Stem cells have been the object of intense, increasing interest due to their biological properties and the promise they hold for medical advances in the treatment of illnesses that are so far incurable. Unfortunately, this field is difficult to understand for people who are not specialized in the health sciences, in part because of a lack of understanding about what a stem cell is and in part because of the ambiguity regarding types of stem cells. We hope to clear up some of that confusion.
What Are Stem Cells?
Most simply, stem cells are cells with two characteristics: (i) they are capable of self-renewal and (ii) they are undifferentiated, that is, they have the capacity of transforming themselves into other types of cells e.g. brain cells, heart cells, bone cells, etc.
Stems cells are further distinguished by their source (prenatal or postnatal) and by their potency (the type of cells into which they can differentiate).
Stem Cell Sources and Potency
Stem cells are found in the tissue of the born (e.g. in the umbilical cord, the placenta, bone marrow, fat tissue, nerve tissue, etc.) and the unborn. Stem cells from the born are often called adult stem cells, though it would be more correct to refer to them as postnatal stem cells, as we will see momentarily.
Before birth, stem cells can be found in amniotic fluid, in embryos (the period from conception to the eighth week of gestation) and in fetuses (from the ninth week of gestation to birth). Stem cells taken before birth should be called prenatal stem cells.
Four categories exist to describe the potency of cells:
1. Totipotent cells can differentiate into any other cell in the body – are not stem cells;
2. Pluripotent cells can differentiate into almost any other cell – are stem cells;
3. Multipotent cells can differentiate into cells of a certain group type – are stem cells; and
4. Unipotent cells can differentiate only into a cell of like type. – are not stem cells.
Both the source of the stem cell and its potency are critical for determining the ethics of its use and its therapeutic value.
Prenatal Stem Cells
Of the prenatal stem cells, human embryonic stem cells (hESCs) garner the most attention because they are pluripotent. hESCs are only contained in human embryos a few days old and harvesting them necessitates the death of the embryo. A brief overview of the embryo is helpful here. An embryo five days old is called a blastocyst. A blastocyst is composed of an exterior wall of cells called the trophoblast inside of which is a cavity filled with fluid and a grouping of cells called the Inner Cell Mass (ICM). The trophoblast will form the child’s placenta, umbilical cord and amniotic membranes. The ICM contains the pluripotent stem cells which will differentiate into the types of cells needed to form tissue, organs and systems.
To obtain the hESCs, the ICM must be extracted, killing the embryo. Because of this, it is not ethical to obtain or to use these cells as it involves the killing of innocent human life.
Postnatal Stem Cells
These are stem cells that are present in various tissues differentiated after birth and develop into specific types of cells. Some are pluripotent and some are multipotent. To obtain them, a sample is taken from a patient, causing only a bit of discomfort, not his death. As they are used to treat grave illness, the therapeutic principle is fulfilled, i.e. a greater good is expected than the inconvenience caused. There is no ethical objection in the production or the use of these cells.
Where’s The Confusion?
Some of the confusion is natural, and some is manufactured. Some confusion is due to the fact that some stem cells can be found both pre- and post-natally. For example, neural stem cells (NSCs) are multipotent stem cells that are able to self-renew and differentiate only into certain types of nerve cells. NSCs are found (i) in the nervous tissue that is in the process of differentiating in embryos and fetuses; (ii) in the already-differentiated tissue of the central nervous system from the fetal stage; and (iii) post-natally at whatever stage of life.
Multipotent stem cells are also referred to at times as tissue-restricted stem cells since they can only differentiate into a limited type of cell. Neural stem cells, therefore, would be a type of tissue-restricted stem cells.
As we can see, the terms “tissue-restricted cells” and “neural stem cells” do not, of themselves, indicate at what stage of human life the cells were obtained, which is why it would be better to refer to the cells as pre- or postnatal, to indicate accurately their source.
Playing with the terms, the company Stemedica  and some legislation have incorrectly referred to prenatal stem cells as “adult” stem cells, with the pretext that they were obtained from tissue more or less differentiated—claiming that the age or “state of change” of the cell, rather than its origin, determines its classification in this regard. But these cells are taken from embryos or aborted fetuses, not post-natally or from an adult. This generates confusion, perhaps intentional, in those who want to support research and treatment with postnatal stem cells which do not present any ethical problems.