Tip cell identity is unaffected after inhibition of CXCR4 signaling. ISH of P1 to P1.5 whole-mount retina indicates that expression of ESM-1 and apelin are limited to tip cells in the developing retinal vasculature. ESM-1 mRNA is still detectable in tip cells after AMD3100 treatment (A). Similarly, apelin is expressed in tip cells at the vascular front in both control and AMD3100-treated retinas (B). Arrowheads indicate tip cells; scale bar represents 100 μm; imaging was performed as described in “ISH of whole-mount retina.”