On July 18, President Bush vetoed the Stem Cell Research Enhancement Act, which passed the Senate with 63 votes to 37. It was the first time he’s used his veto power in his 6 years in office. Bush’s main reason, as press secretary, Tony Snow, articulated was that

The simple answer is he thinks murder’s wrong. The president is not going to get on the slippery slope of taking something living and making it dead for the purposes of scientific research.

Since then, Snow has retracted his remarks about stem cell research being murder, but that has just fueled the controversy surrounding the debate.

In my next few articles, I’m going to talk about the stages of embryonic development during those first 5 days when the fertilized egg divides to form a blastocyst, which is what scientists use in embryonic stem cell research.

The Growth Of Embryonic Stem Cells
Obviously, to produce an embryo, a sperm must fertilize an egg, and each donate 23 chromosomes. The fertilized egg is then called a zygote. Over the course of the next few days, the zygote divides into 2 cells. Those two cells each divide to create 4 cells, and so forth, at a rate of one division every 12-18 hours.

By the time they reach 16 cells, they form a cluster called a morula, which consists of a small group of internal cells surrounded by a group of external cells. The outer cells become trophoblast cells, which will eventually go on to form the chorion, a portion of the placenta once the mass attaches to the uterus. The inner cell mass (ICM) becomes embryonic stem cells and will eventually form the embryo and its various surrounding sacs (yolk, waste and water).

Here’s a picture of 2 developing embryos (identical twins) with the different parts labeled.

When people talk about embryonic stem cell research, they talk about the 4-5 day old blastocyst made up of a few hundred cells. At this point during normal embryonic development, the cell mass has not yet attached to the uterus.

Cell Differentiation
At the point where the cell mass consists of 2, 4, or 8 cells, you can take any one of those cells, put it into a Petri dish full of nutrients, and it will go on to form a blastocyst complete with its own trophoblast cells and inner stem cells. For instance, if you take 1 of the cells from the 8 cell mass, you won’t get 1/8 of an embryo. It will grow into a full embryo that can be implanted into a woman and over 9 months, will grow into a baby.

By days 4-5, the cells have already somewhat specialized into the trophoblast and ICM. If you take an ICM cell at this point, it won’t form a trophoblast cell but it can form any of the types of cells that make up an embryo. In other words, at this stage, the ICM isn’t yet specifically determined to become a particular kind of cell - it can develop into any type of cell. The type of cell it eventually becomes depends on its interactions with other cells.

Unfortunately, up until recently, scientists working with stem cells ended up killing the embryo to get to the cells in the ICM. This week, however, a group of scientists have published a way to harvest stem cells without killing the embryo.

In the next article, I’ll discuss the phenomena of identical twins.

Reference:
Bioethics and the New Embryology, ISBN: 0716773457. Picture taken from pg 17.