give rise to the various types of blood cells. Embryonic Stem Cells A blastocyst (BLAST-oh-sist), is a pre-implantation embryo that develops 5 days after the fertilization of an egg by a sperm. It contains all the material necessary for the development of a complete human being. The blastocyst is a mostly hollow sphere of cells that is smaller than the period at the end of this sentence. In its interior is the inner cell mass, which is composed of 30-34 cells that are referred to by scientists as pluripotent because they can differentiate into all of the cell types of the body. In comon usage, “embryo” can refer to all stages of development from fertilization until a somewhat ill-defined stage when it is called a fetus. Scientists use terms such as “morula” and “blastocyst” to refer to precise, specific stages of pre-implantation development. In order to be as precise as possible, this booklet uses the scientific terms when describing scientific concepts but uses the term “embryo” where more precision seemed likely to confuse rather than clarify. TYPES OF STEM CELLS Embryonic stem cells are derived from the inner cell mass of the blastocyst. In culture, they can self-replicate or produce specialized cell types. TYPES OF STEM CELLS 5 In normal development, the blastocyst would implant in the wall of the uterus to become the embryo and continue developing into a mature organism. Its outer cells would begin to form the placenta and the inner cell mass would begin to differentiate into the progressively more specialized cell types of the body. When the blastocyst is used for stem cell research, scientists remove the inner cell mass and place these cells in a culture dish with a nutrient-rich liquid where they give rise to embryonic stem cells. Embryonic stem cells seem to be more flexible than stem cells found in adults, because they have the potential to produce every cell type in the human body. They are also generally easier to collect, purify and maintain in the laboratory than adult stem cells. Scientists can induce embryonic stem cells to replicate themselves in an undifferentiated state for very long periods of time before stimulating them to create specialized cells. This means that just a few embryonic stem cells can build a large bank of stem cells to be used in experiments. However, such undifferentiated stem cells could not be used directly for tissue transplants because they can cause a type of tumor called a teratoma. To be used for therapies, embryonic stem cells would first need to be differentiated into specialized cell types. Some find embryonic stem cell research to be morally objectionable, because when scientists remove the inner cell mass, the blastocyst no longer has the potential to become a fully developed human being. Sources of Embryonic Stem Cells In Vitro Fertilization: The largest potential source of blastocysts for stem cell research is from in vitro fertilization (IVF) clinics. The process of IVF requires the retrieval of a woman’s eggs via a surgical procedure after undergoing an intensive regimen of “fertility drugs,” which stimulate her ovaries to produce multiple mature eggs. When IVF is used for reproductive purposes, doctors typically fertilize all of the donated eggs in order to maximize their chance of producing a viable blastocyst that can be implanted in the womb. Because not all the fertilized eggs are implanted, this has resulted in a large bank of “excess” blastocysts that are currently stored in freezers around the country. The blastocysts stored in IVF clinics could prove to be a major source of embryonA human blastocyst, which is produced about 5 days after fertilization, is smaller than the period at the end of this sentence. NIH/Mr. J. Conaghan. ic stem cells for use in medical research. However, because most of these blastocysts were created before the advent of stem cell research, most donors were not asked for their permission to use these left-over blastocysts for research. The in vitro fertilization (IVF) technique could potentially also be used to produce blastocysts specifically for research purposes. This would facilitate the isolation of stem cells with specific genetic traits necessary for the study of particular diseases. For example, it may be possible to study the origins of an inherited disease like cystic fibrosis using stem cells made from egg and sperm donors who have this disease. The creation of stem cells specifically for research using IVF is, however, ethically problematic for some people because it involves intentionally creating a blastocyst that will never develop into a human being. Nuclear Transfer: The process called nuclear transfer offers another potential way to produce embryonic stem cells. In animals, nuclear transfer has been accomplished by inserting the nucleus of an already differentiated adult cell—for example, a skin cell—into a donated egg that has had its nucleus removed. This egg, which now contains the genetic material of the skin cell, is then stimulated to form a blastocyst from which embryonic stem cells can be derived. The stem cells that are created in this way are therefore copies or “clones” of the original adult cell because their nuclear DNA matches that of the adult cell. As of the summer of 2006, nuclear transfer has not been successful in the production of human embryonic stem cells,1 but progress in animal research suggests that scientists may be able to use this technique to develop human stem cells