere else.
[Some of this post has appeared previously at The Biology Files and in Biology Digest.]
The opinions expressed here do not necessarily either conflict with or reflect those of the DXS editorial team or contributors.
“You wanna do WHAT?!” Photo courtesy of Justyna Furmanczyk at sxc.hu.
By Tara Haelle, DXS contributor
[Tara Haelle (www.tarahaelle.com) is a health and science writer and a photojournalist based in Peoria, IL after years as a Texan, where she earned her undergraduate degrees and MA in journalism at UT-Austin. She’s the mental health editor for dailyRx.com in addition to reporting on pediatrics, vaccines, sleep, parenting, prenatal care and obesity. This post first appeared on her blog, Red Wine & Apple Sauce focuses on health and science news for moms (www.redwineandapplesauce.com), and you can follow her on Twitter at @health_reporter and @tarasue. She’s also swum with 9 different species of sharks, climbed Kilimanjaro and backpacked in over 40 countries, but that was in the years of B.C. (Before Children). She finds that two-year-olds are tougher to tussle with than tiger sharks.]
I am grateful that their statement was issued with the sensitivity and caution needed for such a controversial practice and decision. Some of the headlines have been frustrating, implying that the AAP said “Circumcision is better.” Um, no. That’s not what they said. They said that the “preventive health benefits of elective circumcision of male newborns outweigh the risks of the procedure.” (To be fair, most headlines basically ran with “benefits trump risks” or some variation thereof.)
In other words, if you choose to do this procedure, the benefits you will gain are greater than the risks involved in the procedure. This is very different from saying “It’s better to be circumcised.” In fact, their policy explicitly points out that they do not officially “recommend” the procedure routinely: “Although health benefits are not great enough to recommend routine circumcision for all male newborns, the benefits of circumcision are sufficient to justify access to this procedure for families choosing it and to warrant third-party payment for circumcision of male newborns.” (That last part just means yes, insurance companies, you should pay for it.)
An analogy: A child with obstructive sleep apnea can have a tonsillectomy/adenoidectomy (called an adenotonsillectomy) to remove their tonsils and adenoids for treatment. The tonsils and adenoids (lumps of issue behind the nose) generally cause the blockage that interferes with a child’s breathing while asleep, so removing them can usually cure the sleep apnea (in 75 to 100 percent of the cases).
There are risks to adenotonsillectomy, namely infection and excessive bleeding. There are risks to sleep apnea, including obesity, heart disease, diabetes, depression and death. For a child with obstructive sleep apnea, the benefits generally outweigh the risks of the procedure. A parent can still elect not to give their child the surgery.
Is it better for the child with sleep apnea to have the surgery? Probably. But perhaps not. It depends on the situation and the child. Is it better for a child without obstructive sleep apnea to have the surgery? Of course not. Why take any risk when there’s no benefit?
Now consider the two primary benefits conferred by circumcision: lower risk of urinary tract infections during the first year and reduced risk of HIV and a several other sexually transmitted infections during heterosexual sex. The risks of circumcision are most commonly bleeding, infection or the wrong amount of tissue snipped off, and this happens in about 1 of every 500 newborn boys (0.2 percent). Other studies found the rates higher, up to 2 to 3 percent, but these complications were still just minor bleeding. They even offered a comparison of a similar surgery as the one I discussed above: complications involving severe bleeding from tonsillectomies occur about 1.9 percent of the time in kids age 4 and under.
For parents with wild imaginations about horror stories, fear not: “The majority of severe or even catastrophic injuries are so infrequent as to be reported as case reports (and were therefore excluded from this literature review). These rare complications include glans or penile amputation, transmission of herpes simplex after mouth-to-penis contact by a mohel (Jewish ritual circumcisers) after circumcision, methicillin-resistant Staphylococcus aureus infection, urethral cutaneous fistula, glans ischemia and death.” Basically, yea, there’s a bunch of really bad stuff that can happen, but it’s really, really, really, really rare. Probably rarer than being struck by lightning. Twice. But that happens too.
So, the risks are pretty low. How beneficial are the benefits? Here’s a condensed run-down from the AAP’s technical report:
Circumcision reduces the odds of contracting HIV during male-female sex by 40 to 60 percent… in Africa. When the CDC calculated that figure with the rate of contracting HIV by heterosexual sex in the U.S., they came up with a 15.7 percent reduction here. It’s something, but nowhere near as good as a condom. Plus, if your kids turns out to be gay, there’s not much evidence that circumcision helps him avoid contracting HIV. (And on the other side of the coin, circumcision can make it a little easier for women to contract HIV from a man, per one study cited in the AAP review.)
