Stem cells?

Stem cells are cells that retain the potential to differentiate into more than one mature cell type and have the capacity to self-renew (make more of themselves). There are many different kinds of stem cells. Pluripotent stem cells are capable of giving rise to any cell type, multipotent stem cells are limited to their specific tissue or a given lineage (neural stem cells, hematopoietic stem cells, mesenchymal stem cells, etc.). Totipotent stem cells can give rise to an entire organism. When people refer to stem cell research, most ume it is on pluripotent stem cells (i.e. embryonic stem cells). Embryonic stem cells are completely a laboratory artifact, they do not exist in normal organisms. Scientists derive pluripotent stem cells from the inner cell m of a very early pre-implantation embryo, this does not hurt it and it could go on to give a complete organism. They culture these cells and select for ones that can be propagated indefinitely given the right conditions. It is these cells that are called embryonic stem cells (ES cells). Any normal cells from an organism will undergo senescence (stop dividing) after a set number of cell divisions and either die or sit around for a while but is completely and irreversibly out of the cell cycle (cannot divide again). Umbilical cord stem cells are multipotent, anyone who says they are pluripotent or ES-cell equivalent are not being truthful, they fail the most important test of pluripotent stem cells in that they cannot contribute in any substantial manner to all the lineages and cell types of an organism or integrate into the germ line. Mostly these are hematopoietic stem cells, and these only give rise to blood cells. The lady who did the lion's share of work on umbilical cord stem cells as "multipotent adult progenitors" (MAPs) that give rise to essentially all cell types and lineages was found to have been involved in major scientific misconduct and her work was never repeated in any other independent lab although many tried. If you want arguments for or against, you can find them on the NIH or NSF websites. At least those are based on science, logic, and even-tempers.