Zebrafish used to visualize blood stem cell generation

Understanding how blood cells are formed is not only important for developing treatments against numerous diseases, but also teaches us more about the fascinating process of turning stem cells into their specialized descendants.  Recent work suggests that the initial stem cell that produces all of our blood’s formed elements (cells) comes in two flavors.  But how do these initial stem cells arise?

Two new studies in the journal Nature have leveraged the unique powers of the zebrafish as a model vertebrate to provide answers to this question.  George Streisinger of the University of Oregon first developed this cute little pet store fish as a tool to study vertebrate development and gene function in the 1970s.  It has since become a prominent player in many areas of biomedical research, and is my model of choice for studying lens development, evolution and cataract.  Its use of external fertilization and a see-through egg makes it ideal for visualizing the early stages of development.  And with basic molecular techniques you can make specific cell types light up with green fluorescent protein (GFP).  This basic approach has now been used to provide further evidence that the initial source of blood stem cells is the lining of the aorta, the largest blood vessel leaving the heart.

Previous studies in mice suggested that hematopoietic stem cells (HSCs: which will become all types of blood cells) arise from the endothelial cells lining the ventral surface of the aorta.  David Travers’ group at UCSD labelled aortic endothelial cells with GFP and used confocal microscopy to show them moving from the endothelium into the bloodstream (Movie 1).  But unlike a proposed mechanism for mammals, these zebrafish HSCs do not enter the arterial bloodstream, but instead move into a neighboring vein.  While this detail differs between zebrafish and mammals, Travers’ work shows that similar molecular signaling coordinates the production of the HSCs in both taxa.  And in a very cool experiment, they used flow cytometry to isolate these new putative HSCs from zebrafish embryos and confirmed that they indeed became blood stem cells.

Movie 1. Live imaging of green HSCs leaving the aortic endothelium.

In the second Nature paper, Kissa and Herbomel from the Pasteur Institute in Paris used confocal microscopy to detail how new HSCs can be removed from the lining of the aorta without damaging the integrity of this tube.  They document that the differentiating HSCs fold over like a burrito, bringing together the neighboring endothelial cells and joining them together before leaving the tube (Figure 1).  This study also confirms that zebrafish HSCs enter the bloodstream through the neighboring vein, not the aorta, and that the process shares similar signaling to mammals.  When the authors used synthetic RNA molecules called morpholinos to stop the expression of a known mammalian signaling molecule called Runx1, the movement of HSCs from the aortic lining was highly reduced.

Figure 1. Detachment of HSCs (labeled in green) from the endothelial lining of the zebrafish dorsal aorta. The arrowhead in panel F shows folding in the HSC pulling together two neighboring endothelial cells before it leaves the aorta.

So what do these papers add to our understanding of HSC generation?  While the source of these cells was already thought to be the endothelial lining of the aorta, these new studies provide the first live visualization and physical description of this process.  And while the physical details of the process differ between zebrafish and mammals, the molecular signaling seems to be the same, suggesting that the zebrafish can be a valuable model for further detailing the generation of HSCs and their development into blood stem cells.  These studies are just one new example of the zebrafish’s growing influence in biomedical studies.

Bertrand, J., Chi, N., Santoso, B., Teng, S., Stainier, D., & Traver, D. (2010). Haematopoietic stem cells derive directly from aortic endothelium during development Nature, 464 (7285), 108-111 DOI: 10.1038/nature08738

Kissa, K., & Herbomel, P. (2010). Blood stem cells emerge from aortic endothelium by a novel type of cell transition Nature, 464 (7285), 112-115 DOI: 10.1038/nature08761