Although stem cells are indeed undifferentiated, the human body contains a large variety of the said cells. Among them, one may find the following:
Totipotent Stem Cells
Totipotent stem cells are capable of forming differentiated cell in the human body. Each totipotent cell is capable of replicating and differentiating in order to form a living human being. In fact, at its early stages, all cells within the human embryo are totipotent up until the sixteenth cell stage or so.
Pluripotent Stem Cells
Pluripotent stem cells have the potential to develop into any of the cell types of the adult organism except those of the placenta which are derived from the trophoblast. Unlike totipotent stem cells, pluripotent stem cells cannot replicate and differentiate to form a human being although they are required to keep every human being alive.
There are three different types of pluripotent stem cells:
Embryonic Stem Cells. (ESCs)
In nature, embryonic stem cells can be found forming the inner cell mass (ICM) of the blastocyst. (A structure formed in early embryogenesis, after the formation of blastocoels before implantation occurs) Embryonic stem cells can be harvested from the developing embryo before implantation occurs and are in fact often taken from the excess embryos produced during in vitro fertilisation (IVF) procedures. Although scientists have managed to extract stem cells from, a mouse embryo without killing it, harvesting embryonic stem cells from human blastocysts often kills the embryo which could have been implanted to produce another baby. The process is therefore controversial and considered murder by many ethicists.
Embryonic Germ Cells (EGCs)
Embryonic germ cells are cells which generally develop to form sperm cells in males and ova in females. They are derived from a specific part of the embryo called the gonad ridge and are isolated from foetuses older than eight weeks of development. While they are similar to embryonic stem cells in many ways, (such as being able to develop into any specialised cell type) they are different in the way in which they grow when cultured in a laboratory. Embryonic stem cell cultures for instance, have been grown for over two years in a laboratory as immortal stem cell lines whereas embryonic germ cells can only survive seventy to eighty cell divisions. This makes them less suitable for establishing cell lines for research. However, their use appears to promise a number of advantages over the use of embryonic stem cells in that they do not seem to form tumours when transferred into the body. This may make them useful as a source of transplant tissue and cell based therapies. Unfortunately, the extraction of embryonic germ cells results in the destruction of a foetus and hence the termination of a potential life. Therefore, embryonic germ cells are only extracted from aborted foetuses and no foetuses are created for research purposes.
Embryonic Carcinoma Cells (ECCs)
These stem cells can be isolated from teratocarcinomas, a tumour that occasionally occurs in a gonad of a foetus. Unlike the other two, they are usually aneuploid.
All these Pluripotent stem cells can be isolated from embryonic or fetal tissue and can be grown by culture in a laboratory. (Using special methods to prevent them from differentiating)
Hematopoietic stem cells
Hematopoietic stem cells are cells which are isolated from the bone marrow and which can renew themselves, can differentiate to a variety of specialized cells, can mobilize out of the bone marrow into circulating blood, and can undergo programmed cell death, called apoptosis. (A process by which cells that are detrimental or unneeded, self-destruct) Hematopoietic stem cells are currently the best studied, such that scientists have even been able to use them to treat certain diseases such as leukaemia. Unlike embryonic stem cells, they are unable to differentiate into any cell type and are only capable of forming any type of blood cells. Hematopoietic stem cells can be found in the bone marrow (their traditional source), peripheral blood, (one of the present sources of HSCs) and umbilical chord blood (Another present source of HSCs). Another important source which is not in clinical use includes the fetal hematopoietic system, a source which has been most extensively studied in the mouse. Their continued presence in an adult person gives the body its repairing and healing ability.
Multipotent stem cells
A Multipotent stem cell can also give rise to other types of cells but it is limited in its ability to differentiate. These other types of cells are also limited in number. Examples of Multipotent stem cells include those present in the brain, which give rise to different neural cells and glia as well as the haematopoietic stem cells found in the bone marrow, which can give rise to different blood cell types, but can not create other cells such as brain cells.
Multipotent stem cells are found in the tissues of adult mammals. It is also thought that they are present in most body organs, where they replace diseased or aged cells. Thus, they function to replenish the body's cells throughout an individual's life.