If fertilization happens outside of the body ( in vitro fertilization), it is possible to remove the internal cells of the embryo at this stage and grow them in the lab. About 1 week after fertilization, cell division has created a tiny embryo containing outer cells that will become the placenta (gray-blue), and inner cells that will become the embryo (yellow). Once the two meet, the egg is fertilized and starts to develop. On the left, you can see the egg and the sperm prior to fertilization.Figure 1 - Early embryonic development and pluripotent cells.So, there is a relatively narrow window during which pluripotent ESCs exist in the embryo. About a week after fertilization, the embryonic cells gradually lose their pluripotency and gradually become the various tissues and organs. Because of this ability, these cells are called pluripotent (“pluri” = a lot “potent” = ability). ![]() Since this small number of cells will become a complete baby, these early cells must have the ability to become every cell in the body, like skin cells, muscle cells, liver cells, or brain cells. About a week after fertilization, the developing embryo looks like a hollow ball of cells, which will later become the placenta, and an internal pile of cells, which will become the embryo itself ( Figure 1). The fertilized egg first divides into two cells, then each of these divides into two more cells and so on until, a few days later, we have a small ball made up of a few dozen embryonic cells. From the moment of fertilization, the fertilized egg goes through a process of cell division. Fertilization brings together the genetic material ( DNA) from both parents, half from the egg and half from the sperm, and this combination of genetic material produces the embryo. Today, scientists can change almost every type of cell into almost every other type of cell! It All Begins in Fertilization, When the Sperm Meets the EggĮmbryonic development begins the moment of fertilization, when the sperm meets the egg ( Figure 1). In 2007, researchers found a way to produce human cells with the abilities of ESCs by reprogramming regular cells so that they become stem cells. While ESCs offer promising and exciting opportunities, like the possibility of growing organs in the lab, the production of ESCs requires human embryos, which involves many technical and ethical problems. Scientists managed to grow these embryonic cells in the lab, and named them embryonic stem cells (ESCs). In the early stages, the microscopic embryo is made up of cells that have the potential to develop into all types of cells. Each cell then divides into two additional cells and so on until, after a few days of cell division, a tiny embryo develops. The fertilized egg cell divides into two cells. Fertilization occurs when the sperm cell meets the egg cell. But all human life begins with the encounter between two cells: the sperm cell from the father and the egg cell from the mother. ![]() ![]() Each tissue and organ of the body contains cells that are specialized to perform specific jobs–the liver contains liver cells, the brain contains neurons, the eyes contain light-detecting cells, and so on. All living things, including humans, are made of cells.
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