Where do stem cells come from?

Stem cells are derived from two main sources: adult and embryo tissue. Embryonic stem cells are derived from 3-5 day old embryos that are in the blastocyst stage. They are extracted from the inner cell mass of the blastocyst or fetal tissue. At this stage in embryonic development, the blastocyst contains approximately 150 cells. The embryos are typically taken from surplus in-vitro fertilization techniques, and used for research after receiving consent. Blastocyst cells are pluripotent, and are able to differentiate into any cell type in the body.  

Adult stem cells typically originate from tissues including umbilical cord tissue, fat, the bone marrow, and blood. The bone marrow is abundant in adult stem cells including hematopoietic stem cells and mesenchymal stem cells. Adipose tissue has a high number of mesenchymal stem cells. The umbilical cord tissue is a rich source of hematopoietic cells. These cells are multipotent, and can only differentiate into a select number of cell types. The quantity and quality of adult stem cells decreases overtime due to aging. It is important to note that when compared in quantity to other cells, there are only a small quantity of stem cells in each tissue. Because of this and their growth rate, it is often difficult to produce adult stem cells in large numbers.