Today’s guest post (originally posted here) is from Katie Hinde, an Assistant Professor in Human Evolutionary Biology at Harvard University. Katie studies how variation in mother’s milk influences infant development in rhesus monkeys. You can learn more about Katie and mammalian lactation by visiting her blog, Mammals Suck… Milk!. Follow Katie on Twitter @Mammals_Suck.
Milk is everywhere. From the dairy aisle at the grocery store to the explosive cover of the Mother’s Day issue of Time magazine, the ubiquity of milk makes it easy to take for granted. But surprisingly, milk synthesis is evolutionarily older than mammals. Milk is even older than dinosaurs. Moreover, milk contains constituents that infants don’t digest, namely oligosaccharides, which are the preferred diet of the neonate’s intestinal bacteria (nom nom nom!) And milk doesn’t just feed the infant, and the infant’s microbiome; the symbiotic bacteria are IN mother’s milk.
Evolutionary Origins of Lactation
The fossil record, unfortunately, leaves little direct evidence of the soft-tissue structures that first secreted milk. Despite this, paleontologists can scrutinize morphological features of fossils, such as the presence or absence of milk teeth (diphyodonty), to infer clues about the emergence of “milk.” Genome-wide surveys of the expression and function of mammary genes across divergent taxa, and experimental evo-devo manipulations of particular genes also yield critical insights. As scientists begin to integrate information from complementary approaches, a clearer understanding of the evolution of lactation emerges.
In his recent paper, leading lactation theorist Dr. Olav Oftedal discusses the ancient origins of milk secretion (2012). He contends the first milk secretions originated ~310 million years ago (MYA) in synapsids, a lineage ancestral to mammals and contemporaries with sauropsids, the ancestors of reptiles, birds, and dinosaurs. Synapsids and sauropsids produced eggs with multiple membrane layers, known as amniote eggs. Such eggs could be laid on land. However, synapsid eggs had permeable, parchment-like shells and were vulnerable to water loss. Burying these eggs in damp soil or sand near water resources- like sea turtles do- wasn’t an option, posits Oftedal. The buried temperatures would have likely been too cold for the higher metabolism of synapsids. But incubating eggs in a nest would have evaporated water from the egg. The synapsid egg was proverbially between a rock and a hard place: too warm to bury, too permeable to incubate.
Ophiacodon by Dmitri Bogdanov
Luckily for us, a mutation gave rise to secretions from glandular skin on the belly of the synapsid parent. This mechanism replenished water lost during incubation, allowing synapsids to lay eggs in a variety of terrestrial environments. As other mutations randomly arose and were favored by selection, milk composition became increasingly complex, incorporating nutritive, protective, and hormonal factors (Oftedal 2012). Some of these milk constituents are shunted into milk from maternal blood, some- although also present in the maternal blood stream- are regulated locally in the mammary gland, and some very special constituents are unique to milk. Lactose and oligosaccharides (a sugar with lactose at the reducing end) are two constituents unique to mammalian milk, but are interestingly divergent among mammals living today.
Illustration by Carl Buell
Mammalian and Primate Divergences: Milk Composition
Among all mammals studied to date, lactose and oligosaccharides are the primary sugars in milk. Lactose is synthesized in mammary glands only. Urashima and colleagues explain that lactose synthesis is contingent on the mammalian-specific protein alpha-lactalbumin (2012). Alpha-lactalbumin is very similar in amino-acid structure to C-type lysozyme, a more ancient protein found throughout vertebrates and insects. C-type lysozyme acts as an anti-bacterial agent. Oligosaccharides are predominant in the milks of marsupials and egg-laying monotremes (i.e. the platypus), but lactose is the most prevalent sugar in the milk of most placental (aka eutherian) mammals. Interestingly, the oligosaccharides in the milk of placental mammals are most similar to the oligosaccharides in the milk of monotremes. Unique oligosaccharides in marsupial milk emerged after the divergence of placental mammals.
Marsupial and monotreme young seemingly digest oligosaccharides. Among placental mammals, however, young do not have the requisite enzymes in their stomach and small intestine to utilize oligosaccharides themselves. Why do eutherian mothers synthesize oligosaccharides in milk, if infants don’t digest them?
