Are there more primitive types of stem cells hidden among blood stem cells, awaiting discovery?

Answer: Almost certainly. Examples:

Press Release 1 Nov 2000 from StemCells, Inc. (Nasdaq: STEM) “First Identification of Purified Blood Stem Cells as a Source of Mature Liver Cells; Published in Nature Medicine Nature 20 June 2002 vol.417. Catherine Verfaillie of the U. Minnesota Medical School and her team claim to have extracted “multipotent adult progenitor cells” from adult bone marrow which can turn into every type of tissue in the body, just like embryonic Stem Cells. Press release 19 July 2004 from Viacell and NETCORD announces the first demonstration that neonatal “unrestricted somatic Stem Cells” (USSCs) can be robustly expanded in vitro (in the lab) to very large numbers and can differentiate, in vivo (in living humans), into a number of tissue types and take on the properties and specific functions of the cells in those tissues: blood, bone, cartilage, heart, liver, and nerve cells. Reference:

• “A new human somatic Stem Cell from placental Cord Blood with intrinsic pluripotent differentiation potential”; published in the peer-reviewed article by Kogler et al., Journal of Experimental Medicine (JEM) 2004 Jul 19;200(2):123-35.

In May 2005, the biotechnology company BioE, based in St. Paul, MN, becomes the first company to commercially market multi-potent Stem Cells extracted from Cord Blood. Their “Multi-Lineage Progenitor Cellsâ„¢ (MLPCsâ„¢)” have differentiated into tissues representative of neural stem cells, nerve cells, liver/pancreas precursors, skeletal muscle, fat cells, bone cells and blood vessels.