Understanding Stem Cell Terminology
By Verdell Goulding

Stem cells have the unique ability to direct the production of many types of cells instead of just one. Embryonic stem cells (ES cells) are the most controversial type of stem cell, and may be best understood in the context of human development.

At the moment of fertilization, an egg and sperm unite to form a single cell called a zygote. This zygote is said to be totipotent , meaning it has the potential to develop into a totally complete organism. It is capable of producing all the cells of the human body, as well as other cells necessary for development (such as the placenta, for example).

In the first few hours following fertilization, this single cell divides to form a cluster of two identical totipotent cells, both of which have the potential to develop into complete organisms. (In fact, this is how identical twins form; if these two cells separate and begin dividing as two separate clusters, the result is two separate, genetically identical people.) After this first division, the cluster of two cells divides to become a cluster of four identical cells, which then divides into eight identical cells, and so on until there has been enough division to form a complete organism.

But if there were not more than this to development, we would simply be a large collection of identical cells. So in addition to innumerable cell divisions, cells must also differentiate from one another to produce all the various cells of the human body (skin cells, muscle cells, bone cells, blood cells, brain cells, etc.).

This process begins as the dividing mass of totipotent cells differentiates to form a hollow sphere called a blastocyst . This blastocyst contains two types of cells: an outer layer of cells which produces the placenta and other tissues necessary for fetal development; and an inner layer of cells which develops into the fetus. The cells of this inner layer are called pluripotent stem cells, which means they are able to produce almost all types of human cells. They are more specialized than totipotent stem cells, however, and generally do not produce all the cells necessary for development (such as those that form the placenta, for example).

To be used for research purposes, the inner and outer layers of the blastocyst must be separated, which destroys the developing embryo, and the pluripotent stem cells are removed. Researchers then try to direct them to produce very specific types of specialized cells (such as a brain cell). The cells produced by the continued divisions of pluripotent stem cells after being removed from the embryo are referred to as stem cell lines .

In contrast to totipotent and pluripotent stem cells, which are present in early development, multipotent stem cells can be found throughout the bodies of children and adults. Commonly referred to as adult stem cells , these are cells that are specialized to a point but still hold potential to specialize further as needed.

For example, blood stem cells are responsible for replenishing the various components of blood (red blood cells, white blood cells, and platelet cells). When any of these component cells dies, the blood stem cell is able to produce a specialized replacement for the specific type of cell that died. In this way, this one kind of adult stem cell is able to produce several different kinds of highly specialized cells.Recent discoveries have indicated that adult stem cells may hold as much (or more) therapeutic potential as the more controversial embryonic stem cells.