Sources of Embryonic Stem Cells

In  Vitro   Fertilization:  The  largest   potential source of blastocysts  for stem cell research is from in vitro  fertilization   (IVF) clinics.  The  process  of  IVF requires the retrieval of a woman’s eggs via a surgical procedure  after  u
ndergoing  an intensive  regimen  of “fertility  drugs,”  which stimulate  her ovaries to pro- duce multiple mature eggs. When IVF is used for reproductive purposes,  doctors typically fertilize all of the donated  eggs in order to maximize their chance of producing  a viable blastocyst that can be implanted  in the womb. Because not all the fertilized eggs are implanted,  this  has  resulted   in  a  large  bank   of “excess” blastocysts that are currently stored in freez- ers around  the country.  The blastocysts  stored in IVF clinics could prove to be a major source of embryonic stem  cells for  use in medical  research.  However, because most of these blastocysts  were created before the advent of stem cell
research, most donors were not asked for their permission  to use these left-over blas- tocysts for research.

Through nuclear transfer, scientists could produce a blastocyst by inserting the nucleus from an adult cell (for example, a skin cell) into an egg without a nucleus. All the stem cells derived from this blastocyst are genetically matched to the adult cell.

The in vitro fertilization  (IVF) technique  could poten- tially also be used to produce  blastocysts  specifically for research purposes.  This would facilitate the isola- tion of stem cells with specific genetic traits necessary for the study  of particular diseases.  For example,  it may be possible  to study the origins  of an inherited disease like cystic fibrosis using stem cells made from egg and sperm donors who have this disease. The cre- ation of stem cells specifically for research using IVF is, however, ethically problematic for some people because it involves intentionally creating  a blastocyst
that will never develop into a human being.

Nuclear  Transfer: The  process  called  nuclear transfer  offers  another  potential way  to  produce embryonic  stem  cells. In  animals,  nuclear  transfer has  been  accomplished by inserting  the  nucleus  of an  already  differentiated adult  cell—for  example, a  skin  cell—into  a  donated egg  that  has  had  its nucleus removed.  This egg, which now contains  the genetic material  of the skin cell, is then stimulated to form  a blastocyst  from  which embryonic  stem cells can be derived. The stem cells that are created in this way are therefore  copies or “clones” of the original adult  cell because  their  nuclear  DNA  matches  that of the adult  cell.

As of the summer  of 2006,  nuclear  transfer  has not been successful in the production of human embryon- ic stem cells,1  but progress in animal research suggests that  scientists  may  be able  to use this  technique  to develop human stem cells in the future.