WHAT ARE STEM CELLS?

Stem cells are a very special category of building-block in the human body, versatile in that they can not only divide to make copies of themselves but also turn into many mature final forms that no longer divide. For example, stem cells from blood or bone marrow can turn into nerve cells, and those from the brain can turn into blood. There is intense interest in medical applications to restore and renew body parts by inducing stem cell grafts to multiply into new types of tissue needed for repair. This is a particularly exciting prospect for diseases of brain degeneration which are presently incurable. This chapter explains the important concepts in simple terms and offers an account of the extent to which this promise is being realised in practice and of the hurdles that still remain. The chapter to follow considers the ethical issues raised by these actual and potential advances.

WHERE DO STEM CELLS COME FROM?

Stem cells are found in embryonic, fetal and adult brain and body. The fertilised egg is definitively ‘totipotent’, meaning that all other types of cell derive ultimately from it. As the embryo develops into a fetus, stem cells become progressively programmed to become specific cell types, and before their final evolution into mature non-dividing cells they are often called ‘progenitor’ or ‘precursor’ cells.