Stem Cells - what are they and why do i care?

Stem cells! Stem cells!! Stem cells!!! It seems every time I open a newspaper or turn on the tv there is some new breakthrough regarding stem cells. But what exactly are these cells? And why does everyone get so excited about them?

In order to understand what a stem cell is we have to first consider what it isn't. Essentially, a stem cell is not a 'somatic' cell, meaning it is not a heart cell, muscle cell, liver cell, lung cell, brain cell or any other 'common' cell. These 'common' types of cells are fully mature and will not become any other kind of cell. They are 'terminally differentiated' -'differentiated' because they have developed from an immature cell and have mature cell characteristics; 'terminally' because they are at the terminus, or end, of their differentiation pathway and will not change into any other cell type. One might say that, "once a somatic cell, always a somatic cell". In short, somatic cells are the mature cells that make up the different tissues (heart, lung, stomach, brain etc) in an adult body. Although stem cells are also present in the adult body, stem cells are not somatic cells.

So now that we know what a stem cell isn't (i.e. a 'terminally differentiated' mature cell), what kind of cell is it? Well, a stem cell is an 'immature' cell that has the ability to generate different types of somatic cells. Generally speaking, a stem cell has the potential to make all the cells of a given tissue. For example, there are blood stem cells (called hematopoietic stem cells) that can generate all the cells in our blood, including red blood cells and white immune cells. Another type of stem cell exists in the brain, called a neural stem cell, and this stem cell is able to generate different brain cells, such as neurons astrocytes and oligodendrocytes. These stem cells are known as 'adult stem cells' and are to be distinguished from embryonic stem cells (which will have to be a topic for a later post). Suffice it to say that embryonic stem cells can generate ALL the cells in the whole body (because they are "pluripotent"), while adult stem cells are more restricted and can generate only the cell types within a specific tissue.


A cell's ability to generate different types of 'common' somatic cells (known as 'multipotency') is not the only defining feature of a stem cell. A stem cell is also able to self-renew. This means that when a stem cell divides it can create another stem cell. This is very important. What this means is that when a stem cell generates another cell type (say when it is repairing injured tissue) it will also generate another stem cell. As such, even while new 'common' cells are being made, the population of stem cells is continually replenished, ensuring stem cells are present for the entire life span of the organism.


So what is the big deal? Why do we need stem cells and why do we need them for our entire life? Well, the body keeps these cells in an immature state just in case there is some kind of damage or injury to one of our organs or tissues. Since stem cells can generate the different cell types in a given tissue the body is able to repair itself. Stem cells are like Superheros that are called into action when evil strikes the body. Also, another important (albeit less heroic) aspect of stem cells is that they ensure old, malfunctioning cells are replaced by new and fresh ones. This maintains the health of our tissues over time and ensures that our liver keeps working as a liver.


I am sure right now you are asking yourself, "than why the heck don't we live forever?". And I say, "good question!". This is because stem cells themselves will age. Although scientists have different ideas on this topic, many believe that stem cells have a lifespan and that they are only able to divide a specific number of times. Maybe you have heard that every heart beats roughly the same number of times and that an elephant and a mouse will have the same number of heart beats in a lifespan; the mouse's heart beats faster and thus uses up these beats much faster than an elephant. Since the elephant uses up its alloted heart beats slower it lives longer. Well, I believe this is analogous to stem cells. A cell can divide x number of times and how fast they divide (and thus how fast they get to the number x) will determine how long they are around. Thus, as we age our stem cells age and they loose their ability to repair our tissues and organs. Consequently, our body ages. Also, it should be mentioned that some kinds of injuries cannot be fixed. This is because not all organs have stem cells, or because something else interferes with the ability of the stem cells to assume their Superhero role and repair the tissue.

Finally, this brings me to the 'why do i care' bit. Well everyone is so excited by stem cells because scientists believe that they can be used to re-grow injured organs, or can be used to repair diseased tissues. For instance, skin stem cells can be used to grow new skin for burn victims. Or blood stem cells can be manipulated or transplanted in order to generate healthy blood cells for leukemia patients. Basically, if we are able to understand how the body controls stem cells than maybe we can control them too. And if we can control stem cells than we can use them to repair our bodies. Heck, maybe one day we really will live forever...