112 
Monitoring Stem Cell Research 
Stem Cdl 
Increasing 
Degree of 
DifTereniiation 
^ rcibusi. Rymmetrical 
^ scl^-reno^val 
I 
/ 
JpiFFER^NTIAJ10^4_^ 
4. 
self- 
\renewal 
O 
o- 
self- 
. renewal 
Specialized Stem Cell I 
(e g mesenchymal) 
Specialized Stem Cell 2 
(e g neural) 
4^ 
-'L- 
^ V 
4 V 
DIFFERENTIATION 
SIGNALS 
_A_ 
4 V ' 
4. V 
[XtfeTentiaied Ditferemiated 
Tissue Cell A Tissue Cell B 
(e^4 cardiomyocytes 
Differentiated Differeniiated 
Tissue Ceil C Tissue Cell P 
(e.g. digodendrocyles) 
Figure 1. Schematic Diaaram of Some Stages in Cell Differentiation 
At the top of the figure is an undifferentiated stem cell; in 
the central box are more “specialized" stem cells (or “precur- 
sor cells" or “progenitor cells”); at the bottom are various dif- 
ferentiated cells that are derived from the specialized stem 
cells. Dashed arrows indicate symmetrical (in the sense that 
both the daughter cells are stem cells) cell divisions that pro- 
duce more stem cells (self-renewal). Solid arrows indicate 
asymmetric cell divisions that produce more differentiated 
daughter cells. (There may also be self-renewal with asymmet- 
ric division — ^not shown here — in which one daughter cell initi- 
ates a differentiation pathway while the other remains a stem 
cell.) Differentiation signals can be supplied by both soluble 
proteins and by specific, cell-surface binding sites. Some of the 
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