Blood stem cells may come in different subtypes

I love showing students new research that will ultimately lead to a revision in their textbooks.  Hey, something has got to make purchasing a new edition every two to three years seem worthwhile.  And it is even more fun when these research headlines come out as we are covering that very topic in class.  A new paper this past week from Cell Stem Cell demonstrating that there may be more than one type of blood stem cell fit the bill.

Your average anatomy and physiology  textbook shows that all of the different cell types in our blood, such as the red blood cells that carry oxygen and the white blood cells that contribute to our immune system, develop from one stem cell type called a hemocytoblast (see figure below).  And because of the importance of understanding the function of blood stem cells to treating many diseases, such as leukemia, this area has attracted lots of research.

A textbook description of blood cell formation.

The hemocytoblast is called a multipotent stem cell because it maintains the ability to differentiate into the different types of blood cells.  This flexible stem cell “commits” to a different developmental pathway by expressing receptor proteins on its surface for different signaling molecules, that will in turn tell it what to become.  The paradigm has been that there is only one type of hemocytoblast, that only becomes committed when a receptor protein is placed on the cell surface.  But studies have hinted at the presence of more than one type of hemocytoblast, and a research team based at the Baylor School of Medicine has now identified two of them.

The researchers were able to identify and purify two mouse bone marrow stem cell types based on a difference in their interaction with a common cellular dye.  When these purified stem cell types were transplanted into mice, the researchers found that each type preferred to make either red blood cells or immune cells.  This preference was maintained in the stem cell population as each individual hemocytoblast type produced copies of itself, suggesting that the bias was programmed into the cell.

It is not known how these two stem cell types differ, or what mechanism leads to the bias in blood cell production.  The researchers found that each stem cell type responded differently to a common signaling molecule used in cellular differentiation, suggesting a possible mechanism.  It is possible that there may still be a homogenous population of hemocytoblast precursor cells that predates the differentiation into these two newly found subtypes.  But clearly, knowing about the presence of blood stem cells with different behaviors will be important for scientists attempting to harness these cells to treat human disease.  I hope to read more about it in the next edition of our textbook.

Challen, G., Boles, N., Chambers, S., & Goodell, M. (2010). Distinct Hematopoietic Stem Cell Subtypes Are Differentially Regulated by TGF-beta1 Cell Stem Cell, 6 (3), 265-278 DOI: 10.1016/j.stem.2010.02.002