Research on Human embryonic stem cells and cloning for stem cells

Abstract

Within five days, the human fertilized egg (oocyte) develops into a blastocyst, a stage which is reached before the embryo implants into the maternal womb. An increasing number of in vitro fertilization (IVF) treatments are being performed to assist infertile couples achieve a pregnancy, and the resulting embryos are often cultured to the 5-day blastocyst stage because it increases the chances for successful implantation. In natural human reproduction, no more than half of the blastocysts succeed to implant and initiate pregnancy. In IVF treatments, the rate of successful implantation is lower (35-40 percent) and hence surplus embryos are produced in order to spare the mother the need for repeated hormonal stimulation and egg collection. Such supernumerary embryos are stored in the frozen state at the temperature of liquid Nitrogen, and if not required for further reproductive use, their only alternative is indefinite storage or destruction. It is estimated that there are 400,000 surplus frozen embryos in fertility clinics in the USA alone.