Harvesting Embryonic Stem Cells     When a sperm fertilizes an ovum, a remarkable cascade of events called embryogenesis is set in motion. The fertilized egg (zygote) divides, and repeated cell division produces more cells. After four or five days, the embryo resembles a sphere, referred to as a blastocyst. The surface of the sphere is formed by cells known as trophoblasts, which will become the placenta. On the inside, the blastocyst is mostly hollow and filled with fluid. But a small clump of cells (about 30--100) is attached to the inner surface of the sphere. This clump, called the inner cell mass, will develop into a fetus.     Most embryos, of course, are created the old-fashioned way: in the mother's body. But through in vitro fertilization techniques, which debuted in the late 1970s, millions of embryos have been produced in laboratories worldwide, to help infertile couples achieve pregnancy. Those embryos that are not implanted or discarded are frozen and stored--sometimes indefinitely. A recent estimate puts the number of frozen embryos at over one million.     For those on a quest for human stem cells, excess embryos present an opportunity. They can be cultured to the blastocyst stage and the cells of their inner cell mass excised. These cells, referred to as embryonic stem (ES) cells, are expected to have the ability to grow into most or nearly all types of cells in the body. (These abilities are referred to as pluripotency and totipotency, respectively).     Recent research [see main text] has shown that if ES cells are cultured under special conditions in a laboratory, they will grow and divide indefinitely. Thus, starting with a single parent cell, one can create a "cell line," a mass of cells descended from the original, sharing its characteristics. Batches of cells can then be separated from the cell line and distributed to other researchers. In this manner, one can generate a continuous supply of ES cells. ----M.T.L. back