In May, Anna Petherick’s post “Multi-tasking Milk Oligosaccharides” revealed that oligosaccharides serve a number of critical roles for supporting the healthy colonization and maintenance of the infant’s intestinal microbiome. Beneficial bacterial symbionts contribute to the digestion of nutrients from our food. Just as importantly, they are an essential component of the immune system, defending their host against many ingested pathogens. The structures of milk oligosaccharides have been described for a number of primates, including humans, and data are now available from all major primate clades; strepsirrhines (i.e. lemurs), New World monkey (i.e. capuchin), Old World monkey (i.e. rhesus), and apes (i.e. chimpanzee).
Among all non-human primates studied to date, Type II oligosaccharides are most prevalent (Type II oligosaccharides contain lacto-N-biose I). Type I oligosaccharides (containing N-acetyllactosamine) are absent, or in much lower concentrations than Type II(Taufik et al. 2012).
In human milk, there is a much greater diversity and higher abundance of milk oligosaccharides than found in the milk of other primates. Most primate taxa have between 5-30 milk oligosaccharides; humans have ~200. Even more astonishingly, humans predominantly produce Type I oligosaccharides, the preferred food of the most prevalent bacterium in the healthy human infant gut- Bifidobacteria (Urashima et al 2012, Taufik et al. 2012).
Human infants have bigger brains and an earlier age at weaning than do our closest ape relatives. Many anthropologists have hypothesized that constituents in mother’s milk, such as higher fat concentrations or unique fatty acids, underlie these differences in human development. But only oligosaccharides, a constituent that the human infant does not itself utilize, are demonstrably derived from our primate relatives (Hinde and Milligan 2011). At some point in human evolution there must have been strong selective pressure to optimize the symbiotic relationship between the infant microbiome and the milk mothers synthesize to support it. The human and Bifidobacteria genomes show signatures of co-evolution, but the selective pressures and their timing remain to be understood.
Vertical Transmission of Bacteria via Milk
In the womb, the infant is largely protected from maternal bacteria due to the placental barrier. But upon birth, the infant is confronted by a teeming microbial milieu that is both a challenge and an opportunity. The first inoculation of commensal bacteria occurs during delivery as the infant passes through the birth canal and is exposed to a broad array of maternal microbes. Infants born via C-section are instead, and unfortunately, colonized by the microbes “running around” the hospital. But exposure to the mother’s microbiome continues long after birth. Evidence for vertical transmission of maternal bacteria via milk has been shown in rodents, monkeys(Jin et al. 2011), humans(Martin et al. 2012), and… insects.
A number of insects have evolved the ability to rely on nutritionally incomplete food sources. They are able to do so because bacteria that live inside their cells provide what the food does not. These bacteria are known as endosymbionts and the specialized cells the host provides for them to live in are called bacteriocytes. For example, the tsetse fly has a bacterium, Wigglesworthia glossinidia,* that provides B vitamins not available from blood meals. Um, if you are squeamish, don’t read the previous sentence.
*I submit the tsetse fly and its bacterial symbiont (Wigglesworthia glossinidia) for consideration as the number one mutualism in which the common name of the host and the Latin name of the bacteria are awesome to say out loud! Bring on your challenger teams.
Hosokawa and colleagues recently revealed the Russian nesting dolls that are bats (Miniopterus fuliginosus), bat flies (Nycteribiidae), and endosymbiotic bacteria (proposed name Aschnera chenzii)(2012). Bat flies are the obligate ectoparasites of bats (Peterson et al. 2007). They feed on the blood of their bat hosts, and for nearly their entire lifespan, bat flies live in the fur of their bat hosts. Females briefly leave their host to deposit pupae on stationary surfaces within the bat roost.
Bat flies are even more crazy amazing because they have a uterus and provide MILK internally through the uterus to larva! Male and female bat flies have endosymbiotic bacteria living in bacteriocytes along the sides of their abdominal segments (revealed by 16S rRNA). Additionally, females host bacteria inside the milk gland tubules, “indicating the presence of endosymbiont cells in milk gland secretion”.
The authors are not yet certain of the specific nutritional role that these bacterial endosymbionts play in the bat fly host. The bacteria may provide B vitamins, as other bacterial symbionts of blood-consuming insects are known to do. My main question is what is the exact role of the bacteria in the milk gland tubules? Are they there to add nutritional value to the milk for the larva, to stowaway in milk for vertical transmission to larva, or both?
