Science of Pregnancy Week Two: Fertility

Your egg, preparing for debut

Girls are born with ~1-2 million eggs in their ovaries. In adulthood, a fraction of these eggs ripen. In a typical menstrual cycle, one egg is launched each month on a great lazy river ride. It’s released from the ovary, swept up in the tendrils of the fallopian tube, and ushered by a gentle tide toward the uterus.

When you had your period last week, your body flushed out an egg that was not fertilized during that adventure. But your body is already at work preparing the next contestant.

If you are trying to conceive, your most fertile period is in this window: the week leading up to, and including, ovulation.  Check an ovulation calendar to identify when you are most likely to become pregnant.

During this time, several hormones are working like greek goddesses in your system, bestowing vital gifts to prepare your egg for its journey: protective shield, snack bags, a safe landing zone— and plenty of sperm-grabber, just in case.

Meiosis Part Two: Two Halves

Plato looked at love as two half-beings coming together as one. Genetically, we ARE two half-beings fused together as one. Isn’t that rad?

Having sex to become pregnant: Why it is necessary?

The gorgeous plumes of a peacock are a classic example of a trait that emerged solely for sex appeal.

“Some are fancy on the outside.  Some are fancy on the inside.  Everybody’s fancy.  Everybody’s fine.  Your body’s fancy, and so is mine.”  –Fred Rogers (♫ listen here)

“Some mollusks (not many) can have children merely by sitting around and thinking about it.”   — E.B. White, Is Sex Really Necessary?

As it turns out, snail sex (a snail is a type of mollusk) is really far out:  Some can self-fertilize, some stab their mates during foreplay with harpoon-like “love darts,” and most snails have two sex organs, so they can do it both ways at the same time.  As E. B. White observed, “mollusks are infinitely varied in their loves, their hates and their predilections.” 

Nurture begins in the womb

Epigenetics is the study of the chemical reactions that govern which genes get turned on or off. Wikipedia image credited to the National Institutes of Health (NIH).

Am I hungry? Have I just gotten sloshed? Am I in outer space?

All of these factors affect how I am feeling, and less obviously, how my genes are functioning.

If I am a pregnant lady, factors like these become critical because they impact the activation and silencing of genes that coordinate the delicate orchestration of my baby’s development.

Remember, genes are the same in all of our cells, but our cells and body parts look and behave differently because certain genes within them are switched on or off. And in order for the cells of a developing embryo to emerge is a person, genes need to be switched on and off at just the right moment.

What’s controlling these switches? It’s not the genes themselves. Epigenetic signals –(click this for great videos and articles on epigenetics from the University of Utah’s Genetic Science Learning Center) –are the conductors that cue genes in and out at just the right time. They change in function of what we eat, smoke, breathe and drink.

The Science of Pregnancy Timeline: Week 4– How does the embryo ‘know’ what it should be shaped like?

Adapted from Gray's Anatomy, this surface illustration of a rabbit embryo looks just like a human's around day 13, with the central wedge forming the primitive streak (pr).

“It is not birth, marriage or death, but gastrulation, which is truly the most important time in your life.” –Biologist Lewis Wolpert

Like teens backpacking for a year before college, the immature cells of the embryo go a-travelling before they commit to becoming specific cell types like eye cells and liver cells. As it turns out, where they end up plays a huge role in what they’ll become.

In the first weeks of pregnancy, as the embryo grows from the size of a poppy seed to that of a lemon seed, its duplicating cells are moving and folding in patterns.

Image shared via Flickr by user Mammaoca2008.

If you’ve ever made a paper crane you know that you start by making creases in the paper that establish a center point, making subsequent folds perfectly symmetrical. So it is with the bodies of most all of earth’s creatures, which have two symmetrical sides. The origami analogy applies well to the early development of the embryo: The body becomes organized as layers of cells migrate, fold, and turn inward.

Around 13 days after conception, the cells of the pear-shaped embryo begin to gather at the midline, establishing the axis of the embryo called the “primitive streak.”

This development marks the beginning of “gastrulation,” (scientists again with the unsexy wordsmithing), — the establishment of the basic, 3-layered body plan.

