Which is NOT a main source of stem cells?

Adult liver is considered the option that does not serve as a main source of stem cells. While the other options, such as adult bone marrow, specifically created embryos, and umbilical cord blood, are well-documented sources of stem cells, adult liver tissues are not typically recognized for their stem cell populations.

Adult bone marrow is a prominent source of hematopoietic stem cells, which can differentiate into various blood cells. Specifically created embryos, or embryonic stem cells, have the potential to develop into any cell type in the body, which is fundamental for research and therapeutic applications. Umbilical cord blood is rich in stem cells as well, mainly hematopoietic stem cells, which can also contribute to blood and immune cell development.

On the other hand, while the liver does contain some progenitor cells, it is not considered a primary or readily accessible source for stem cell research or therapy compared to the other listed options. This distinction highlights the unique role that specific tissues, like bone marrow and umbilical cord blood, have in regenerative medicine and developmental biology.