116 
Monitoring Stem Cell Research 
fate of these cells. Failure to control the cells may yield ex- 
perimental results that are difficult or impossible to reproduce. 
The following more specific observations make clear the di- 
mensions of this difficulty. 
A. Initial Stem Cell Preparations Can Contain Multiple Cell 
Types 
Isolation of adult stem cells from source tissues such as 
bone marrow, brain, or muscle initially yields a heterogeneous 
cell preparation. The initial preparation contains the several 
cell types found in the source tissue, and it may also include 
red blood cells, white blood cells, and (possibly) circulating 
stem cells, owing to the presence of blood flowing through the 
tissue in question. Initial mixtures of cells may then be treated 
in various ways to remove unwanted contaminating cells, 
thereby increasing the proportion of stem cells in the prepara- 
tion. But seldom, if ever, does one produce an adult stem cell 
preparation that is 100 percent stem cells, unless the adult 
stem cell preparation has been “ single-cell cloned ” in vitro 
(see below). 
The way in which human embryonic stem cells have been 
produced from ICM cells also raises a question about the “spe- 
cies homogeneity” of the initial cell preparations. In the past, 
human embryonic stem cells were isolated and maintained by 
in vitro growth on top of irradiated (so that they no longer di- 
vide) “feeder layers ” of mouse cells. It is thought that the 
feeder cells secrete factor(s) that enable the stem cells to di- 
vide while maintaining a relatively undifferentiated state. Al- 
though the mouse cells have been treated to prevent their cell 
division, should any of them happen to survive, human embry- 
onic stem cells prepared in this way may contain some viable 
mouse cells.* More recently, several groups have shown that it 
is possible to grow ESCs on feeder layers of human cells, in- 
cluding fibroblasts obtained firom skin biopsies, or without any 
feeder cell layer at all.” One way to be certain that human em- 
* The issue of possible mouse virus contamination is dealt with in Section F, 
below. 
PRE -PUBLICATION VERSION 
