Research and Therapy Developments 
113 
specialized stem cells inside the dashed box, for example, 
mesenchymal stem cells, can be isolated from tissues after 
birth and correspond to adult stem cells. Scientists are cur- 
rently investigating whether, at least in some cases, the proc- 
ess can be reversed, that is, whether specialized cells may, on 
appropriate signals, dedifferentiate to become precursor or 
even fully undifferentiated stem cells. 
The terminology used to describe different stem cell types 
can be confusing. As used in this chapter, stem cells are self- 
renewing, cultured cells, growm and preserved in vitro, that 
are capable — upon exposure to appropriate signals— of differ- 
entiating themselves into (usually more than one) specialized 
cell types. Stem cells may be classified either according to 
their origins or according to their developmental potential. 
Stem cells may be obtained from various sources: firom em- 
bryos, from fetal tissues, from umbilical cord blood, and firom 
tissues of adults (or children). Thus, depending on their origin, 
stem cell preparations may be called adult stem cells .* embry- 
onic stem cells, embryonic germ cells , or fetal stem cells. Adult 
stem cells [see (4)] are cells derived from various tissues or or- 
gans in humans or animals that have the two characteristic 
properties of stem cells (self-renewal and potency for 
differentiation). Embryonic stem ceRs (ESCs) [see (2)] are 
derived from cells isolated firom the inner cell mass of early 
embryos. Embryonic germ cells (EGCs) [see (1)] are stem cells 
derived from the primordial germ cells of a fetus . Fetal stem 
ceRs (not further discussed in this chapter, but included for the 
sake of completeness) are derived from the developing tissues 
and organs of fetuses; because they come (unlike EGCs) from 
already differentiated tissues, they are (like adult stem cells) 
‘‘non-err±>ryonic,” and may be expected to behave as such. 
Depending on their developmental potential, cells may be 
called pluripotent . multipot ent . or unipotent. Cells that can 
produce aR the cell types of the developing body, such as the 
ICM cells of the blastocyst, are said to be pluripotent. The 
somewhat more specialized stem cells, of the sort found in the 
As already noted in Chapter 1, "adult stem cells" is something of a misno- 
mer. The cells are not themselves "adult." As non-embryonic stem cells, they 
are, however, partially differentiated and many of them are multipotent. (See 
discussion in the text that follows shortly.) 
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