Flat mounts of oxygen-induced retinopathy models. a. Retinal flat mount of a postnatal (p) day 17 (p17) mouse that had been in constant 75% oxygen from p7 to p12 and then in room air for 5 days. Lectin staining permits visualization of the retinal vessels. Note central “vaso-obliterated” retina and intravitreal neovascular buds at the vascular– avascular junction. This model may mimic ROP of the 1950s and what occurs in regions that do not regulate oxygen. b. Lectin-stained retinal flat mount from a p18 rat pup eye that underwent repeated fluctuations between 50% and 10% oxygen every 24 hours for 14 days, followed by 4 days of room air exposure. In this model, there is first a delay in physiologic retinal vascular development causing peripheral avascular regions of retina followed by intravitreal neovascularization at the vascular-avascular junction. This model is the most representative model of type 1 ROP. A beagle model of OIR has some characteristics of both rodent models.