Stem cells are unlike other cells in the body because they can replenish other cells as they deteriorate. They replicate frequently and can become other types of cells, such as muscle or brain cells. Two types of human stem cells exist: embryonic and adult.
Embryonic Stem Cells
Embryonic stem cells come from human blastocysts, which are newly fertilized eggs. The embryos used in stem cell research generally are conceived through in vitro fertilization and donated for research. They are then cultured in a laboratory.
Umbilical Cord Stem Cells
Scientists have discovered the blood within the umbilical cord is rich in stem cells. Some parents of newborns opt to harvest and store the blood from their baby’s umbilical cord for possible future transplants.
Adult Stem Cells
Adult stem cells are located within the brain, blood and blood vessels, bone marrow, skeletal muscle, heart, liver, intestines, ovaries, testes, skin and teeth. Scientists believe within each organ or tissue location, the stem cells reside in a specific place called a “stem cell niche.”
As stem cells divide, they can become other types of cells. Hematopoietic stem cells become varying types of blood cells, including red blood cells, T-cells and macrophages. Mesenchymal stem cells turn into cells of other tissues, such as fat, bone and cartilage. Neural stem cells reside in the brain and give rise to neurons and other brain cells. Epithelial stem cells are located in the digestive tract and can eventually aid in digestion. Epidermal stem cells, found in hair follicles and skin layers, become protective skin cells.
Some experiments have shown stem cells to evolve into unexpected organ or tissue cells (i.e., a brain cell that transforms into a blood cell). This phenomenon is called transdifferentiation. Adult stem cells can also "reprogrammed" to become like embryonic stem cells. These cells are known as induced pluripotent stem cells.
Stem cells can be cultured in a laboratory and transplanted into people with leukemia or bone-marrow cancer. Scientists also can use stem cells to understand the origins of cancer and birth defects and to test new drugs.