Arhgef15 stabilizes vessels in neonatal retinal vasculature. (A-C) X-gal staining combined with isolectin B4 (iB4) staining of Arhgef15⁺/lacZ retinas at P5. Arhgef15 is predominantly expressed in arterial ECs (closed arrowheads), whereas weaker expression was detected in venous (open arrowheads) and capillary ECs, including tip/stalk cells (arrows). (D) Quantitative PCR analysis of HUVECs cultured in the presence or absence of 50 ng/mL VEGF-A (12 hours) after 12 hours of serum starvation (n = 4). (E-N) Whole-mount IHC of P5 retinas with the indicated Abs. Arhgef15 KO mice showed delayed radial growth and sparse vasculature in the central retina (E-F). Although the number of tip cells (red points in G and I) and proliferation (H,J) were not affected, Arhgef15 KO mice showed markedly increased empty sleeves (Col IV(+) iB4(−); arrowheads in K-N). (O-R) Quantification of each parameter (n = 4). Scale bars: 500 μm (E-F), 200 μm (A), and 50 μm (B-C,G-N).