Flow means 'go' for proper lymph system development

July 27, 2015, University of Pennsylvania School of Medicine
Lymph flow drives development of lymphatic valves in collecting lymphatic vessels in the embryonic mouse mesentery. Two transcription factors: PROX1 (green) and FOXC2 (red) are highly unregulated in the valve endothelial cells, and are required for development of the valves. The extracellular matrix protein Laminin-alpha5 (blue) is expressed in the valve leaflets and in the arterial basement membrane of the underlying artery. Credit: Daniel Sweet, PhD and Mark Kahn, PhD, Perelman School of Medicine, University of Pennsylvania

The lymphatic system provides a slow flow of fluid from our organs and tissues into the bloodstream. It returns fluid and proteins that leak from blood vessels, provides passage for immune and inflammatory cells from the tissues to the blood, and hosts key niches for immune cells. How this system develops hasn't been well understood, but now researchers from the Perelman School of Medicine at the University of Pennsylvania have found from experiments in mice that the early flow of lymph fluid is a critical factor in the development of mature lymphatic vessels.

"Once the primary are in place, an enormous amount of maturation has to happen, and what we've found is that the maturation process is triggered by physical forces from the earliest flow of in a developing embryo," said senior author Mark L. Kahn, MD, the Edward S. Cooper, M.D./Norman Roosevelt and Elizabeth Meriwether McLure Professor in the department of Medicine.

The findings, published online ahead of the August issue of the Journal of Clinical Investigation, represent a big step forward in the basic scientific understanding of lymphatic system development. They also should contribute to a better understanding of lymphatic disorders, including the lymphedema that affects many women following breast cancer surgery.

Mice With No Lymph Flow

The project was prompted in part by recent studies of cultured lymphatic vessel endothelial cells that suggested that fluid forces could be an important factor in the maturation of lymphatic vessels. But there was no straightforward way to test this hypothesis in live animals.

"No one had been able to come up with a way to stop lymph flow in embryonic animals, without preventing their lymphatic vessels from developing in the first place," said first author Daniel T. Sweet, PhD, a postdoctoral fellow in the Kahn Laboratory.

In humans and other mammals, the lymphatic system is a low-flow system that, unlike the blood circulatory system, has no central pump. Instead it relies on muscular contraction of the mature lymphatic vessel wall and small valves in lymphatic vessels to squeeze fluid along and prevent backflow.

Larger collecting lymphatic vessels receive many small inflows from lymphatic capillaries in surrounding tissues. "You can't just close one of the vessels to block flow downstream—there are many other tributaries coming in," said Sweet.

The solution to testing the role of flow in developing lymphatic vessels turned out to be a line of transgenic mice, developed earlier by the Kahn laboratory. Lacking a gene called Clec2, the mice fail to produce a certain receptor on blood platelets, with a remarkable result: veins that normally help drain the leak back into it. Like an ocean tide surging into a river, this upstream flow of blood stops the usual downstream flow of lymph.

Effects of No-flow

With fluorescent tracers, Sweet confirmed that the Clec2-knockout mice, as they developed, had no net flow of lymph. The animals did grow normal-looking primary network of lymphatic vessels, but as prior lab-dish studies had hinted might happen, those vessels failed to develop valves.

Unexpectedly, Sweet, Kahn and colleagues found that two other major features of the also failed to mature properly in these no-flow mice.

Normally after the initial phase of development of lymphatic vessels, a remodeling phase occurs in which the system makes itself more efficient. The vessels form a hierarchical structure in which smaller vessels upstream feed into larger vessels downstream. In the no-flow mice, this remodeling failed to occur.

What's more, the researchers found that smooth muscle cells, which normally form a thin lining in mature lymphatic vessels, and perform contractions that help lymph flow, instead formed an abnormally thick lining, as if the usual signal to shut off cell proliferation were missing.

"We started off thinking that flow might have a role just in valve formation, but ended up realizing that flow is really controlling all aspects of the maturation of these lymphatic vessels," said Sweet.

To demonstrate that these effects were due to the lack of lymph flow, not to a more direct impact of Clec2 deletion on lymphatic vessels, the researchers shut off Clec2 just in platelets, not in other cells, and saw the same results.

Cell culture studies, performed with Juan M. Jiminez, PhD, in the laboratory of collaborator Peter F. Davies, PhD, a professor of Pathology and Laboratory Medicine, also confirmed that the physical flow of fluid across lymphatic vessel cells could on its own stimulate gene expression changes closely resembling those known to direct lymphatic valve development.

