Fig. 2: Endothelial FLRT2 regulates specifically venous sprouting and CNS angiogenic expansion.

a Representative images of main arteries and veins (identified by their morphology) from control and Flrt2^iΔEC P7-P8 retinas stained with IB4. Red dots indicate branch points from the corresponding mother vessel. b Quantification of the number of branch points in arteries (left) and veins (right) per vessel length. n = 19 control and 16 mutant animals (arteries); 20 control and 17 mutant animals (veins). Two-tailed Mann-Whitney test, p = 0.333 (arteries); Two-tailed unpaired t-test, p = 0.0001 (veins). c Expanded P7 retina stained for FLRT2 and collagen IV (Col IV) showing FLRT2 expression in retinal vessels. Note the expression of FLRT2 at the EC membrane in the vein but its absence in the artery. d Representative images of retinal vascular front from control and Flrt2^iΔEC mice stained with IB4. Red dots indicate cellular protrusions identified as angiogenic sprouts. e Quantification of number of sprouts per 100 μm of retinal vascular front. n = 22 control and 18 mutant mice. Two-tailed unpaired t-test, p = 0.0002. f Vascular fronts from control and Flrt2^iΔEC P7-P8 retinas showing blood vessels labelled with IB4 and EC nuclei stained for ERG. g Quantification of the number of tip cells per stalk cells at the vascular front. n = 10 control and 8 mutant mice. Two-tailed unpaired t-test, p = 0.002. h Glut1 staining visualizing vessel sprouts in P7-P8 control and Flrt2^iΔEC cerebral cortices. i Quantification of the number of sprouts per vessel density in the cerebral cortex. n = 5 control and 6 mutant mice. Two-tailed unpaired t-test, p < 0.0001. Scale bars: 50 μm (a, f), 90 μm (c), 40 μm (d), 100 μm (h). Data are shown as mean ± SEM. **P < 0.01, ***P < 0.001, ns = not significant.