Two weeks after fertilization, gastrulation is in full swing. (Note: most pregnancy timelines call this “week four,” since they measure from the date of the mother’s last menstrual period.)

The embryo divides into three layers:
• The outer layer will become the skin, hair, brain, nerves and spinal cord.
• The middle layer will form muscles and bones, heart and lungs, kidneys, blood vessels, testicles or ovaries.
• The inner layer will become the stuff betwixt your piehole and your corn hole, like the tongue, tonsils, and digestive system.
(For those of you taking the quiz, these are the ectoderm, mesoderm, and endoderm.)

The migrations of cells in the early embryo are critical, because cells that “know” where they are begin to behave differently, and it’s this behavior that drives development. Two ways a cell can “know” where it is: Certain signals or proteins may be more or less ample according to their location along up-and-down or front-and-back axes, making it possible for cells to “read” their location in the scheme of the body the way we read latitude and longitude lines on a map. Or, a cell “knows” its location by “communicating” with its nearest neighbors, by sending or receiving chemical signals.  Larry the skin cell working in a subcutaneous cubicle knows he is third in line for the “corner office” on the surface of the elbow, and he is waiting to move up as his predecessors retire.  And Larry himself will change as he rises in the ranks.

Cells behave differently according to where they are located:  They change shape, they attract or repel one another, they influence cells around them. Some cells self-destruct, as they do to create the space between forming fingers. A cell appropriately placed in the mesoderm, becoming part of the team that will build the heart, will begin to activate the protein-making genes that will make it a muscle cell.

Three weeks after conception (in pregnancy week 5), the embryo begins to have curves and bumps, thanks to differentiation, and is working hard to create organs. It now has muscle cells and blood cells, neurons and glial cells. The baby will be born not just with 10 fingers and 10 toes, but with some 250 cell types. These cell types, the same 250 or so that we have as adults, have identical genes. But they behave differently, and acquire distinct traits because they are expressing different genes, and thus creating different proteins, according to their placement in the body and their relationships with nearest neighbors.

Pregnancy timeline: 4 to 8 days after conception

Four days after fertilization, a wad of 16 to 32 identical cells (the berry-like “morula”) approaches the uterine cavity.

A week after fertilization, the blastocyst lands, thanks to cellular communication. Image by Tevah Platt.

But in the following day or two, it starts to look more like a dented soccer ball, with an outer ring of cells surrounding an inner cluster. Scientists have an unsexy name for this cell-wad: The blastocyst.

The geographic parsing of inner and outer cells in the blastocyst is exciting because it is the first step toward differentiation (the amazing sorting of cells into different cell types: for example, skin cells, blood cells, eye cells, etc.). The inner cells will later form the embryo, and the other cells will form the placenta.

These stem cells are the ones that are valued by researchers because they haven’t reached a point on the path of development at which they acquire super-specialized characteristics and become committed cell types like eye cells or liver cells. They are no longer “totipotent” (able to become any kind of cell); they are now “pluripotent” (able to become one of many kinds of cell in the future).

At this stage, communication across cells has ramped up with the creation of ion channels, gap junctions and protein channels– all fancy words for doors and windows through which cells can essentially chat with their neighbors.

Molecules that go in and out of these windows are the messages that circulate among the cells, allowing parts of the body to work together.

Seven or eight days after fertilization, the cells of the blastocyst also begin to coordinate with the cells in mom’s body.

For example, the blastocyst receives chemical signals from the lining of the uterus that guide them to a safe spot for landing. In turn, they secrete enzymes that clear the ground for implantation. Cellular cooperation will be critical over the next 9 months, not only for the exchange of nutrients and oxygen between mother and embryo, but for the orchestration of the embryo’s development.

We don’t often think about it, but we know from our everyday actions that our cells work together all the time. Our hands and mouths cooperate every time we eat a hunk of cheese. When Lady Gaga put on a suit made of meat, her eye cells and arm cells had to communicate with the brain cells that made that decision. Ask any person who is quadriplegic and has had his nerve cells cut off and you realize that relationships within the body run everything.

Once we have cells that can communicate, we have cells that know where they are, that can be called upon to make stuff, and that can become certain types of cells.

