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Monitoring Stem Cell Research 
developed organs or tissues of the body, are said to be muiti- 
potent if they produce more than one differentiated tissue cell 
type, and unipotent if they produce only one differentiated tis- 
sue cell type. 
We introduce in this chapter an additional term: stem cell 
preparation. A stem cell preparation is a population of stem 
cells, prepared, grown, and preserved under certain condi- 
tions. Because different laboratories (or even the same one) 
can have different preparations of the same type of stem cell, it 
is important to recognize the potential differences between 
particular preparations of embryonic stem cells.* It will some- 
times be important to call attention to this fact, by speaking of 
a ‘'preparation of ES cells" (or a preparation of adult stem cells) 
rather than of “ES cells," pure and simple. We will use the 
term “stem cell preparations" when we are speaking of a di- 
verse group of stem cell cultures, when we are speaking of 
stem cell cultures that contain an admixture of other types of 
cells, or when the developmental homogeneity of the stem 
cells in the population has not been defined. 
Adult and embryonic stem cell populations have also been 
called “stem cell lines." In the past, the term “cell line" de- 
noted a cell population (usually of cancer cells containing ab- 
normal chromosome numbers or structure, or both) that could 
grow “indefinitely" in vitro. Embryonic and some adult stem 
cell preparations are capable of prolonged growth beyond 50 
population doublings in vitro while retaining their characteris- 
tic stem cell properties and initially with no change in the 
chromosome numbers and structure. It is not yet known 
whether any preparation of human ES cells (generally believed 
to be much longer-lived than adult stem cells) will continue to 
grow “indefinitely," without breaking down. 
Under the influence of various cell-differentiation signals, 
embryonic stem cells differentiate into numerous distinct types 
of more specialized cells. Some of these are specialized stem 
cells that can also self-renew, while retaining their ability also 
to differentiate into multiple cell types. Recent research has 
* Embryonic stem cell cultures prepared from different embryos of a single 
inbred mouse strain are more likely to have closely similar biological proper- 
ties than will ESC cultures from genetically different individual human be- 
ings. 
PRE -PUBLICATION VERSION 
