Components of the SDF-1 signaling system are expressed in the retinal vasculature. ISH was performed on whole-mount postnatal day 1 or 2 retinas, and fluorescently labeled isolectin was used to counterstain the entire vasculature. ISH of whole-mount neonatal retina shows CXCR4 mRNA enriched in tip cells (A, arrowheads) and in the developing arteries (B, arrows). CXCR7 is expressed at low levels in the developing arteries (C, arrows) and absent from tip cells (C, inset box). SDF-1 mRNA is detected in the larger vessels of the developing retina (D, arrows) and in the retinal astrocyte layer (merged image shows ISH and anti-GFAP stain; E). Boxed area shows representative area of overlap with anti-GFAP staining of the astrocytes. Black scale bars represent 100 μm. Images were acquired as described in “ISH of whole-mount retina” using a 20× (A-D) or a 40× (E) objective. For ISH and merged images, RGB levels were adjusted using Photoshop; for merged images, fluorescent images were inverted and hue, levels, and transparency were adjusted to make both light and fluorescent images discernible.