What is a Cell?
Cells are the smallest structural and functioning units of all living organisms. The body consists of millions of cells; among them include brain cells, skin cells, liver cells, and fat cells. All of these cells have unique functions and features and work together in order to maintain life.
So, what is a stem cell?
Of an organism’s cells, probably some of the more unique are stem cells. Stem cells are the “original” form of every cell present inside an organism. They are simple unspecialized cells made by every organ in an animal’s body which then develop and specialise such that they adapt to the function which the body develops them to carry out.
Stem cells have two important characteristics that distinguish them from other cells. Firstly, they are unspecialized cells which renew themselves for long periods of time through cell division by means of mitosis. By this cell division they can either form two stem cells, thus increasing the size of the stem cell “pool” or produce one daughter cell that goes on to differentiate and one daughter cell that retains its stem cell properties. This allows for further cell division to occur. Secondly, stem cells, under certain physiologic or experimental conditions, can be induced to become cells with special functions such as beating cells of the heart muscle or the insulin-producing cells of the pancreas. It is this property that has triggered the medical researchers’ interest and hence the branch of medical research related to stem cells.
All stem cells, regardless of their source have three general properties:
They are capable of dividing and renewing themselves for long periods
This means that unlike the body’s specialised cells (such as muscle cells, nerve cells, and blood cells), which do not replicate frequently, stem cells are capable of replicating many times by mitosis to form identical unspecialised cells. By this cell division they can either form two stem cells, thus increasing the size of the stem cell “pool” or produce one daughter cell that goes on to differentiate and one daughter cell that retains its stem cell properties. This allows for further cell division to occur. This repeated replication is called proliferation and can be easily observed when a starting population of stem cells is allowed to grow in a lab. It will be found that as long as the resulting cells, like the parent cells continue to be unspecialised, the starting population of stem cells will be fully capable of long-term self-renewal, and in a few months, will yield millions of cells.
They are unspecialised
This means that a stem cell does not have any tissue specific structures that allow it to carry out specific functions in the body. For instance, a stem cell can neither work with its neighbours to pump blood around the body or to move a muscle (like a muscle cell) nor can it fire electrochemical signals around the body (like a nerve cell), nor can it carry oxygen around the body in the form of an oxyhaemoglobin molecule (like a red blood cell). Nevertheless, it can form specialised cells including all those mentioned above.
They can give rise to specialised cell types
Stem cells are capable of differentiation. That is, when unspecialised stem cells give rise to specialised stem cells. This property of the cells is of great scientific interest, and has triggered the branch of medical research related to stem cells. As yet, differentiation is not yet fully understood, although scientists are beginning to understand that signals inside and outside cells trigger stem cell differentiation. The internal signals are controlled by a cell's genes, which are interspersed across long strands of DNA, and carry coded instructions for all the structures and functions of a cell. The external signals for cell differentiation include chemicals secreted by other cells, physical contact with neighbouring cells, and certain molecules in the microenvironment.
In short, stem cells are cells that are capable of retaining the ability to reinvigorate themselves through mitotic cell division and which can differentiate into a diverse range of specialized cell types.
Their unique properties make it impossible for stem cells to not play an important role in the growth and development of animals and humans. Normal growth and development, including the maintenance of tissues and organs in the body, requires the production of new cells by means of cell division. However, specialised cells, such as blood and muscle cells, are unable to divide. Therefore, they are instead replenished by stem cells from the stem cell “pool” which form specialised cells by means of differentiation in order to replace the lost specialised cells. Stem cells, therefore, play a crucial role in supporting tissues such as blood, skin, and gut that undergo continuous turnovers (cell replacement), and muscle, which can be built up according to the body's needs and is often damaged during physical exertion.