Stem Cells, Biotechnology
Every tissue and organ in the human body is made up of different types of cells. Cells that make up skin, for example, are different from those that make up the heart. This makes it impossible for cells that make up one tissue or organ to be transferred to another tissue or organ.
However, all cells share one thing in common - they come from one cell source. In the early stages of human development, these cells can become any tissue or organ - that is, they have not yet become specialized. These cells are called stem cells.
Stem cells have two important characteristics that make them different from other types of cells. First, as noted above, all stem cells are unspecialized, and renew themselves for long periods of time through cell division. Second, under certain biochemical cues they can be made to differentiate (see below). This means that they can divide into cells with special functions, such as the beating cells of the heart muscle or the insulin-producing cells of the pancreas.
Types of stem cells
Stem cells come in three forms: embryonic stem (ES) cells, embryonic germ cells, and adult stem cells. ES cells come from embryos; embryonic germ cells come from testes, and adult stem cells can come from bone marrow. Scientists work mostly with ES cells and adult stem cells.
ES cells are found in the early beginnings of human life - at the blastocyst stage of human development. This stage is from four to five days after the union of the sperm and the egg, before the embryo implants in the uterus. The 20 or so stem cells in the blastocyst are called pluripotent, which means they are capable of forming all embryonic tissues, but they cannot form a complete organism without support from the placenta.
ES cells come from embryos, fetuses and some can come from the blood of the umbilical cord after birth.
The process by which the stem cell can become a specific cell type is called differentiation. Stem cell differentiation begins when they are exposed to certain biochemical cues - either physiological or experimental. Biochemical cues in different parts of the body stimulate stem cells to grow into the specific cells needed in that location.
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