So what exactly are stem cells?

As much as stem cells have been in the news, I don't think I've ever seen them defined in those reports. So, before we get into the details of how induced pluripotent stem cells are made, it's best to go over the basics.

Our bodies are made up of cells. Each of these cells, with some exceptions (red blood cells, B- and T-cells, and the gametes) contain the entirety of an organism's genome. If every cell carries the same genes, then why is a heart cell not a retina cell or a bladder cell or a skin cell? The different cell types arise during a process called differentiation. During differentiation, some genes are turned on while others are turned off. The expression pattern of the genes changes at every step of differentiation and the actions of these genes determine the fate of the cell.

Stem cells are defined by two main qualities: self-renewal and potency. Self-renewal means that they can divide many times to make more stem cells while staying undifferentiated. Potency means that, with the right signals, they can differentiate to become many different types of cells. Both of these features are of great interest to biomedical researchers. Self-renewal means they can keep stem cells in culture for a long time and continue making more as they need them. Potency means they can turn the cells into the type of specific tissue or organ they're interested in, either to study how it works or what causes a disease, or to transplant into a person to replace diseased cells.

There are different levels of potency. The most potent cell is the zygote, the first cell that's formed when an egg is fertilized by a sperm and a new organism comes into being (and the morula, which is formed by the first few cell divisions). The zygote/morula is totipotent because it can become every type of cell necessary for the organism to develop into an adult member of the species. In humans, this includes not only the 200 cell types that make up the body, but also the extra-embryonic structures (placenta, part of the amniotic sac, etc.).

A blastocyst getting ready to implant.

The morula quickly develops into a blastocyst, which consists of a hollow sphere (trophoblast, purple) with a mass of cells on the inside (the creatively named inner cell mass, green). The outer sphere becomes the placenta while the inner cell mass will go on to form the tissues of the body. At this point, the inner cell mass cells, also known as embryonic stem cells, are pluripotent, meaning the can become any type of cell in the body, but no longer become the extra-embryonic cell types.

Hematopoiesis (click here for bigger version)

The inner cell mass eventually becomes all 200 types of human cells, first by differentiating into the three germ layers, and then into more and more specific lineages. There are stem cells within these lineages that are called "adult stem cells." The word "adult" does not imply that they are only found in adult organisms, but it refers to the maturity of the cell. Adult stem cells are multipotent, meaning the can only become a subset of cell types. One example is the blood stem cell (hematopoietic stem cell), which goes through multiple steps to become different blood components: red blood cells, white blood cells, and platelets.

Now, for a very important note. I mention that adult stem cells are not only found in adult organisms because not all sources of adult stem cells are ethical. In particular, I am always wary at the mention of neuronal stem cells. As far as I am aware, the only source of human neuronal stem cells is from an aborted baby. It is possible that they might be harvested from the body of a spontaneously miscarried baby, but I have never seen confirmation of this being done.

Recently, I heard a favorite podcaster praising work using neuronal stem cells. With a sick feeling in my stomach, I looked up the article he referenced, which turned out to be from a pro-life site. They'd gotten their information from The Telegraph, which didn't mention the source of the cells, but a quick google search brought me to the same report on CNN.com, which did:

The company was looking for 12 recently paralyzed patients for a study of its product: purified human neural stems cells derived from donated fetal brain tissue.

Notice they said "from donated fetal brain tissue." Wanting to know the circumstances under which this tissue was donated, I went to the website of the company behind this stem cell line, StemCells, Inc., but the information was not posted and no amount of internet searching turned up an answer.

As I said, it's possible that the fetal tissue was from a spontaneous miscarriage...but it's unlikely. It's much easier to schedule in the harvesting of tissues during an abortion...and the tissues are much fresher.

This is why Catholics and other pro-lifers must be Pro-life 3.0. We need to be able to know the warning signs and dig deeper behind the reports. Otherwise, we risk getting duped into supporting unethical research and medical treatments.

So, back to the science. The final products of all this differentiation are the mature cells of the organs and tissues of the body, the terminally differentiated cells. And terminal they are, because they live and die as the same type of cell. It was believed there was no going back...or is there? In my next post, I'll talk about what induced pluripotent stem cells are and how they're made.