It is important to be clear, first of all, about the embryo from which stem cells are extracted. It is not implanted and growing in a woman's uterus. It is not a fetus. It has no recognizable human features or form. It is, rather, a blastocyst, a cluster of 180 to 200 cells, growing in a petri dish, barely visible to the naked eye. Such blastocysts are either cloned in the lab or created in fertility clinics. The bill pending in Congress would fund stem cell research only on excess blastocysts left over from infertility treatments.
The blastocyst represents such an early stage of embryonic development that the cells it contains have not yet differentiated, or taken on the properties of particular organs or tissues -- kidneys, muscles, spinal cord, and so on. This is why the stem cells that are extracted from the blastocyst hold the promise of developing, with proper coaxing in the lab, into any kind of cell the researcher
wants to study or repair.