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2. Embryonic Germ Cells. 
Human embryonic germ cells are isolated from the primor- 
dial germ tissues of aborted fetuses. Gearhart^ has summa- 
rized the results of recent research with human and mouse EG 
cells. One study focused on regulation of imprinted genes in 
EG cells: it showed “that general dysregulation of imprinted 
genes will not be a barrier to their (EG cell) use in transplanta- 
tion studies. In addition, Kerr and coworkers^^ showed that 
cells derived from human EG cells, when introduced into the 
cerebrospinal fluid of rats, became extensively distributed over 
the length of the spinal cord and expressed markers of various 
nerve cell types. Rats paralyzed by virus-induced nerve-cell 
loss recovered partial motor function after transplantation with 
the human cells. The authors suggested that this could be due 
to the secretion of transforming growth factor-a and brain- 
derived growth factor by the transplanted cells and subse- 
quent enhancement of rat neuron survival and function. 
Until recently, work with human EG cells came primarily 
from one laboratory. Recently the isolation and properties of 
human EG cells have been independently confirmed. Because 
human EG cells share many (but not all) properties with ESCs, 
these cells offer another important avenue of inquiry. 
3. Embryonic Stem Cells from Cloned Embryos (Cloned ESCs). 
Although it has yet to be accomplished in practice, somatic 
cell nuclear transfer (SCNT) could create cloned human em- 
bryos from which embryonic stem cells could be isolated that 
would be genetically virtually identical to the person who do- 
nated the nucleus for SCNT: hence cloned ESCs [see (7)]. In 
theory, using such cloned embryonic stem cells from individual 
patients might provide a way aroimd possible immime rejec- 
tion (see below), though in practice this could require individ- 
ual cloned embryos for each prospective patient — a daunting 
Previous work had shown that variation in imprinted gene expression was 
observed in cloned mice, and that it might be partly responsible for their 
subtle genetic defects. So it was reassuring that the pattern of imprinted 
gene expression appeared to be normal in EG cells. 
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