Stem Cells Explained, Part 1: What Are They?

Stem cell biology has arguably been one of the most controversial topics in biology in the early 2000s. Because it's also the field in which I have done the majority of my professional work, it's a topic I really wanted to start with early on in this blog. I began what I thought would be one post on the basics of stem cells, but it has quickly ballooned into a series, which I will release over the course of the next couple of week. And I'm sure this will just be the start of my coverage on the field. For the purpose of this initial series, though, I thought I would try to explain what exactly a stem cell is, and, possibly more importantly, what stem cells mean in the context of the media articles and advertisements that mention them.

So, what exactly is a stem cell, anyway?

For the purpose of this introduction, I am going to define stem cells as "the cells that make all the other cells". There is a much more precise, functional definition of what makes a stem cell which is used by scientists, and which I will cover later this week. However, for the purpose of today's discussion, this is a good starting point.

To explain the different types of stem cells (yes, there is more than one kind), I find it's easiest to think about the reason they exist in the first place: to help build the human body. To explain, I am going to take a very brief detour through the process of human development. (Note: I realizing I am glossing over the details of an incredibly complex process, but I believe this will serve the purpose of helping to understand stem cells.)

Each human being starts as a single cell, formed by the fusion of an egg and a sperm: this is called the zygote. From this single cell comes the embryo, then the fetus, then the child, and finally the adult. This single cell has the potential to ultimately give rise to every other cell that the body may need to function. Not only that, it can also produce the cells which feed the embryo and help form the placenta. Because this single cell has the potential to create such a comprehensive number of cell types, it is referred to as totipotent (lit. "total capacity").

http://www.accessexcellence.org/RC/VL/GG/blastocyst.php

This original single cell divides, and divides, and divides again, etc. After a few days, it has formed a structure called the blastocyst. This structure has two defined sets of cells; the outside cells that feed/nurture the embryo (but which don't ultimately contribute to the body), called the trophoblast; and the inner cells which will ultimately develop into the fetus, which are called the inner cell mass. Cells in the inner cell mass still have the potential to become any type of cell in the body; however, they can no longer make the extra cell types needed to nurture the embryo. Because they have slightly less potential than the original starting cell, they are called pluripotent ("several capacities").The stem cells present in this inner cell mass are referred to as embryonic stem cells; these are the stem cells which cause so much ethical controversy, and I will discuss them in more detail in a future post.

Over the nine months of pregnancy, these pluripotent stem cells give rise to all of the other cell types in the body. This is a gradual process, and the cells become more and more specialized over time. The process is also complicated, controlled by precise signals between cells, and the physical location of the cells in the embryo. However, ultimately, the fetus will develop the different systems it needs to function: blood, bone, muscle, nerves, skin, etc. As these different systems develop and the cells become more specialized, they also gradually lose their ability to transform into any cell type: pluripotency has been lost. As cells mature and divide, they trade off their stem cells capabilities in return for a specialization.

However, just because different body parts have developed, doesn't mean that the body will never need to make more cells. Red blood cells, which carry oxygen in the blood, have a life span of around 4 months. Bone and cartilage cells will be needed to help the body grow, and to repair damage that occurs through life. Skin cells get shed and re-grow. Therefore, each tissue type holds on to a small group of cells that have the responsibility to make all the different cells which might be needed in future. These are called adult or multipotent stem cells ("many capacities"). Their stem cell function is restricted to their specific functional system: hematopoietic (blood) stem cells make blood cells, neural stem cells can make all the different cell types of the nervous system; and mammary stem cells can make the cells which make up breast tissue. Despite their more limited repertoire, they still maintain the defining characteristics of stem cells: they can give rise to all of the cell types in their specific tissue, including making more of themselves.