Activation of retinal microglia rather than microglial cell density correlates with retinal neovascularization in the mouse model of oxygen-induced retinopathy

Background: Retinal neovascularization has been intensively investigated in the mouse model of oxygen-induced retinopathy (OIR). Here, we studied the contribution of microglial cells to vascular regression during the hyperoxic phase and to retinal neovascularization during the hypoxic phase. Methods: Mice expressing green fluorescent protein (GFP) under the Cx3cr1 promoter labeling microglial cells were kept in 75% oxygen from postnatal day 7 (P7) to P12. Microglial cell density was quantified at different time points and at different retinal positions in retinal flat mounts. Microglial activation was determined by the switch from ramified to amoeboid cell morphology which correlated with the switch from lectin negative to lectin positive staining of GFP positive cells. Results: Microglial cell density was constant in the peripheral region of the retina. In the deep vascular layer of the central region, however, it declined 14 fold from P12 to P14 and recovered afterwards. Activated microglial cells were found in the superficial layer of the central avascular zone from P8 to P12 and from P16 to P18. In addition, hyalocytes were found in the vitreal layer in the central region and their cell density decreased over time. Conclusion: Density of microglial cells does not correlate with vascular obliteration or revascularization. But the time course of the activation of microglia indicates that they may be involved in retinal neovascularization during the hypoxic phase.