Scientific background

What are stem cells?

15.3 Stem cells are biological materials present in all human beings, and in other animals.^[1] Stem cells have two characteristics that distinguish them from other cell types: they are able to differentiate into specialised cell types; and they are able to renew themselves, allowing stem cell populations to be maintained for long periods through cell division.^[2]

15.4 Stem cells are typically characterised according to the tissue from which the cells are derived. As described below, stem cells may be obtained from embryos, foetal tissue and certain adult tissue. Depending on the tissue source, stem cells may have particular characteristics for potential development, which scientists call ‘pluripotent’ or ‘multipotent’.

Embryonic stem cells

15.5 A fertilised ovum and the cells comprising an embryo in the earliest stages following fertilisation—up until about the eight-cell stage—are totipotent.^[3] These cells have the capacity to form the placenta and other supporting tissue necessary for the development of an embryo in utero, as well as post-embryonic tissues and organs.

15.6 Embryonic stem cells appear at the blastocyst stage of embryonic development, approximately four days after fertilisation. A blastocyst is a hollow sphere of about 120 cells with an outer layer (which later develops into the placenta and other supporting foetal tissue) and an inner cell mass. The inner cell mass comprises embryonic stem cells, which are pluripotent. Pluripotent embryonic stem cells are capable of giving rise to almost all of the different types of cells found in humans, but cannot produce the placenta and other supporting tissues necessary for foetal development in the uterus. Thus, if placed in a woman’s uterus, pluripotent embryonic stem cells do not have the capacity to develop into a human being.

15.7 Embryos from which stem cells may be obtained can be acquired in different ways. The most widely used source is surplus embryos from assisted reproductive technology (ART) programs. In Australia, embryonic stem cell lines may be lawfully derived only from such surplus embryos. Embryos could also be created specifically for use in research; for example, through somatic cell nuclear transfer^[4]—commonly referred to as ‘cloning’. To date, however, only one research team, based in South Korea, has reported successfully developing an embryonic stem-cell line from a cloned human embryo.^[5]

Figure 15–1 Stem cell cultivation

Copyright in this illustration is held by the University of Wisconsin Board of Regents. The illustration is reproduced with permission.

Foetal stem cells

15.8 Foetal stem cells may be isolated from primordial germ cells in the incipient gonads (ovaries and testes) of aborted foetuses and are often referred to as ‘embryonic germ cells’. Embryonic germ cells are pluripotent. Multipotent stem cells—that is, stem cells that can give rise to various types of cells but only within a certain tissue type—have also been derived from foetal neural tissue and from umbilical cord blood.^[6]

Adult stem cells

15.9 Adult stem cells^[7] exist in human organs and tissues and are responsible for the normal replacement and repair of different organs and tissues. Currently, about 20 different types of adult stem cells have been identified, though scientists have found such cells difficult to identify, isolate and grow in culture.^[8] Adult stem cells are multipotent. They are thought to be less flexible than embryonic stem cells, and to be capable of differentiating into a more restricted range of specialised cells. However, there is some evidence that adult stem cells may be able to give rise to cell types outside their own lineage—that is, cells of a different tissue type.^[9]

Potential of stem cell research

15.10 There has been widespread discussion about the potential applications of stem cell research.^[10] Research into the events that lead to cell specialisation in humans and the stages of human development may increase scientific understanding of the causes of birth defects and abnormal cell activity, such as cancer.

15.11 Stem cells may also be used to generate cells and tissue for transplantation in the treatment of diseases of the nervous system such as Alzheimer’s disease and Parkinson’s disease, as well as treatment for spinal cord damage, strokes and burn injuries. The use of stem cells to regenerate damaged organs or to create new organs for transplantation purposes has also been proposed. Such therapeutic applications of embryonic stem cells are unlikely to be available for some years.^[11] However, some therapeutic applications of adult stem cells are already in use; for example, bone marrow transplants and regrowth of skin cells for burn victims.

15.12 In the future, stem cell lines might be used for pharmacological testing of candidate drugs—known as cell-based drug screening—although research use of embryonic stem cell lines for this purpose is speculative and controversial. Promising drugs might then be further tested on animals and finally in clinical trials.