What are the sources of stem cells?
Embryonic stem cells (ESCs).All humans start their lives from being a single cell, called the zygote, which is formed by the union of sperm and egg. The zygote divides and at 4- to 5-days-old, the embryo is known as a blastocyst. Each blastocyst consists of 200 cells. The inner cell mass contains about 50 pluripotent cells which go on to form all the cells of the body. Embryonic stem cells come from excess fertilized eggs stored in freezers at in-vitro fertility (IVF) clinics. Currently, there are over 400,000 unused frozen embryos in U.S. fertility clinics, which could be donated for research purposes with the informed consent of the donors. Adult stem cells are undifferentiated cells that are found in small numbers in most adult tissues. The primary roles of adult stem cells in the body are to maintain and repair the tissues in which they are found. They are usually thought of as multipotent cells, giving rise to a closely related family of cells within the tissue. An example is hema