Stem Cells
Stem cells are characterized as undifferentiated cells that have the ability to differentiate into a variety of specialized cell types  In most cases, stem cells originate either during embryonic development or are found in adult tissue. Adult stem cells are found throughout the body in various tissues such as skin, liver, blood and brain. These cells can rejuvenate into many types of cells within the host organ and even have potential to rebuild an entire organ if damaged.  Embryonic stem cells originate from embryos that have been fertilized by means of in vitro fertilization.  Just as adult stem cells have the ability to differentiate, embryonic stem cells will give rise to three primary germ layers during embryogenesis.