So, it’s time for my next blog post and to try and explain the world of stem cells to anyone that’s interested! From the last blog post, we now know (hopefully :P) what stem cells are and why they are important, so now we can move on to asking what makes stem cells unique?
Stem cells differ from other kinds of cells in the body. All stem cells—regardless of their source—have three general properties: they are capable of dividing and renewing themselves for long periods; they are unspecialised; and they can give rise to specialised cell types.
Stem cells are capable of dividing and renewing themselves for long periods. Unlike muscle cells, blood cells, or nerve cells—which do not normally replicate themselves—stem cells may replicate, or proliferate many times almost indefinitely. A starting population of stem cells that proliferates for many months in the lab can yield millions of cells. If the resulting cells continue to be unspecialised, like the original stem cells, the cells are said to be capable of long-term self-renewal.
Scientists are trying to understand two fundamental properties of stem cells that relate to their long-term self-renewal:
- Why can embryonic stem cells proliferate for a year or more in the laboratory without differentiating, but most non-embryonic stem cells cannot; and
- What are the factors in living organisms that normally regulate stem cell proliferation and self-renewal?
Discovering the answers to these questions may make it possible to understand how cell proliferation is regulated during normal embryonic development or during the abnormal cell division that leads to cancer. Such information would also enable scientists to grow embryonic and non-embryonic stem cells more efficiently in the laboratory, which in the long-term will help with regenerative medicine.
The specific factors and conditions that allow stem cells to remain unspecialised are of great interest to scientists. It has taken scientists many years of trial and error to learn to derive and maintain stem cells in the laboratory without them spontaneously differentiating into specific cell types. For example, it took two decades to learn how to grow human embryonic stem cells in the laboratory following the development of conditions for growing mouse stem cells. Likewise, scientists must first understand the signals that enable a non-embryonic or adult stem cell population to proliferate and remain unspecialised before they will be able to grow large numbers of unspecialised adult stem cells in the laboratory.
Stem cells are unspecialised. One of the fundamental properties of a stem cell is that it does not have any tissue-specific structures that allow it to perform specialised functions. For example, a stem cell cannot work with its neighbours to pump blood through the body (like a heart muscle cell), and it cannot carry oxygen molecules through the bloodstream (like a red blood cell). However, unspecialised stem cells can give rise to specialised cells, which includes any cell in the body.
Stem cells can give rise to specialised cells. When unspecialised stem cells give rise to specialised cells, the process is called differentiation. While differentiating, the cell usually goes through several stages, becoming more specialised at each step. Scientists are just beginning to understand the signals inside and outside cells that trigger each step of the differentiation process. The internal signals are controlled by a cell’s genes and the external signals for cell differentiation include chemicals secreted by other cells, physical contact with neighbouring cells, and certain molecules in the microenvironment.
Many questions about stem cell differentiation remain. For example, are the internal and external signals for cell differentiation similar for all kinds of stem cells? Can specific sets of signals be identified that promote differentiation into specific cell types? Addressing these questions may lead scientists to find new ways to control stem cell differentiation in the laboratory, thereby growing cells or tissues that can be used for specific purposes such as cell-based therapies or drug screening.
Adult stem cells typically generate the cell types of the tissue in which they reside. For example, a blood-forming adult stem cell in the bone marrow normally gives rise to the many types of blood cells. It is generally accepted that a blood-forming cell in the bone marrow—which is called a hematopoietic stem cell—cannot give rise to the cells of a very different tissue, such as nerve cells in the brain.
However, experiments over the last several years have claimed to show that stem cells from one tissue may give rise to cell types of a completely different tissue through something called cellular reprogramming, but we will address this another time. This remains an area of great debate within the research community. This controversy demonstrates the challenges of studying adult stem cells and suggests that additional research using adult stem cells is necessary to understand their full potential as future therapies.