Blood vessels constructed by vasculogenesis are immature. For the maturation of vessel structure, mural cells (MCs), such as pericytes or smooth muscle cells, have to adhere to endothelial cells (ECs). In the maturation process involved in blood vessel formation, the ECs which form the tube, recruit supporting MCs, by releasing PDGF-BB. MCs subsequently adhere to ECs resulting in the formation of a structurally-stable blood vessel. It has been proposed that this cell adhesion between ECs and MCs is induced when angiopoietin-1, produced from MCs, stimulates Tie2, a receptor tyrosine kinase on ECs. During angiogenesis, blood vessels should be able to adjust their caliber, in order to allow them to respond adequately to the changes in demand for oxygen and nutrients made by the organs and tissues. This caliber adjustment is involved in the maturation process during angiogenesis. Apelin, produced from ECs via activation of Tie2 stimulates APJ, a GPCR expressed on ECs induces non-leaky enlarged blood vessels. Besides such molecular signals for blood vessel maturation, recently it has been clarified that phalanx cells among ECs induce maturation of blood vessels.
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