How Natural Twins Are Formed
There are two types of twins: monozygotic (one egg) which produces identical twins and dizygotic (two egg) which produces fraternal twins. With fraternal twins, there are two separate fertilization processes, each with its own sperm and egg. With identical twins, there is only one fertilization process, but at some point, the inner cell mass (ICM) splits apart to form two completely separate embryos.

Identical twins happen in about 1 in 400 births. About 33% happen before day 5 after fertilization, 66% happen between days 5-9, and splits after day 9 are rare and increase the risk of conjoined twins.

Prior to day 5 - If the ICM splits prior to day 5, each embryo will develop its own trophoblast cells, and 2 separate chorion will form.

Between days 5-9 - At this stage, the trophoblast cells have already differentiated themselves from the ICM, so if the ICM splits at this point, there will only be 1 chorion that will encase the two embryos.

At day 9, another layer forms around the embryo called the amnion. This will eventually become the amniotic sac which surrounds the embryo with amniotic fluid. If the twinning division has occurred before day 9 - and in most cases it has - each embryo will form its own amnion. If, however, the ICM splits after day 9, there will only be 1 amnion. With only one chorion and one amnion, the chance of the embryos conjoining and possibly sharing organs or body parts increased dramatically.

Stem Cells As the Precursor To All Cells
Looking at twin development, we can see that even if the cell mass divides between days 5-9, each division will go on to form a fully developed human being. The ICM remains undifferentiated during this time and can form any part of the embryo.

It’s also safe to say that at this stage, the mass of stem cells bears no resemblance to what we think of when we think of a human being. It’s just a bunch of cells, not unlike a cluster of skin cells or organ cells. If you saw it at this stage, you wouldn’t be able to tell that it would go on to become a human, nor does it possess any type of consciousness or any of the other characteristics we use to describe what makes someone a “human being” or “person.”

In my third installment, I’ll talk more about when cells start to differentiate during the 2-3 weeks of development and the neural tube starts to form.

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

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.

The leading company providing cryopreservation has only 200 clients and has yet to use any frozen eggs. Worldwide, there have been somewhere between 150-200 live births from egg freezing, not enough to reliably determine a success rate. The American Society for Reproductive Medicine still calls the technique “investigational.”

On the other hand, the rate of live births from proven assistive reproduction technologies like in vitro fertilization is 30.2% for 35- to 37- year-olds and 20.2% for women 38 to 40. You’ll likely have better odds simply using your remaining fresh eggs. It’s not until you’re over 40 that IVF success rates drop to 11%.

It’s interesting to see just how far we’ve come since the first test tube baby, Louise Brown, was born on July 25, 1978, weighing at 5lb 12 oz. By 1999, 300,000 women around the world had conceived through IVF.

Geneticists have been looking for less intrusive ways to extract fetal cells, and now, a solution may be on the horizon. In 1998, Dennis Lo and his colleagues found that fetal DNA floats freely in the mother's bloodstream. The main problem is separating the mother's DNA from the fetus'. If the fetus is a boy, they look for a Y chromosome, but if the fetus is a girl, they need other criteria.

Recent research has found two ways to do this:

1. Lo and his colleagues found that fetal DNA can be detected with unmethylated Maspin, a tumor-supressing gene that's active in fetal cells of a placenta but not in the mother.
2. Sinuhe Hahn and coleagues found that the fetus' genetic materials which circulate in the mother's bloodstream are actually much shorter than the mother's. In a study of 32 pregnant women, when the group checked their results against chorinic villus sampling, they identified mutations with almost 100% accuracy.

Both tests simply require a sampling of the mother's blood.

Other geneticists are trying to find ways to extract whole fetal cells, rather than just the fetal DNA. Researcher Ester Guetta and colleagues have identified several types of fetal cells that circulate with placenta cells in the mother's blood stream. While extracting a lot of those cells is difficult to do, she's been able to take blood samples that hold 1-2 fetal cells per 20 ml of blood and multiply that number five fold in the lab over the course of 5-7 days. She's used the methodology to predict the fetus' gender with approx 93% accuracy.

Source: Science News (7/22/06) (subscription)

The researchers found that women whose cortisol levels significantly increased over their baseline during the first three weeks of pregnancy were 2.7 times more likely to suffer a pregnancy loss than women with no cortisol increase. Nine of 10 pregnancies to women with high cortisol levels ended in miscarriage, compared with only four of 12 to women with stable cortisol.

(Source: New Scientist)