Stem cells are found at different stages of human development and can be categorized based on the level of development, the range of cells produced, or the types of cells produced. These categories often overlap with each other.
The earliest stem cells in the human body, embryonic stem cells, are produced after an egg is fertilized. Fetal stem cells are produced after an embryo has attached to the walls of the uterus. Adult stem cells are the only stem cells found in the adult human body—these stem cells are responsible for replacing cells lost naturally throughout life. Adult stem cells are found throughout the body in areas such as the skin, muscle, and bone. As we age, our stem cell counts decline, resulting in a lesser ability for the body to naturally heal itself.
Stem cells can be further categorized by their ability to differentiate into the various cells of the body. Pluripotent stem cells are capable of differentiating into stem cells of all the germ layers of the body. These germ layers include the mesenchymal, ectodermal, and endodermal layers. Fetal and embryonic cells are pluripotent stem cells. Multipotent stem cells, on the other hand, have a limited range of cells that they can differentiate into (bone, adipose, cartilage, nervous tissue, etc.). Multipotent stem cells are the stem cells found in the adult body.
Mesenchymal stem cells differentiate into tissues of the mesenchymal germ layer, forming bone, cartilage, muscle, and fat cells. The stem cells used in Stem Cell Therapy come from this layer. Ectodermal stem cells differentiate into tissues of the ectodermal germ layer, forming skin and nerve cells that make up the dermis, nervous system, sensory organs, and gut. Endodermal stem cells differentiate into tissues of the endodermal germ layer, forming a majority of the digestive and urinary tracts, as well as the liver, gall bladder, pancreas and the mucosal lining in the lungs.