Understanding Stem Cells

What Are Cells?
In biology, cells are the basic structural and functional units that all living organisms are comprised of. All organisms are classified as being either unicellular or multicellular. Just like plants and animals, humans are multicellular - which means we are made up of more than one cell. A unicellular organism consists of only one cell - bacteria, for example, are unicellular life forms.

There are many different types of cells. Within the human body there over 200 distinct kinds of cells.

What Are Stem Cells?
Different cells can have different functions. Some cells are have a more general function than other cells. Certain cells are very specialised to perform a particular task /set of tasks. For example: red blood cells are specialized cells and their a primary function is to carry oxygen throughout the human body. Specialised cells can only perform the functions they are made for, which means a red blood cell can't do the job of another type of cell. Another aspect of these specialized cells is that they cannot replicate and create new copies of themselves. So a red blood cell cannot divide into two new red blood cells.

Stem cells are unique in two ways:

1) Stem cells are essentially 'undifferentiated' or 'unspecialised' cells', which means they don't have a prescribed function, however, they are able to replicate and turn into different 'specialized' cells. A good metaphor for stem cell is to think of them as 'blank' cells which can replicate when needed into specialized cells.

2) Unlike specialised cells which cannot replicate, stem cells can replicate, or 'self-renew' to produce more stem cells. This is only possible in it's 'blank' stage. Once a stem cell turns into a specialized cell, that new specialized cell cannot replicate into new stem cells.

It's important to understand, stem cells are not the only cells in the body that can replicate. We have many varied cells that replicate for regeneration. But what makes stem cells so distinctive the fact they they can not only replicate, but they can replicate into almost any kind of specific cell. Other cells, like skin cells for example, can only replicate to form new skin cells (aka new copies of itself). A skin cell cannot become a red blood cell, or a muscle cell. But stem cells can turn into red blood cells, or liver cells, or skin cells, or cells of another specific organ in your body.

Where Are Stem Cells Found?
In mammals, there are two broad types of stem cells. One being embryonic stem cells, which are found in the early stages of an embryo. This is really fascinating because 'early' in this regard means really early. Embryonic stem cells are found 4-5 days after fertilization (aka conception of the sperm and ova) occurs.

The second type of stem cells are adult stem cells. Adult stem cells are found throughout the body, including the bone marrow, brain, bone, blood, muscles and organs of the body.

Image A: shows human embryonic stem cells in their undifferentiated state.
Image B: shows neurons derived from human embryonic stems (i.e. neurons formed from stem cells).

(Image credit & copyright: Eugene Russo)