Stem cells - young and energetic entities
Our bodies, made up of 60 trillion cells, started out as a single cell (fertilized egg) in our mother's womb. This one cell underwent many rounds of cell division to become our body. The same is true of any living thing. The bodies of organisms are collections of highly varied cells that started out from a single cell.
Last time we talked about how cells taking on their own respective roles is known as "differentiation". A single cell repeatedly undergoes cell division and differentiation to create a part of the body, which is a group of various types of cells. At this time, similar sorts of organs and tissue will be made from one type of cells that are lumped together as a certain group. For example, bones and cartilage, muscles and ligaments mainly start out as a single type of cells, and similarly, the cells that make up the blood, like red blood cells and white blood cells, were of a common type originally. For this reason, cells that form the basis for creating subgroups of this type came to be called "stem cells". The meaning of "stem" here is like the trunk of a tree before it splits into branches.
Although they are lumped together under the term "stem cells", there are actually a number of different types. For example, one type is the mesenchymal stem cells, which develop into tissue in the group including bone, cartilage, and muscle. Another type is the hematopoietic stem cells, which develop into the cells that make up the blood.
Let's run over the main points of what type of cells stem cell are. Stem cells are generally defined as those meeting the following two criteria: Firstly, they possess the ability to produce things that are the same as themselves (replication competence); secondly, they possess the ability to differentiate into different types of cells (pluripotency). As previously mentioned, the stem cells that become bone and cartilage of course possess the ability to replicate themselves, and they also could be said to be cells that possess the ability to create other types of tissue and cells, at least those like bones and cartilage. In the same way, blood stem cells can multiply themselves and form the basis for producing white blood cells and red blood cells. They possess both replication competence and pluripotency.
A fertilized egg itself has the ability to differentiate into a great many different types of tissue (all of the types of tissue possessed by the human body), but that in itself does not mean that it has replication competence. When cell division starts in the mother's womb, the process of growth is initiated, and the cell ceases to be a fertilized egg. We cannot call this replication (If the fertilized egg really did replicate itself, a large number of babies might be born). For this reason, cells that possess similar functions to the fertilized egg are created by aptly altering the fertilized egg and increasing it as cells that can be cultured. You may have heard of these cells before; they are known as embryonic stem cells (ES cells). Like other stem cells, these ES cells possess the ability to replicate themselves, and they also possess the ability to develop into a far greater range of tissue and organ types than other stem cells. Actually, because they can produce an animal or human being, the handling of ES cells raises unavoidable ethical issues.
Of course, our bodies do not have any stem cells that correspond to ES cells. However, stem cells that can produce bones or blood or skin are still alive within the bodies of adults. A newborn baby or a child has a number and proportion of stem cells that far exceeds that in an adult, but stem cells still firmly remain in the bodies of adults. Steady advances are being made in technologies that produce the equivalent of body tissues by skillfully increasing the number of such stem cells. Technologies that increase and maintain stem cells in this manner, or lead them to differentiate in a certain direction, are indispensable to regenerative medicine.