Several Questions About Embryonic Stem Cells

1. What are embryonic stem cells?

Embryonic stem cells are cells derived from embryos. Most embryonic stem cells are derived from embryos that develop from eggs that have been fertilized in an in vitro fertilization clinic and then donated for research purposes with informed consent of the donors. They are not actually derived from eggs fertilized in a woman's body.

2. How are embryonic stem cells cultured in the laboratory? 

Human embryonic stem cells (hESCs) are generated by transferring cells from a preimplantation-stage embryo into a plastic laboratory culture dish that contains a nutrient broth known as culture medium. The process of generating an embryonic stem cell line is somewhat inefficient, so lines are not produced each time cells from the preimplantation-stage embryo are placed into a culture dish.

However, if the plated cells survive, divide and multiply enough to crowd the dish, they are removed gently and plated into several fresh culture dishes, which will be repeated many times and for many months. Each cycle of subculturing the cells is referred to as a passage. Once the cell line is established, the original cells yield millions of embryonic stem cells.

3. How to identify embryonic stem cells?

At various points during the process of generating embryonic stem cell lines, scientists test the cells to see whether they exhibit the fundamental properties that make them embryonic stem cells. This process is called characterization.

Scientists use several kinds of tests, such as examining the chromosomes under a microscope, so as to assess whether the chromosomes are damaged or if the number of chromosomes has changed,  determining whether the cells can be re-grown, or subcultured, after freezing, thawing, and re-plating and testing whether the human embryonic stem cells are pluripotent.

4. How are embryonic stem cells stimulated to differentiate?

As long as the embryonic stem cells in culture are grown under appropriate conditions, they can remain undifferentiated (unspecialized). When cells are allowed to clump together to form embryoid bodies, they begin to differentiate spontaneously, forming into muscle cells, nerve cells, and many other cell types.

Scientists try to control the differentiation of embryonic stem cells to generate cultures of specific types of differentiated cells. They change the chemical composition of the culture medium, alter the surface of the culture dish, or modify the cells by inserting specific genes. Through years of experimentation, some basic protocols or "recipes" for the directed differentiation of embryonic stem cells have been established.

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