A recent paper published in Nature, A dual origin for blood vessels, describes experiments that trace the development of endothelial cells in mice. By targeting expression of a fluorescent protein to cells of the yolk sac, Plein et al. were able to show that endothelial cells lining blood vessels develop from two different embryonic cell types: the mesoderm and the yolk sac.
So why does the yolk sac being a newly discovered origin of endothelial cells matter? Is it just esoteric biology?
“Contrary to previous assumptions, it seems the cells that line blood vessels are derived from more than one source. In addition to their known developmental path, they can arise from progenitors of embryonic blood cells.”
Endothelial Cells: Role in Immunity
The yolk sac contains cells that eventually populate the bone marrow and give rise to blood cells. Previous studies have shown that tissue-resident macrophages are also descendants of this cell population. Apparently, some of these yolk sac progenitor cells revert to endothelial cells and are sprinkled through the vasculature. They seem to have a different genetic signature and may respond to stimuli differently than the cells that are developed directly from the mesoderm.
From an immunological perspective, these findings are interesting because tissue-resident macrophages and these endothelial cells arise from the same progenitor cells of the yolk sac.
Endothelial cells share some characteristics with macrophages, such as a receptor for acetylated low-density lipoprotein (AcLDL), CD31, and TLR4. Perhaps these shared markers are due to the shared development pathway. If so, there may be more of a role for endothelial cells in immunity than is fully appreciated.