Embryonic Stem Cells

Embryonic stem cells are derived from the inner cell mass of the blastocyst. In culture, they can self-replicate or produce specialized cell types.

A blastocyst  (BLAST-oh-sist), is a pre-implantation embryo  that  develops 5 days after the fertilization  of an egg by a sperm. It contains  all the material  necessary for the development  of a complete human being. The blasto- cyst is a mostly hollow sphere of cells that is smaller than the period at the end of this sentence.
In its interior is the inner cell mass, which is composed of 30-34 cells that are referred  to by scientists as pluripotent because they can differentiate into  all  of  the  cell  types  of  the  body.  In comon  usage, “embryo” can refer to all stages of devel- opment  from  fertilization   until  a  somewhat ill-defined stage when it is called a fetus. Scientists use terms such as
“morula” and  “blastocyst” to refer  to precise,  specific stages of pre-implantation development. In order to be as precise as possible, this booklet  uses the scientific terms when describing  scientific  concepts  but  uses  the  term “embryo” where  more  precision  seemed likely to  con- fuse rather  than  clarify.
In normal development, the blastocyst  would implant in the wall of the uterus  to become the embryo  and continue developing into a mature organism.  Its outer cells would  begin to form the placenta  and the inner cell mass would  begin  to differentiate into  the pro- gressively more specialized cell types of the body.

When  the blastocyst  is used for stem cell research, scientists remove the inner cell mass and place these cells  in  a  culture  dish  with  a  nutrient-rich  liquid where they give rise to embryonic stem cells. Embryonic  stem cells seem to be more flexible than stem cells found in adults, because they have the potential to  produce  every  cell type  in the  human body. They are also generally easier to collect, purify and maintain  in the laboratory than adult stem cells.

Scientists can induce  embryonic  stem cells to repli- cate themselves in an undifferentiated state for very long periods of time before stimulating them to cre- ate specialized cells. This means that just a few embryonic  stem cells can build a large bank of stem cells to be used in experiments. However,  such undifferentiated stem cells could not be used direct- ly for  tissue  transplants because  they  can  cause  a type of tumor  called a teratoma. To be used for ther- apies,  embryonic  stem cells would  first  need  to  be differentiated into specialized cell types.

Some find embryonic stem cell research to be morally objectionable,  because  when  scientists  remove  the inner cell mass, the blastocyst no longer has the poten- tial to become a fully developed human being.