Such embryos are produced in the 'test-tubes' for infertile couples, but often more numbers of embryos are produced than needed and stored in case there is abortion of implanted embryo. By having multiple embryos there in no requirement of taking out egg or sperm and going though the whole process.

Embryonic stem cells, derived from surplus embryos, can be programmed to become any cell type of the body. 

They also have the capacity to keep proliferating indefinitely in a culture medium.

Embryonic stem cells are derived from the cells that make up the inner cell mass of the blastocyst. Embryonic stem cells are undifferentiated cells that are unlike any specific adult cell and are of great interest to science because of their ability to develop into virtually any other cell type made by the human body.

These embryonic stem cells can proliferate indefinitely in culture and can produce unlimited source of cells that can differentiate into specific and important adult cells such as bone, muscle, liver or blood cells.

Mouse embryonic stem cells are capable of generating to any cell types in the body. So, they are called as pluripotent and have unlimited potential as far as growth and differentiation. Human embryonic stem cell lines are currently being studied and several research teams are working to determine whether or not they possess the same properties as mouse embryonic stem cells.

Important application of human embryonic stem cell would be in the field of drug discovery. As these cells have the ability to grow and produce specific cell types.

Embryonic stem cells help to know the developmental events in humans, which would help to prevent or treat birth defects, infertility and pregnancy loss. A thorough knowledge of normal development could ultimately allow the prevention or treatment of abnormal human development.