How many types of stem cells are there?
There are two basic types of stem cells, embryonic and adult. Embryonic stem cells (ESCs) are only found in four- to 14-day-old embryos. At this point the embryo is a microscopic cluster of about 150 cells called a blastocyst. Between 30 and 50 cells inside the blastocyst are stem cells. After the embryonic stems cells have changed or specialized, they are referred to as adult or tissue stem cells. There are many varieties of adult stem cells, including blood-forming stem cells, skin stem cells, muscle stem cells, nervous system stem cells, etc. Adult stem cells are found in specific body tissues and are also found in the umbilical cord.