The studies described above represent new frontiers in lactation research. The capacity to secrete “milk” has been evolving since before the age of dinosaurs, but we still know relatively little about the diversity of milks produced by mammals today. Even less understood are the consequences and functions of various milk constituents in the developing neonate. Despite the many unknowns, it is increasingly evident that mother’s milk cultivates the infant’s gut bacterial communities in fascinating ways. A microbiome milk-ultivation, if you will, that has far reaching implications for human development, nutrition, and health. Integrating an evolutionary perspective into these newly discovered complexities of milk dynamics allows us to reimagine the world of “dairy” science.
Hosokawa et al. 2012. Reductive genome evolution, host-symbiont co-speciation, and uterine transmission of endosymbiotic bacteria in bat flies. ISME Journal. 6: 577-587
Jin et al. 2011. Species diversity and abundance of lactic acid bacteria in the milk of rhesus monkeys (Macaca mulatta). J Med Primatol. 40: 52-58
Martin et al. 2012. Sharing of Bacterial Strains Between Breast Milk and Infant Feces. J Hum Lact. 28: 36-44
Oftedal 2012. The evolution of milk secretion and its ancient origins. Animal. 6: 355-368.
Peterson et al. 2007. The phylogeny and evolution of hostchoice in the Hippoboscoidea(Diptera) as reconstructed using fourmolecularmarkers. Mol Phylogenet Evol. 45 :111-22
Taufik et al. 2012. Structural characterization of neutral and acidic oligosaccharides in the milks of strepsirrhine primates: greater galago, aye-aye, Coquerel’s sifaka, and mongoose lemur. Glycoconj J. 29: 119-134.
Urashima, Fukuda, & Messer. 2012. Evolution of milk oligosaccharides and lactose: a hypothesis. Animal. 6: 369-374.
An historic view interpretation of the placenta (source).
She gave me a few minutes to meet my daughter before she reeled me back into a state that was my new reality. “You’re not finished Jeanne. You still need to birth your placenta.” What?!?! More pushing? But I was lucky and the efforts required to bring my placenta ex vivo were minimal.
This is the second placenta my body helped make. OK, so it doesn’t EXACTLY look like meatloaf…
The idea of a placenta, which is the only human organ to completely and temporarily develop after birth, was fascinating. That thing sitting in a rectangular periwinkle bucket was what allowed me to grow another human.. inside of my body! There was no way I was not going to check it out, as well as create a permanent record of its relatively short-lived existence.
My first impression was that it looked like “meatloaf.” Not necessarily a well made meatloaf, but perhaps one that is made by my mother (sorry mom). But, alas, chaos reigned and I wasn’t able to really take a good look. However, for my second birth and hence second placenta, my midwife indulged me with a more detailed look and a mini-lesson.
Baby’s eye view: Where geekling deux spent 39 weeks and 4 days.
Her gloved hands, still wet with my blood and amniotic fluid, slid into the opening that was artificially created with a tool resembling a crocheting needle. She opened the amniotic sac wide so I could get a baby’s eye view of the crimson organ that served as a nutritional trading post between me and my new bundle of joy.
She explained that the word “placenta” comes from from the Greek word plakoeis, which translates to “flat cake” (however, I’m sure if my mom’s meatloaf was more common in ancient Greece, the placenta would be named differently). “It’s one of the defining features of being a mammal,” she explained as I was working on another mammalian trait – getting my baby to nurse for the first time.
That was about all I could mentally digest at the time, but still, more than three years later, the placenta continues to fascinate me, mostly due to the fact that it is responsible for growing new life. It’s a natural topic for this long overdue Pregnancy101post, so let’s dive in!
Development of the placenta
It all starts when a fertilized egg implants itself into the wall of the uterus. But, in order to fully understand how it works, we should start with an overview of the newly formed embryo.
The very early stages of us (and many other things that are alive).
The trophoblast invades the uterus, leading to implantation of the blastocyst.