Safely lodged in the uterus, the ever-dividing cells can turn to the work of organizing themselves, laying out the basic plan of the body, and slowly and gently, beginning the process of differentiation that will give rise to bones and ears and knuckles.

Pregnancy timeline: One to three days after conception

During the first days of pregnancy, cells divide to create duplicates of the original, fertilized egg. As genes become activated, the cells begin to communicate by sending and receiving chemical signals.

Compared to cells that will later allow baby to babble and barf, the first, original cell doesn’t have much to do. Before fertilization, the egg doesn’t need a lot of protein channels, or windows and doors through which it could “chat” with its neighbors. It has only to look for one thing in its environment: The wiggly-tailed sperm. It is set up with the chemical matrix to sense sperm, to help that first wiggler to traverse into its nucleus, and to create a barrier to deflect also-rans in the sperm race.

About a day after conception, the egg divides for the first time, making an exact copy of itself.

Only now that the egg is fertilized and is making its way down the fallopian tube will it develop the chemical recognizers it will need at the end of the week to lodge into the warm wall of the uterus. Creating them any sooner would be a waste of molecular time and energy, like putting wheels on a car that might never be driven.

Making their way womb-ward, the cells that began with the egg divide every 15 hours or so. Two days after fertilization, 2 cells become 4. On day 3, 4 cells become 8.

At about the 8-cell stage, the machinery within each cell starts to click into gear and turn on. The cells have not differentiated into distinct types like eye cells or liver cells; they are at this stage still “totipotent,” or capable of becoming any type of body cell.

But now, in addition to replicating their chromosomes in order to duplicate, they begin to activate genes to create little protein machines or structural scaffolding for the cells. And they begin to produce chemicals that are communicators and recognizers– transmitters and receivers of molecular messages.

Communication between little cells will drive the development of the embryo as a whole.

We could learn a lot from an embryo.

Photo by Yorgos Nikas, Wellcome Images, images@wellcome.ac.uk, shared via Flickr.

The lesson of the embryo: The steps we take toward becoming something magnificent may, along the way, look decidedly un-magnificent. In the moment, we look nothing like what we are becoming.

Growing in the womb we look primitive, unusual, and grotesque. We look like turds, bean sprouts, aliens, fish.

We may look like (or be) a clumsy teenager before we emerge as artists and athletes.

It is the patience of watching improbable pieces come together that gives us the ability to build planes or go to the moon, to sculpt masterpieces or to make dinner.

The big. ass. egg.

Womanhood is powerful. If ever you seek a concrete image to illustrate this, look at a human egg flanked by sperm.

The first thing you will notice is that the egg is huge. Sperm cells swarm like tiny rockets around a super planet.

The biological big bang

Baking soda and vinegar: Lots of fun.  Peanut butter and chocolate: Extremely delicious.  Egg and sperm: The most amazing pair in the universe!

They put on an awesome show.

How does 1 cell divide and grow to become a human being that is trillions of cells big?

Mitosis is the spark of life that starts from conception, or the merging of egg and sperm.  It is the most fundamental way in which we grow in our bodies, and the materials of mitosis come from, initially, the mother.

Weekly reading: The Tentative Pregnancy

Barbara Katz Rothman’s The Tentative Pregnancy: How Amniocentesis Changes the Experience of Motherhood is about women’s experiences with prenatal diagnosis.

Published in 1986 (with a new introduction and appendices in the 1993 edition), this work was an early exploration of the ways in which the technology of amniocentesis alters women’s experience of pregnancy.  “The possibility of a bad diagnosis,” Rothman writes, “casts its shadow over the early months and the flow of time in pregnancy itself is changed with mid-pregnancy diagnosis.  Most important, the mother’s developing relationship with her fetus is affected by the new technology of reproduction.”

The author bases the work on interviews with mothers, recipients of genetic counseling and genetic counselors.

Topics include the scope and ambiguity of prenatal diagnoses; the risks and benefits of amniocentesis, and its emotional consequences.  The author includes a practical appendix, “Guidelines for Personal Decision-Making,” directed to women who are currently pregnant.