Embryonic stem cells

Embryonic stem cells are taken from the inner part of a young embryo, also known as a blastocyst. An embryo reaches the blastocyst stage about 4-5 days after fertilization. At that point they contain about 50-150 cells.

Embryonic stem cells can differentiate into the three "germ layers": ectoderm, endoderm and [[mesoderm]. The 220 types of cells in humans are all based on these germ layers. Because of their origin, embryonic stem cells can proliferate indefinitely and can be used to study the differentiation of any type of human cell.

Embryonic stem cells currently in use have all been derived from leftover embryos from in vitro fertilization - embryos already slated for destruction. Some scientists have begun research into a process known as therapeutic cloning, which would allow production of embryonic stem cells that would match a patient's own DNA. However, this process has not yet been carried out.

Pro-life conservatives consider taking stem cells from human embryos as tantamount to abortion, even though the cells cannot possibly be obtained from an abortion procedure and the embryos used to obtain the cells were already slated for destruction.. President Bush has allowed limited funding of stem cell research. However, most of the lines currently funded have been damaged by growth conditions that are sub-optimal. President Bush vetoed a measure that would have expanded funding to include other lines created from discarded embryos from in vitro fertilization - leaving those embryos to be destroyed without purpose.