Q. What are human embryonic stem cells?

A. The human body consists of approximately 200 cell types. Each of these cells has a special task to perform: A brain cell differs from a liver cell, a muscle cell from a heart cell, etc. Because of these differences, these cells are sometimes called "differentiated."

When human embryos begin to develop, their cells are not yet differentiated. Scientists now know that all of the body’s different cell types STEM from a master cell with the remarkable potential to generate all the cell types the body needs.

During the first week of development, when the human embryo is still a hollow sphere, these master cells emerge. These cells are called “embryonic stem cells,” a term coined by UCSF researcher Gail Martin. Martin co-discovered the cells in mice in 1981.

Scientists are now able to isolate human embryonic stem cells (usually from donated human embryos left over following fertility treatments) and study them in the laboratory. Their goals:

• To find the signals (the right mix of growth factors, for example) that make these master cells specialize into different cell types.
  
• To craft and transplant exactly the right cells that patients need to stabilize or perhaps even cure such diseases as diabetes, heart disease, Parkinson’s, ALS (Lou Gehrig’s disease), spinal cord injury and osteoporosis.
  
• To study the steps of embryonic stem cell differentiation into specialized cells, in order to identify the genetic missteps that cause diseases such as diabetes, amyotrophic lateral sclerosis (ALS) and some cancers, and that account for some cases of birth defects and infertility.

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