Circumcised men are about 30 to 40 percent less likely to get any type of human papillomavirus (HPV), including both the relatively harmless strains and the ones that can lead to cervical cancer or raise your risk of cancer of the mouth, throat, penis and anus. Now, the CDC has recommended that boys get the HPV vaccine, but the vaccines available do not cover all the strains. Gardasil takes care of four of them, including the two responsible for about 70 percent of cervical cancer (HPV-16 and HPV-18) and the two responsible for 90 percent of genital warts. Cervarix only takes care of HPV-16 and HPV-18. So, circumcision would offer some protection against getting the HPV strains that the vaccines don’t cover, most of which — but not all — are not linked to cancer or warts.
There’s some evidence that circumcision reduces risk of herpes (HSV-2) by about 28 to 34 percent, based on two studies in Africa.
Evidence for protection against syphilis is weak. There’s no evidence that circumcision decreases the risk of contracting gonorrhea or chlamydia.
There’s good evidence that uncircumcised boys get more urinary tract infections that circumcised boys, in part because bacteria can hang out in that moist area under the hood. The AAP estimates that 7 to 14 of every 1,000 uncircumcised boys will get a UTI before their first birthday, compared to 1 to 2 out of 1,000 circumcised boys. With such a low rate overall, in either population, the AAP notes that “the benefits of male circumcision are, therefore, likely to be greater in boys at higher risk of UTI, such as male infants with underlying anatomic defects such as reflux or recurrent UTIs.” (These are mostly the boys that get UTIs anyway.)
So, those are definitely some benefits to circumcision, especially if your little guy will have sex one day (which, presumably, you want him to do at some point in the far off, I-don’t-want-to-think-about-it future). It’s also fair to say that good sex education and condom use would make those benefits almost moot (not the UTIs, which are pretty low risk, and not all HPV strains, which sometimes infect even with condom use).
In any case, these two benefits, a lower risk for UTIs and some STIs, then become the risks of not being circumcised. The former is — usually — not very serious. There are some very serious urinary tract infections, and untreated ones can damage the kidneys. And they’re certainly not fun. They aren’t, however, usually life or death situations. HIV (somewhat still) is. Of course, boys are still at a pretty high risk for getting HIV if they sleep with someone who has it and don’t use a condom, circumcised or not. But every bit of protection helps, right?
Unless it requires lopping off part of a little boy’s penis. There. I said it. Because that’s what many parents are simply uneasy about, regardless of the health benefits, which are great or marginal, depending on your perspective. And that’s why the AAP stopped short of recommending circumcision as a routine procedure.
They did include in their review several studies related to sexual satisfaction and sensitivity, one of the complaints that “intactivists” bring up. The AAP summarizes it pretty nicely: “The literature review does not support the belief that male circumcision adversely affects penile sexual function or sensitivity, or sexual satisfaction, regardless of how these factors are defined.”
But it’s not possible to take into consideration, in scientific, mathematical terms, the primary complaint of those who oppose circumcision, which is that the man these little boys become may have wanted that little flap over the tip. And this is one of those gray areas that give parents pause. Once you cut that hood, you can’t put it back. How many circumcised men regret what their parents did? Well, probably not vast numbers, or circumcision rates would have plummeted.
So, this is where we end up. There are some decent benefits. There are very few and mostly minor risks to the procedure. And there’s big, giant, gray unknown area of “what if’s” and “could have been’s” for the boys who get snipped. It’s disingenuous to compare the practice to female circumcision, as some do, since neither its intent nor its effect is to influence sexual satisfaction. But whether it’s the right thing to do…? The AAP says it’s up to mom and dad. (Which, in many households, like mine, probably means mostly dad.)
“Parents ultimately should decide whether circumcision is in the best interests of their male child,” they wrote. “They will need to weigh medical information in the context of their own religious, ethical, and cultural beliefs and practices. The medical benefits alone may not outweigh these other considerations for individual families.”
What are those other considerations? Well, whether you want your little guy to have a foreskin. Or, whether you don’t know if he does or doesn’t want it and figure he should decide that in 18 years. Maybe daddy’s not circumcised and you both want him to look like daddy. (I know many people who circumcised for this reason alone.)
About the only certain thing that can be said about circumcision, based on the AAP’s policy statement and research and what we know about opposition to the practice, is that this controversy will be with us for years to come.
The opinions in this post do not necessarily reflect or disagree with the opinion of the DXS editorial team.