Century-old Debate On Origin Of Mammalian Network Of Lymphatic Vessels Settled

Embryonic veins are the source of cells that multiply and migrate in the embryo to give rise to the entire network of lymphatic vessels, a finding that suggests that these cells might one day be used to grow new vessels.

The St. Jude team used various mouse models to demonstrate that the lymphatic vasculature arises in the embryo from veins by means of continuous release from the veins of cells that multiply and then migrate to different parts of the body.

Conclusively determining how the lymphatic vasculature develops in the embryo is an important step in fully understanding the mechanisms that form this vital network of vessels, according to Guillermo Oliver, Ph.D., a member of the St. Jude Department of Genetics and Tumor Cell Biology.

"Identifying the origin of any specific cell type is critical to understand the basic processes guiding the development of an organ. The detailed characterization of the formation of a normal healthy lymphatic vasculature is central to our efforts to prevent, diagnose, and hopefully cure lymphatic vasculature disorders," explains Dr. Oliver.

"This work is a major contribution in the long-term strategy of promoting the growth of new lymphatic vessels in people suffering from different forms of lymphatic disease that are either inherited or acquired after birth," Oliver said. "The detailed characterization of the formation of a normal, healthy lymphatic vasculature is central to our efforts to prevent, diagnose and cure lymphatic vasculature disorders."

Oliver is senior author of a report on this work that appears in the October 1 issue of "Genes & Development." His laboratory previously discovered that the gene Prox1 plays a critical role in development of the lymphatic vasculature.

The lymphatic vasculature drains fluids that normally escape through the thin walls of the tiny blood vessels called capillaries, which provide nutrients for the cells forming the body's tissues and organs. The lymphatic vasculature reabsorbs much of this fluid, called lymph, from the spaces surrounding the cells.

Failure of lymph transport promotes lymphoedema, a disfiguring, disabling and occasionally life-threatening disorder with limited treatment options. A defective lymphatic vasculature could also promote obesity, according to a previous finding by Oliver's team.

The lymphatic system is essential for the immune response to infectious agents; for example, during inflammation, immune system cells travel to sites of inflammation by way of lymphatic vessels.

The St. Jude team studied the origin of the mammalian lymphatic vasculature using a powerful technique called Cre/loxP-based lineage-tracing. This technique enables researchers to label specific progenitor cells in the embryo and follow them as they reach their final destinations in the developing body. Progenitor cells are parent cells that multiply and give rise to distinct populations of cells with different, specific functions in the body.

The researchers suspected, based on previous work done at St. Jude and elsewhere, that the entire mammalian lymphatic system arises from cells called lymphatic endothelial (LEC) progenitors located in the embryonic veins and that express (activate) the Prox1 gene. These LEC progenitor cells were thought to form lymph sacs--structures from which the entire lymphatic vasculature is derived. However, there was no proof of this; competing theories held that other cells, such as hematopoietic (blood-forming) cells contribute to formation of the lymphatic vasculature.

To conclusively address this question, the researchers did lineage tracing to follow the labeled LECs as they left the veins, formed the lymph sacs and spread through the entire mouse embryo. Using additional animal models, the investigators also determined that the loss of hematopoietic cells did not interfere with the early critical phase--formation of the lymph sacs--of lymphatic development. This finding indicated that hematopoietic-derived cells were not a main contributor to development of the mammalian lymphatic vasculature, according to Oliver.

"We showed that venous-derived LECs sprout from the lymph sacs, multiply and then migrate to their ultimate destination to give rise to the entire lymphatic vasculature; and that hematopoietic cells do not contribute significantly to this process," said R. Sathish Srinivasan, Ph.D. "Therefore, we concluded that in mammals, such as mice and humans, the lymphatic vasculature arises directly and solely from the embryonic veins." Srinivansan is a postdoctoral fellow in Oliver's laboratory and did most of the work on this project.

"This finding supports the original theory presented in the early 20th century by the eminent researcher Florence Sabin, who proposed that the origin of the mammalian lymphatic vasculature is the venous system," Oliver said. "She didn't have the technology to prove her idea back then. We did it for her 100 years later."

Other authors of this study include Miriam Dillard and Oleg Lagutin (St. Jude); Fu-Jung Lin, Sophia Tsai and Ming-Jer Tsai (Baylor College of Medicine, Houston) and Igor Samokhvalov (Center for Developmental Biology, Kobe, Japan).

This work was supported in part by the National Heart, Lung, and Blood Institute, a Cancer Center support grant and ALSAC. Srinivasan is a recipient of a Lymphatic Research Foundation postdoctoral fellowship.