Human embryonic stem cells are made from 4- to 6-day-old embryos that have been created in laboratories, usually fertility clinics. The inner cells from the ball-shaped embryos are isolated and placed in a dish along with the nutrients they need to grow.
Embryonic stem cells are currently made from very early preimplantation embryos originally created for in vitro fertilization, although researchers are working on other techniques. These embryos, or blastocysts, are hollow balls of cells with a small thickening at one side. With the right combination of skill and luck, scientists can separate these cells, place them in laboratory dishes and grow them. This process destroys the embryo.
The inner cells are placed in sterile dishes with the nutrients they need to grow. In a matter of months, the 30 or so cells collected from each blastocyst can divide to make millions and millions of embryonic stem cells. All the embryonic stem cells generated from a single embryo are called an ESC line.
Making a cell line is easier said than done. Most attempts to grow human embryonic stem cells fail. When the cells do grow, scientists must make sure that the cells have the correct number of chromosomes and pass a series of other tests for characterising them. For example, the cells must be able to form all three major types of tissue.
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How are embryonic stem cell lines made and grown?. Nat Rep Stem Cells (2007). https://doi.org/10.1038/stemcells.2007.17