Jackie Skiba, an undergraduate research student in our lab, has been taking the new cryostat out for its shakedown run this summer:

Jackie Skiba preparing thin sections of zebrafish larvae

Stained section through a 3-day old zebrafish eye

Mole crab molts on the beach in Southern Shores, North Carolina

Adult mole crabs start mating in early spring and go through to fall.  Like other arthropods, males copulate with females and fertilize eggs internally.  The females then hold the developing embryos under their abdomens for 2-3 weeks (see image below), after which the larvae leave the females and live on their own in near shore areas.  The larvae then leave the water column to settle back into the surf zone in June/July and September/October, where the juveniles and adults feed on small plankton and detritus in the swash zone of the beach.  The molts I have been finding on the beach this week are all about 2 cm in length, and may be from actively mating crabs.  We did find some crabs in the swash zone back in April, but not this large number of molts.  Are the females molting prior to mating, like in blue crabs?

Female mole crab with egg mass

A very interesting study examined whether female mole crabs time the release of their larvae.  They found that the larvae do in fact leave the females in quick 5-15 minute bursts just after it becomes dark.  What is really interesting is that the larvae themselves control this timing, as the rhythmicity is also seen in egg masses that have been removed from the females.  And this rhythm continues in constant darkness, showing that it is due to some internal clock in the embryos, not simply a response to darkness.

These little ubiquitous beach crabs can pull off some impressive tricks.

This was also the second year of ARVO’s meeting blog, and my second year of contributing.  This turned out to be a good way to share information from meeting veterans, and learn some new faces and names.

Here is a taste of our trip (thanks to my colleague Patty Saunders for serving as trip photographer).  Still to come, some video and food highlights:

Our welcome to the beach after 15 hours on the road

Breaking camp the first morning

Kayaking out of Manteo harbor with the Queen Elizabeth II in the background

Official portrait on the Manteo dock

Lots of beachcombing, and a mini-study on how Oregon Inlet affects shell deposition on the beach

Beach seining yielded some small blue crabs, croaker (or spot), silverside and shrimp. The water was cold, but it was worth it.

Juvenile dolphins playing off the beach just south of Oregon Inlet

. . . and a large group of royal terns. You can see Bodie Light wrapped up in the distance while it gets a refurbished Fresnel lens (right side of picture).

Birding on Pea Island

At the top of Hatteras Light, after a great history lesson in the failures of beach stabilization by a Lighthouse volunteer

One of many beautiful sunsets over the dunes

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

So I present my first Flip video, edited on Flip software and annotated on YouTube.  The topic is the three types of breakers found on sandy beaches – something we are talking about in my marine bio class this coming Monday.  Just to entice you, there are dolphins, and my daughter says something funny at the end.  Helpful comments always appreciated.

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