Stem Cells/Stem Cell Transplantation

The human body is composed of a vast number of cells, on the order of 1013 to 1014. These are grouped, based on functional and morphological characteristics into various types of tissue composed of similar cells. Yet, for all humans, there was a time when there was but one cell. This cell was the product of syngamy (fertilization): the union of egg and sperm. This one cell divided and, in response to internal and environmental cues, various cells arising from these divisions became what we call stem cells. Stem cells are distinguished by their ability to proliferate indefinitely and their ability to give rise to numerous more specialized cell types. In contrast, differentiated cells have a limited ability to renew themselves and are limited in the types of cells they may differentiate into.

The above description characterizes a progression from one cell to many which is accompanied by increasing specialization and diminishing potential. The potential available to a cell's successors is known as its plasticity. The most specialized cells in the human body perform a particular task very well and divide very few times, if at all, before dying. These cells are said to have no plasticity: they are committed to a certain fate.

Contrast this with the original cell of the zygote (the product of syngamy). This one cell can give rise to all the cells of the human body as well as all the cells needed for development of the maternal environment (placenta, etc.). It is said to be totipotent in its plasticity: Its successors may be any cell type.

At some point early in embryonic development, cells have differentiated into those committed to maintaining the developmental environment and those destined to be a part of the developing human [Page 1608]being. These cells are said to have pluripotent plasticity as they can give rise to many, but not all, of the cell types needed to bring a human being into the world. These pluripotent cells are known as stem cells. More specifically, they are embryonic stem cells. Embryonic stem cells are sometimes referred to as totipotent because although they cannot differentiate into all tissue types, they can differentiate into any tissue type seen in humans after the time of birth.

Interest in stem cells is driven by their potential use in stem cell transplantation as part of regenerative medicine. This branch of science seeks to remedy those diseases and dysfunctions caused by the absence of, or defects in, certain specialized cells or groups of such cells organized as tissue or organs. Examples of such diseases are diabetes (pancreatic dysfunction) and Parkinson's diseases (neurological degeneration). The goal is to take stem cells and induce them, through chemical and physical means, to specialize (differentiate) into the sorts of cells, tissues, or organs needed to remedy the disease or dysfunction. The ideal stem cell will be easily acquired and amenable to efforts at directed differentiation.