Introduction 
9 
B. Embryonic Germ (EG) Cells 
EG cells are stem cells that are isolated from the gonadal 
ridge of a developing fetus. These are the cells that ultimately 
give rise to sperm cells or egg cells, depending on the sex of 
the fetus. The EG cells are collected from the bodies of five-to- 
nine-week-old fetuses that have been donated after induced 
abortions.* * In federally funded research, collection of the EG 
cells is governed by existing federal regulations for fetal-tissue 
donation, designed (among other things) to ensure the separa- 
tion of the decision to terminate pregnancy from the decision 
to donate the fetal tissue for research.^ 
Cell lines established from either of these two sources (ES 
and EG cells, from embryos and fetal gonads, respectively) 
have demonstrated two important properties: great ability to 
multiply and form stable lines that can be characterized, and 
great flexibility and plasticity. Their progeny can differentiate 
in vitro into cells with characteristics of those normally derived 
from all three embryonic germ layers (ectoderm, endoderm, 
and mesoderm), which layers (in vivo) give rise in turn to all 
the different types of cells in the body. Because they are so 
flexible, it also seems likely that they could be used to produce 
cell preparations that could then be transplanted (assuming 
that the recipient’s immune response could be managed) to 
repopulate a part of the body such as the pancreas or spinal 
cord that has lost function due to disease or injury. As with 
stem cells derived from the various tissues of the adult body, 
ES cells and EG cells seem to hold out hope for an era of re- 
generative medicine. 
from the womb, but this procedure, though technically feasible, has a very 
low yield and is rarely done. 
* Abortion is legal throughout the United States, pursuant to a series of fed- 
eral Supreme Court decisions, the most important cases being Roe v. Wade 
(1973) and Casey v. Planned Parenthood (1992). 
PRE-PUBLICATION VERSION 
