Sea invertebrate sheds light on evolution of human blood, immune systems -- ScienceDaily"The mammalian and Botryllus blood-forming systems also share hundreds of homologous genes, even though the two species are separated by over 500 million years of evolution," said former postdoctoral scholar Benyamin Rosental, PhD. Rosental shares lead authorship of the study with graduate student Mark Kowarsky. The senior authors are Irving Weissman, MD, the Virginia and D.K. Ludwig Professor for Clinical Innovation in Cancer Research and professor of pathology and of developmental biology; Stephen Quake, PhD, the Lee Otterson Professor in the School of Engineering and professor of bioengineering and of applied physics; and senior research scientist Ayelet Voskoboynik, PhD. The researchers isolated the Botryllus stem cells that are the foundation of its blood and immune system, as well as the progenitor cells they make on their way to becoming adult blood and immune cells. "Out of all the invertebrates, the Botryllus blood stem cells and progenitors are the most similar to vertebrate blood cells, so it is possible, if not likely, that they are the 'missing link' between vertebrates and invertebrates," said Weissman, who also directs the Stanford Institute for Stem Cell Biology and Regenerative Medicine and the Ludwig Cancer Center at Stanford.