During human foetal brain vascularization, activated CD31(+)/CD105(+) endothelial cells are characterized by the emission of filopodial processes which also decorate the advancing tip of the vascular sprout. Together with filopodia, both the markers also reveal a number of plasma membrane-derived microvesicles (MVs) which are concentrated around the tip cell tuft of processes. At this site, MVs appear in tight contact with endothelial filopodia and follow these long processes, advancing into the surrounding neuropil to a possible cell target. These observations suggest that, like shedding vesicles of many other cell types that deliver signalling molecules and play a role in cell-to-cell communication, MVs sent out from endothelial tip cells could be involved in tip cell guidance and/or act on target cells, regulating cell-to-cell mutual recognition during vessel sprouting and final anastomosis. The results also suggest a new role for tip cell filopodia as conveyor processes for transporting MVs far from the cell of origin in a controlled microenvironment. Additional studies focused on the identification of MV content are needed to ultimately clarify the significance of tip cell MVs during human brain vascularization.
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