Tag Archives: differentiation

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

8 Aug

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

5 Aug

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

4 Aug

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.

Lady Gaga Revisited: How one cell becomes an entire person

3 Aug

DISCLAIMER: This latest entry has not yet been rubber stamped by the White Coats who check this blog for scientific accuracy. Corrections to lies below will come soon.  Reader comments, questions and corrections are welcome.

Months ago we began this blog by introducing a puzzle:

As if by wizardry, Lady Gaga arose from a single cell that was smaller than the period at the end of this sentence.  This ought to strike us as incredible, not only because Lada Gaga is composed of trillions of cells, but because her cells have various features and functions, so that her eye balls are distinct from her tuchus.  And yet the cells in her body, whether they make up her liver, legs or lashes, all contain the exact same DNA.

If somehow Lady Gaga had only type of cell—that is, she had grown to full size with cells that never “differentiated,” – we postulate that she would look something like the Fruit of the Loom grape man.

That is to say, she would be a blown up version of the blob she was when she was about 32-cells big, making her way toward her mother’s uterus by way of the fallopian tube.  The sac of cells at this stage is called the morula, latin for mulberry, because it looks like a cluster of seeds.

So why isn’t Lady Gaga a grape man?
Read the answer

Where did Lady Gaga come from?

29 Sep

Photo by Creative Commons user The Cranky Geek, http://www.flickr.com/photos/jolissa/

Lady Gaga comes from New York. She has grandparents in West Virginia. Her ancestors came from Italy.

But the story we want to tell (with apologies, Gaga fans, for the bait and switch) is how Lady Gaga and the rest of us were at one time just one single cell.

That is the big “aha” moment in genetics: The realization that our parents have each given us half of themselves, and it comes together in a kind of a miraculous moment when the small rocket of a sperm penetrates the atmosphere of the giant planet that is the egg.      

The genomes combine and they begin to work together to multiply and to become, well, an inexplicable grassroots system of communication and sharing that builds something so large and complex and highly functional that in the end it forms a human being with huge capability and imagination that can make glam videos and outrageous contributions to fashion over decades and decades.

Imagine that a little cell can dream itself into Lady Gaga.  How that happens and stays with us is, for us, the real story about where we come from. 

This is the cool story of being embodied. 

Whether or not you relate this to divinity there’s an awesomeness to the fact that we start as single cells and grow into something that’s 10 to 20 trillion cells big.  That includes some 220 cell types that differentiate into eye cells and liver cells and hair cells…  All of that from a single cell: That’s a pretty neat trick. And that is part of the epic story that we will be telling during the life of this blog.