Vasculature Growth - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 608

Table 10.3. Mechanisms of endothelial leading, tip and trailing, stalk cell selection and their
main features and roles (Source: [ 886 ]). Endothelial cells produce multiple Notch receptors
(Notch-1, Notch-3, and Notch-4) and transmembrane Notch ligands (DLL1, DLL4, Jag1, and
Jag2). Activated VEGFR2 induces DLL4 expression in tip cells that activates Notch on adjacent
stalk cells. Ligand binding causes cleavage of Notch receptor by ADAM10, ADAM17, and
presenilins, thereby liberating an intracellular fragment (Notch ICD ). In stalk cells, Notch precludes
VEGFR2 activity, downregulates the expression of VEGFR3, and upregulates that of VEGFR1
and soluble VEGFR1, a sink for VEGFa that then represses VEGFR2 activity, thereby preventing
tip cell fate. In addition, Notch induces expression of DLL4 and Notch-regulated ankyrin repeat-
containing protein (NRARP). The latter supports Wnt signaling in stalk cells, thus maintaining
adhesion sites between endothelial cells; it also fosters inhibitory feedback of Notch. In tip cells,
Notch is blocked by Jagged-1 produced by stalk cells; it impedes DLL4-Notch connections on
tip cells when Notch is glycosylated. Binding of VEGFc to its VEGFR3 receptor, which is highly
expressed in tip cells, enables lymphangiogenesis.

Tip cell Stalk cell

VEGF signaling DLL4-Notch signaling

VEGFa/c Low VEGFR signaling

VEGFR2/3
Low Notch signaling
PDGFb, Unc5b

Highly motile Weakly motile

Guide sprouting Lumenogenesis

Maintenance of between-cell junctions
Connection to parent vessel

Establishment of blood flow requires the formation of a vascular lumen. Cellular
structures are converted into tubes by vacuole formation and intracellular and
subsequent intercellular fusion of large vacuoles. Vascular tubulogenesis is initiated
by the acquisition of the apicobasal polarity of endothelial cells that is regulated
by cell-matrix interactions and signaling via partitioning defective protein PAR3
and VEGFR receptor. Tubulogenesis is controlled by EGF-like domain-containing
protein EGFL7 expressed by endothelial cells.
Interactions between tip cells regulate the fusion of adjoining sprouts to form
a continous lumen. Tip cells abandon exploration upon encountering tips of other
sprouts or existing capillaries, i.e., when they can form new vascular connections.
Junctional contacts are then built at merging regions. Newly formed vessels are
stabilized by recruitment of pericytes and deposition of matrix proteins into a
basement membrane.