As soon as a male sperm cell fuses with a female egg cell, fertilization occurs and the cells begin to multiply. But, they remain contained within a tiny sphere. As the cells continue to divide, they are given precise instructions depending on their location within that sphere, and begin to transform into specific cell types. This process, which is called cellular differentiation, actually seals the fate every cell in our body, sort of like how we all have different jobs – some of us are transport things, some of us are involved in policing the neighborhoods, some of us build structures, some of us communicate information, some of us deal with food, some of us get rid of waste, etc. Every cell gets a job (it’s the only example of 100% employment rates!).
Now back to the cells in the fertilized egg. As they start to learn what their specific job will be, the cells within the sphere will start to organize themselves. After about 5 days after fertilization, the sphere of cells becomes something called a blastocyst, which readies itself for implantationinto the wall of the uterus.
The act of implantation is largely due to the cells found on the perimeter of the blastocyst sphere. These cells, collectively known as the trophoblast, release a very important hormone – human chorionic gonadotropin (hCG) – that tells the uterus to prepare for it’s new tenant. (If you recall, hCG is the hormone picked up by pregnancy tests.) Around day 7, the trophoblast cells start to invade the lining of the uterus, and begin to form the placenta. It is at this point that pregnancy officially begins. (Here is a cool video, created by the UNSW Embryology Department, showing the process of implantation.)
Structure of the placenta
Eventually the trophoblast becomes the recognizable organ that is the placenta. Consider the “flat cake” analogy, with the top of the cake being the fetal side (the side that is in contact with the baby), and the bottom of the cake being the maternal side (the side that is in contact with the mother).
Cross section of the placenta: Blood vessels originating from the fetus sit in a pool of maternal blood, which is constantly replenished my maternal arteries and veins. The red represents oxygenated blood, and the blue represents de-oxygenated blood.
Projecting from the center of the fetal side of the placenta are two arteries and one vein, coiled together in a long, rubbery rope, often bluish-grey in color. This umbilical cord serves as the tunnel through which nutrients and waste are shuttled, and essentially serves to plug the baby into the mother’s metabolic processes. At the umbilical cord-placenta nexus, the umbilical cord arteries and vein branch out into a network of blood vessels, which further divide into a tree-like mass of vessels within the placenta.
These tree-like masses originating from the umbilical cord (and thus fetus) sit in a cavity called the intervillous space, and are bathed in nutrient-rich maternal blood. This maternal blood, which provides the fetus with a means for both nutrient delivery and waste elimination, is continually replenished via a network of maternal arteries and veins that feed into the intervillous space. Furthermore, these arteries and veins help to anchor the placenta into the uterine wall. One of the most interesting aspects about the mother-feus relationship is that the blood vessel connection is indirect. This helps to prevent a detrimental immune response, which could lead to immunological rejection of the fetus (sort of like how a transplanted organ can become rejected by the recipient).
Functions of the placenta
Just like a plant needs sunlight, oxygen, and water to grow, a baby needs all sorts of nutrients to develop. And since a baby also produces waste, by nature of it being alive and all, there is an absolute requirement for waste removal. However, because we can’t just give a developing fetus food or a bottle, nor are we able to change diapers in utero, the onus lies completely on the biological mother.
This is where the placenta comes in. Because the fetus is plugged into the circulatory system of the mother via the umbilical cord and placenta, the fetus is provided with necessary nutrients and a mechanism to get rid of all the byproducts of metabolism. Essentially, the placenta acts as a waitress of sorts – providing the food, and cleaning it all up when the fetus is done eating.
But it’s not just about nutrition and waste. The placenta also serves as a hormone factory, making and secreting biological chemicals to help sustain the pregnancy. I mentioned above that the placenta produces hCG, which pretty much serves as a master regulator for pregnancy in that it helps control the production of maternally produced hormones, estrogen and progesterone. It also helps to suppress the mother’s immunological response to the placenta (along with other factors), which cloaks the growing baby, thereby hiding it from being viewed as a “foreign” invader (like a virus or bacteria).
