In the adult individual, a variety of tissues have also been found to harbour stem cell populations. Biologically, hEG cells have many properties in common with hES cells ( Shamblott et al., 2001).

In humans, hEG cells were first established in culture in 1998, shortly after the first hES cells, from tissue derived from an aborted fetus ( Shamblott et al., 1998).

The only other cells with proven pluripotency similar to that of ES cells are embryonic germ (EG) cells, which as their name implies, have been derived from ‘primordial germ cells’ that would ultimately form the gametes if the fetus had not been aborted. hES cells are also immortal, expressing high levels of a gene called telomerase, the protein product of which ensures that the telomere ends of the chromosomes are retained at each cell division and the cells do not undergo senescence. They are therefore distinct from the ‘totipotent’ fertilized oocyte and blastomere cells deriving from the first cleavage divisions. Although hES cells can form all somatic tissues, they cannot form all of the other ‘extraembryonic’ tissues necessary for complete development, such as the placenta and membranes, so that they cannot give rise to a complete new individual. HES cells are derived from the so‐called ‘inner cell mass’ of blastocyst stage embryos that develop in culture within 5 days of fertilization of the oocyte ( Thomson et al., 1998 Reubinoff et al., 2000). Broadly speaking, two types of stem cell can be distinguished: embryonic stem (ES) cells which can only be derived from pre‐implantation embryos and have a proven ability to form cells of all tissues of the adult organism (termed ‘pluripotent’), and ‘adult’ stem cells, which are found in a variety of tissues in the fetus and after birth and are, under normal conditions, more specialized (‘multipotent’) with an important function in tissue replacement and repair. ‘Stem cells’ are primitive cells with the capacity to divide and give rise to more identical stem cells or to specialize and form specific cells of somatic tissues. In order to discuss the moral aspects of the isolation and use of hES cells, which is the aim of the present article, it is first essential to understand exactly what these cells are, where they come from, their intended applications and to define the ethical questions to be addressed. The European Science Foundation has established a committee to make an inventory of the positions taken by governments of countries within Europe on this issue ( European Science Foundation, 2001). The UK has been the first to pass a law governing the use of human embryos for stem cell research. The question ‘Can these cells be isolated and used and, if so, under what conditions and restrictions’ is presently high on the political and ethical agenda, with policies and legislation being formulated in many countries to regulate their derivation.

The most controversial variant of this is the transfer of a somatic cell‐nucleus from a patient to an enucleated oocyte (unfertilized egg) in order to produce hES cells genetically identical to that patient for ‘autologous’ transplantation (so‐called ‘therapeutic’ cloning) this may prevent tissue rejection. To date, most embryos used for the establishment of hES cell lines have been spare embryos from IVF, but the creation of embryos specifically for deriving hES cells is also under discussion. On the other hand, the use of hES cells is highly controversial because they are derived from human pre‐implantation embryos. On the one hand, these ‘super cells’ have a major clinical potential in tissue repair, with their proponents believing that they represent the future relief or cure of a wide range of common disabilities replacement of defective cells in a patient by transplantation of hES cell‐derived equivalents would restore normal function. Human embryonic stem cells (hES cells) are currently discussed not only by the biologists by whom they were discovered but also by the medical profession, media, ethicists, governments and politicians. Key words: cell therapy/cloning/embryos/ethics/stem cells Introduction

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