Natural history of choroidal neovascularization after surgical induction in an animal model

Purpose: To study an expanded time course of surgically induced choroidal neovascularization (CNV) in a porcine model applying fluorescence angiography and immunohistology. Methods: Twenty-two porcine eyes underwent vitrectomy, a retinal bleb was raised and the detached retina perforated using endodiathermy. Bruch's membrane was perforated with a retinal perforator at a site where the overlying neuroretina was normal. Eyes were enucleated in a time interval between 30 min and 42 days after the perforation, and the pigs were subsequently killed. Immediately prior to enucleation, fundus photographs and fluorescein angiograms were obtained. Sections of paraffin-embedded eyes were immunohistochemically stained. Results: On fluorescein angiography, membranes aged 14 days or less exhibited leakage in 10/11 cases while the remaining showed persistent staining. The propensity to leak diminished with time and only 1/3 of the oldest membranes leaked. In eyes enucleated immediately after surgery, neuroretinas overlying the induced lesions were intact without apparent atrophy of cells. At day 3, macrophages and myofibroblasts formed membrane-like structures in the subretinal space. At day 7, the outer surface of the membrane was covered by retinal pigment epithelium (RPE) cells and the neuroretinas had suffered damage in the form of outer segment loss. In the time period 14-42 days, the CNV membrane became completely enveloped by RPE cells. The degree of membrane vascularization increased with time and was at its maximum after 42 days. Intact outer segments were identified over the oldest membranes. Conclusion: The formation of surgical CNV membranes followed the normal reparatory pathway and the degree of vascularization of CNV membranes continued to increase during the 42 days. However, propensity to leak diminished with time. We believe that this was because of the fact that RPE cells completely enveloped older membrane and thus prevented leakage from the newly formed vessels. Photoreceptor outer segments, which had atrophied after 7 days, were able to regenerate over CNV membranes and could be identified again after 42 days.