Another hormone produced by the placenta is human placental lactogen (hPL), which tells the mother to increase her mammary tissue. This helps mom prepare for nursing her baby once it’s born, and is the primary reason why our boobs tend to get bigger when we are pregnant. (Yay for big boobies, but my question is, what the hell transforms our rear ends into giant double cheeseburgers, and what biological purpose does that serve?? But I digress…)
Despite the fact that the mother’s circulatory system remains separate from the baby’s circulatory system, there are a clear mixing of metabolic products (nutrients, waste, hormones, etc). In essence, if it is in mom’s blood stream, it will very likely pass into baby’s blood stream. This is the very reason that pregnant mothers are strongly advised to stay away from cigarettes, drugs, alcohol, and other toxic chemicals, all of which can easily pass through the placental barrier lying between mother and fetus. When moms do not heed this warning, the consequences can be devastating to the developing fetus, potentially leading to birth defects or even miscarriage.
There are also situations that could compromise the functions of the placenta – restriction of blood supply, loss of placental tissue, muted placental growth, just to name a few – reducing the chances of getting and/or staying pregnant. This placental insufficiency is generally accompanied by slow growth of the uterus, low rate of weight gain, and most importantly, reduced fetal growth.
And it’s not just the growth of the placenta that is important – where the placenta attaches to the uterus is also very important. When the placenta grows on top of the opening of the birth canal, the chances for a normal, vaginal birth are obliterated. This condition, known as placenta previa, is actually quite dangerous and can cuase severe bleeding in the third trimester. 0.5% of all women experience this, and it is one of the true medical conditions that absolutely requires a C-section.
Then, there is the issue of attachment. If the placenta doesn’t attach well to the uterus, it could end up peeling away from the uterine wall, which can cause vaginal bleeding, as well as deprive the baby from nutrient delivery and waste disposal. This abruption of the placenta is complicated by the use of drugs, smoking, blood clotting disorders, high blood pressure, or if the mother has diabetes or a history of placental abruption.
Conversely, there are times when the blood vessels originating from the placenta implant too deeply into the uterus, which can lead to a placenta accreta. If this occurs, the mother generally delivers via C-section, followed by a complete hysterectomy.
Cultural norms and the placenta
There are many instances where the placenta plays a huge role in the culture of a society. For instance, both the Maori people of New Zealand and the Navajopeople of Southwestern US will bury the placenta. There is also some folklore associated with the placenta, and several societies believe that it is alive, pehaps serving as a friend for the baby. But the tradition that seems to be making it’s way into the granola culture of the US is one that can be traced back to traditional Chinese practices: eating the placenta.
Placentophagy, or eating one’s own placenta, is very common among a variety of mammalian species. Biologically speaking, it is thought that animals that eat their own placenta do so to hide fresh births from predators, thereby increasing the chances of their babies’ survival. Others have suggested that eating the nutrient-rich placenta helps mothers to recover after giving birth.
However, these days, a growing number of new mothers are opting to ingest that which left their own body (likely) through their own vaginas. And they are doing so though a very expensive process involving dehydrating and encapsulating placental tissue.
Why would one go through this process? The claims are that placentophagy will help ward of post partum depression, increase the supply of milk in a lactating mother, and even slow down the ageing process. But, alas, these are some pretty bold claims that are substantiated only by anecdata, and not actual science (see this).
So, even though my placentas looked like meatloaf, there was no way I was eating them. If you are considering this, I’d approach the issue with great skepticism. There are many a people who will take advantage of maternal vulnerabilities in the name of cold hard cash. And, always remember, if the claims sound to good to be true, they probably are!
Thanks for tuning into this issue of Pregnancy101, and enjoy this hat, and a video!
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.
[Editor's note: We are pleased to be able to run this post by Dr. Kate Clancy that first appeared at Clancy's Scientific American blog, the wonderful Context and Variation. Clancy is an Assistant Professor of Anthropology at the University of Illinois. She studies the evolutionary medicine of women’s reproductive physiology, and blogs about her field, the evolution of human behavior and issues for women in science. You can follow her on Twitter--which we strongly recommend, particularly if you're interested in human behavior, evolutionary medicine, and ladybusiness--@KateClancy.]
Over the course of my training to become a biological anthropologist with a specialty in women’s reproductive ecology and life history theory, or ladybusiness expert, I have learned a lot about miscarriage. Only it wasn’t miscarriage, it was spontaneous abortion. Except that some didn’t like the term spontaneous abortion and used intrauterine mortality (Wood, 1994). Or fetal loss. Fetal loss is probably the most common.
There is also pregnancy loss (Holman and Wood, 2001). You can use that term, too. Oh, or aContinue reading →