The Beginning: Embryonic vs. Adult Stem Cells
When stem cells were first identified, only two types of stem cells were identified, adult stem cells and embryonic stem cells. However, a diverse range of stem cell types now exist, because human embryonic stem cells have become further characterized, the diversity of adult stem cell types has substantially expanded, and induced pluripotent stem cells (iPSCs) have since been discovered.
Now, researchers debate the number of stem cells in existence, because the number depends on whether stem cells are characterized functionally (for example, neural stem cells, mesenchymal stem cells, and hematopoietic stem cells) or characterized by source tissue (for example, cord blood stem cells, dental pulp stem cells, and amniotic stem cells).
What is clear is that a wide range of stem cell types now exist, which has added substantial complexity to the stem cell landscape.