Stem cellsEdexcel A-Level Biology B: Revision notes

A stem cell is an undifferentiated cell that can divide by mitosis to make more stem cells (self-renewal) and can differentiate into specialised cells. Stem cells are classified by the range of cell types they can form (their potency):

• Totipotent: can form every cell type, including the extra-embryonic tissues such as the placenta. Found in the zygote and the cells of the very early embryo.
• Pluripotent: can form every cell type of the body but not the placenta. Found in the inner cell mass of the blastocyst.
• Multipotent: can form only a limited range of related cell types, for example haematopoietic stem cells in bone marrow, which give blood cells.

Every cell has the same genome, so the loss of potency during development is caused by epigenetic modification and the action of transcription factors, not by loss of genes.

1. The totipotent cells of the early embryo develop into a blastocyst.
2. Epigenetic modifications (DNA methylation, histone modification, non-coding RNA) silence some genes, such as those needed for placental tissue. The inner cell mass is pluripotent.
3. Further modification restricts cells to a smaller range of types (multipotent).
4. Eventually the cell expresses only the genes for one function and becomes a fully differentiated somatic cell. The pattern is passed on by mitosis.

Pluripotent stem cells from embryos (usually spare IVF embryos at the blastocyst stage) can be grown and directed to form many cell types. They offer new medical advances, for example replacement cells for spinal cord injury, diabetes (beta cells), heart disease or blindness, and models for testing drugs.

There are ethical considerations:

• Obtaining the cells destroys the embryo, which some regard as a potential human life.
• The embryo cannot give consent.
• Others argue that spare IVF embryos would be discarded anyway, that the embryo is at a very early stage and that the benefit to patients is great.
• Embryonic cells are genetically different from the patient, so there is a risk of rejection.

iPS cells are made by reprogramming a differentiated cell. Fibroblasts (connective tissue cells, easily taken from skin) are given named genes, such as Oct4, Sox2, Klf4 and c-Myc, often using a virus as a vector.

These genes code for transcription factors that switch on pluripotency genes and reverse the epigenetic modifications that had silenced them, so the cell becomes pluripotent.

Advantages over embryonic stem cells: no embryo is destroyed; the cells are genetically identical to the patient, so there is less risk of rejection and no need for immunosuppressants; and they can be obtained from the patient.

Risks: some of the genes (for example c-Myc) can cause cancer, viral vectors may disrupt other genes, and leftover undifferentiated cells can form tumours.