Curiously, these cells appeared to need not just a forward movement of fluid, but at least a brief reversal, to stimulate gene expression changes. "It's like three steps forward, one step back, and we think that this flow-reversal is what induces the signaling," said Sweet.

The researchers now plan to study in more detail how cells turn the mechanical forces from lymph flow into the changes in gene expression needed for the maturation of lymphatic vessels. Another follow-on will be to investigate whether similar flow-related mechanisms drive the maturation of vessels and valves in the blood circulatory system.

Kahn expects that the research will also yield useful insights for treating lymphatic disorders, even those that occur in adults. One of the most common is the arm-swelling lymphedema that can follow the removal of armpit lymph nodes during . "We suspect that the flow of lymph works even after maturity, to maintain the appropriate molecular and genetic environment for these lymphatic vessels and valves," said Kahn. "Interrupting that flow might therefore have long-term adverse effects on those vessels and valves, so it raises the question of whether we need to restore the flow quickly at the time of lymph node removal."

Explore further: Impact of Type 2 diabetes on lymphatic vessels identified

Related Stories

Impact of Type 2 diabetes on lymphatic vessels identified

July 14, 2015
Approximately 28 million Americans live with Type 2 diabetes, a condition characterized by high blood sugar levels. Until now, the disease's effect on the body's lymphatic vessels has been unknown. A study by University of ...

Researchers describe regulatory protein controlling the patterning of the lymphatic system

June 2, 2015
The lymphatic vasculature is vital for the function of the immune system, but its development is poorly understood in comparison to that of the blood vasculature from which it arises. The vessels that make up the lymphatic ...

The origin of the lymphatic vasculature uncovered

March 12, 2015
In a new study, published in Cell Reports, researchers at Uppsala University describe a novel mechanism by which lymphatic vessels form during embryonic development. The finding may open new possibilities for repairing damaged ...

Unraveling the link between brain and lymphatic system

June 15, 2015
In a study published in The Journal of Experimental Medicine, researchers working at the Wihuri Research Institute and the University of Helsinki report a surprising finding that challenges current anatomy and histology textbook ...

How cancer tricks the lymphatic system into spreading tumors

May 11, 2015
Swollen lymph nodes are often the earliest sign of metastatic spread of cancer cells. Now cancer researchers and immunologists at Sweden's Karolinska Institutet have discovered how cancer cells can infiltrate the lymphatic ...

Lymphatic cells grown in the lab for the first time

May 20, 2015
For over one hundred years, scientists have debated the question of the origins of the lymphatic system - a parallel system to the blood vessels that serves as a conduit for everything from immune cells to fat molecules to ...

Recommended for you

Exercise-induced hormone irisin triggers bone remodeling in mice

December 13, 2018
Exercise has been touted to build bone mass, but exactly how it actually accomplishes this is a matter of debate. Now, researchers show that an exercise-induced hormone activates cells that are critical for bone remodeling ...

Law professor suggests a way to validate and integrate deep learning medical systems

December 13, 2018
University of Michigan professor W. Nicholson Price, who also has affiliations with Harvard Law School and the University of Copenhagen Faculty of Law, suggests in a Focus piece published in Science Translational Medicine, ...

Drug targets for Ebola, Dengue, and Zika viruses found in lab study

December 13, 2018
No drugs are currently available to treat Ebola, Dengue, or Zika viruses, which infect millions of people every year and result in severe illness, birth defects, and even death. New research from the Gladstone Institutes ...

Faster test for Ebola shows promising results in field trials

December 13, 2018
A team of researchers with members from the U.S., Senegal and Guinea, in cooperation with Becton, Dickinson and Company (BD), has developed a faster test for the Ebola virus than those currently in use. In their paper published ...

Pain: Perception and motor impulses arise in brain independently of one another

December 13, 2018
Pain is a negative sensation that we want to get rid of as soon as possible. In order to protect our bodies, we react by withdrawing the hand from heat, for example. This action is usually understood as the consequence of ...

Researchers give new insight to muscular dystrophy patients

December 13, 2018
New research by University of Minnesota scientists has revealed the three-dimensional structure of the DUX4 protein, which is responsible for the disease, facioscapulohumeral muscular dystrophy (FSHD). Unlike the majority ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.