The origin of blood stem cells

The development of cell biology research coincides with the advance of microscopes in the 19th century. It was finally possible to directly observe the various blood cell types and to witness their proliferation and differentiation (Mazzarello, 1999). On the basis of his observations, the German pathologist Franz E.C. Neumann (1834–1918) suggested that the site of blood formation was the bone marrow (BM). He also proposed the pioneer theory in which one cell might be at the origin of all blood cell lineages. The Russian scientist Alexander A. Maximow (1874–1928) also developed and introduced the theory of a common cell for the complete blood-building system or hematopoiesis (Maximow, 1909). The concept of Hematopoietic Stem Cells (HSCs), although very controversial at the time, was born and has led to the beginning of stem cell research (Ramalho-Santos and Willenbring, 2007).