Mechanical and Chemical Regulation of Arterial and Venous Specification - Mechanical and Chemical Signaling in Angiogenesis - page 12

Fig. 3 Signaling mechanisms underlying arterial and venous specification. See text for details

experimental manipulation has not been reported. Moreover, upstream regulators
that induce the activation of COUP-TFII or PI3K/Akt signaling in endothelial cells
are unknown. Therefore, it is not clear if there are any inductive processes
involved in venous fate determination.
One hallmark of veins is the presence of valves. However, no molecular signals
have been uncovered that are involved in the formation of venous valves.
Furthermore, it remains unknown how the formation of valves is suppressed in
arteries.
As early as the fifteenth century, it was already proposed that there may be
cross-talk between arterial and venous vessels by Jacopo Berengario da Carpi
based on his anatomical observation. However, there is currently no evidence for
the existence of paracrine factors that mediate a putative cross-checking of the
phenotypes by each other.

5 Speculative Remarks

Although significant advances have been made in understanding mechanical and
chemical mechanisms underlying arterial and venous phenotype and function, it is
far from a complete understanding how this regulatory system works. Thus, I end
my chapter with ''speculative remarks'' instead of customary concluding remarks.
Based on what we currently know about this subject, I speculate on three
hypothetical models (Fig. 4 ): Morphogenetic, habituation, and integrative models.