Abstract
Mural cells of the vascular system include vascular smooth muscle cells (SMCs) and pericytes whose role is to stabilize and/or provide contractility to blood vessels. One of the earliest markers of mural cell development in vertebrates is α smooth muscle actin (acta2; αsma), which is expressed by pericytes and SMCs. In vivo models of vascular mural cell development in zebrafish are currently lacking, therefore we developed two transgenic zebrafish lines driving expression of GFP or mCherry in acta2-expressing cells. These transgenic fish were used to trace the live development of mural cells in embryonic and larval transgenic zebrafish. acta2:EGFP transgenic animals show expression that largely mirrors native acta2 expression, with early pan-muscle expression starting at 24 hpf in the heart muscle, followed by skeletal and visceral muscle. At 3.5 dpf, expression in the bulbus arteriosus and ventral aorta marks the first expression in vascular smooth muscle. Over the next 10 days of development, the number of acta2:EGFP positive cells and the number of types of blood vessels associated with mural cells increases. Interestingly, the mural cells are not motile and remain in the same position once they express the acta2:EGFP transgene. Taken together, our data suggests that zebrafish mural cells develop relatively late, and have little mobility once they associate with vessels.
Highlights
New blood vessels form during angiogenesis from angioblasts that migrate into position and differentiate into endothelial cells
In the stage of angiogenesis, endothelial cells attract perivascular mural cells including pericytes found on smaller vessels, and smooth muscle cells (SMCs) found on larger vessels
We find that there is extensive coverage of the ventral aorta and associated vessels in the adult acta2:EGFP transgenic fish, suggesting that these early pericyte-like acta2:EGFP positive cells mature into a conventional smooth muscle layer as the fish grows (Figure 2B)
Summary
New blood vessels form during angiogenesis from angioblasts that migrate into position and differentiate into endothelial cells These ‘naked’ endothelial tubes undergo a maturation process. The mutual attraction of the mesenchymal and endothelial cells results in the two layers forming close contacts, followed by maturation of the mesenchymal cells into smooth muscle or pericyte cells. Both pericytes and SMCs require Sonic hedgehog signalling (Shh) for normal vascular development [7],and for the induction of Angiopoietin expression [8,9]. Signalling through Notch and Sphingosine phosphate pathways promotes the investment of mural cells on endothelial tubes [12,13]
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