Unique cell shape keeps lymphatic vessels and plant leaves stable

The cells that make up the walls of the finest of all lymphatic vessels have a lobate, oak leaf-like shape that makes them particularly resilient to changes in fluid volume. A similar cell shape also supports mechanical stability in plants. This has been shown by researchers from Uppsala University in an article published in the journal Nature.

The lymphatic system consists of a network of lymph vessels that maintains the body's fluid balance and supports the immune system. The finest of all these lymphatic vessels are called lymph capillaries. They have walls that are made up of just a single layer of lymphatic endothelial cells that are permeable to fluids, cells and large molecules from surrounding tissues for transport to other parts of the body.

These vessels need to be able to let fluid through efficiently while also being resilient to sudden changes in fluid volume in the surrounding tissue, such as in the case of swelling, without rupturing.

Stretched cells took on a lobate shape

In the current study, the researchers investigated how the thin layer of lymphatic endothelial cells can withstand changes in vessel caliber when the capillary takes up fluid. This turned out to be due to the cells' capacity to continuously change their peculiar shape.

"It has been known for a long time that capillary lymphatic endothelial cells have a lobate shape, rather like oak leaves or jigsaw puzzle pieces," says Taija Mäkinen, professor at Uppsala University, who led the study.

"However, the reason for this unique morphology was not known, and it had never previously been successfully replicated in cultured cells. In our study, we exposed a thin layer of cultured lymphatic endothelial cells to repeated multidirectional stretching and found that the cells began to acquire a lobate appearance.

"There was also a greater overlap between the cells, which meant that each cell's contact surface with their neighboring cells increased."

Similar to the cells in plant leaves

Lobate shapes can also be found in a completely different type of cell—those on the surfaces of plant leaves. There, their role is to withstand turgor pressure, i.e. the fluid pressure required for plants to grow and hold themselves upright.

"The lobate shape of the plant cells is controlled by a specific signaling pathway and a corresponding pathway is also present in lymphatic endothelial cells. When we tested blocking this signaling pathway in cultured cells, the stretch-induced overlap between the cells was reduced," says Mäkinen.

"In mice that lack one of the signaling molecules in the pathway, not only was the shape of the lymphatic endothelial cells altered but the integrity and function of the lymphatic capillaries was also impaired. This indicates that the lymphatic capillaries need this overlap for the vessel to be able to expand without rupturing when the fluid pressure rises," says Mäkinen.

The lobate appearance of both plant cells and mammalian lymphatic endothelial cells stands out as a distinctive feature among the diversity of cell shapes that have been observed in nature.

"The researchers have interpreted the results from their study as this shape being due to their specialized function: to be able to withstand changes in fluid volumes. This reveals a biological design principle that exists in very diverse species to increase structural stability."