Müller cell changes precede vascularization of the pigment epithelium in the dystrophic rat retina

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Abstract
In the Royal College of Surgeons rat with inherited retinal dystrophy, photoreceptor cell degeneration is accompanied by retinal pigment epithelial (RPE) cell alterations and Müller cell changes such as increased expression of glial fibrillary acidic protein (GFAP). Vascular changes such as vascularization of the RPE, vascular proliferation, and formation of vitreoretinal membranes (VRMs) are observed later. To study the relationship of Müller cell changes to the vascular alterations in the dystrophic retina, we used immunoperoxidase techniques and antibodies against GFAP and vimentin. Our study showed that during photoreceptor degeneration, Müller cells expressed small amounts of GFAP. As degeneration progressed, GFAP expression increased and morphological alterations occurred in Müller cells. Müller cell apical processes extended and proliferated in the subretinal space and contacted the apical surface of duplicated RPE cells. Later, GFAP reactive fibers surrounded retinal vessels apposed to the RPE. As the vessels became enmeshed within the RPE, the GFAP-positive perivascular processes disappeared. Eventually, the RPE-associated vessels became displaced into the inner retina where VRMs were sometimes observed. Immunoblots showed increased GFAP in dystrophic as compared with control retinas. Studies of vimentin distribution in the dystrophic retina showed results similar to the GFAP study. Moreover, the vimentin study suggested increased number of Müller cell processes in the dystrophic as compared with control retinas. The close temporal and anatomical relationships among Müller cell, RPE, and vascular changes in the dystrophic rat suggest a role for Müller cells in retinal neovascularization and proliferative retinopathy.

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