Cells are the smallest living units of living systems. They can respire, respond, and reproduce. A single human cell, however, or even a conglomerate of human cells, does not a human being makes. In fact, the human body is made up of trillions of cells that are organized into specialized tissues and organs, and many of these cells die each day and need to be replaced by new ones. Unspecialized and undifferentiated cells that are dormant are called upon to renew or repair the body; these are the so-called stem cells.
A given human being is never the same conglomerate of cells from day to day, but somehow remains the same person. It is the nervous system that provides the property (or, perhaps more correctly, the illusion) of sameness, i.e., the continuity of self. It is the organization and the activity of neuronal cells that allows for the emergence of the mind--for awareness, memory, and self-identity. It is the nervous tissue, therefore, that is most identified with "humanness." For this reason, the taking of stem cells from embryos in which no nervous tissue has yet appeared has received more acceptance than the taking of stem cells from more developed embryos or fetuses.
Recent evidence indicates that even in the nervous system some cell turnover can occur, but this seems limited to a few specific areas and has not been observed in a non-pathological or non-treated human patient. In other words, evidence so far has only been gathered from autopsies of chemotherapeutically-treated patients. The assumption that brain cell turnover occurs normally rests mainly on animal studies and, even there, turnover is very limited.
Stem cells are undifferentiated cells and have the potential to become many cell types and to form various tissues by mitosis (multiplication) and differentiation (specialization).......
