Mitogenesis and retinal pigment epithelial cell antigen expression in the rat after krypton laser photocoagulation

Polypeptide growth factors, such as the acidic and basic fibroblast growth factors (aFGF and bFGF), may be important in the pathogenesis of subretinal neovascularization. The authors studied the relationship of aFGF and bFGF expression to retinal pigment epithelial (RPE) cell and choriocapillary endothelial cell proliferation in krypton-laser-treated regions of the retina, RPE, and choroid of a model of subretinal neovascularization in the pigmented rat they developed.

Multiple krypton laser burns were applied to the posterior poles of the eyes of pigmented rats according to a protocol described for producing subretinal neovascularization in these animals. At intervals up to 80 days after treatment, the retinas, RPE, and choroid of these animals were examined by [3H]-thymidine autoradiography and electron microscopic immunocytochemistry, using antibodies to aFGF, bFGF, cytoplasmic retinaldehyde-binding protein, opsin, and various basement membrane macromolecules.

Nuclear radiolabeling became evident in these layers when the label was injected as late as 75 days after photocoagulation, but the number of labeled nuclei was greatest when isotope was injected 2-5 days after laser treatment. Although there were labeled nuclei in the retina, RPE, and choroid, the frequency of labeling as a fraction of the total number of nuclei appeared to be greatest in the RPE and choriocapillaris. Non-laser-damaged RPE cells were immunocytochemically strongly positive for cytoplasmic retinaldehyde-binding protein, but were negative for aFGF and bFGF. After laser treatment, many RPE cells lost their cytoplasmic retinaldehyde-binding protein positivity but stained strongly for aFGF and bFGF within intracellular structures of variable shape and homogeneous appearance. Although these structures had an appearance suggestive of basement membrane, they did not stain with antibodies to collagens type IV or V, to laminin, or to heparan sulfate proteoglycan core protein. They also did not stain with an antibody to the N-terminal peptide of opsin. Choriocapillary endothelial cells were unreactive with antibodies to aFGF, bFGF, or cytoplasmic retinaldehyde-binding protein either before or after laser treatment. FGF-positive RPE cells persisted for the 80-day duration of the experiment.

Because the presence of FGF-positive RPE cells coincides temporally with increased nuclear thymidine labeling in the RPE and choriocapillaris, aFGF and/or bFGF may be at least partly responsible for initiating the process of cell proliferation and subretinal neovascularization in these animals.