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.

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2 Responses to “Pregnancy timeline: 4 to 8 days after conception”

  1. Ginny haberman August 7, 2011 at 7:13 am #

    What a gentle, understandable, but still accurate way you have of explaining it all. Also like the short chunks you publish in – enough to digest without feeling stuffed. Looking forward to the next 38 weeks or so.

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