Light-induced deposits in Bruch's membrane of protoporphyric mice.

Photosensitization of choriocapillary endothelium with blood-borne photosensitizers, such as protoporphyrin IX, has been proposed as a mechanism for the choriocapillary sclerosis and Bruch's membrane deposits seen in age-related macular degeneration. Utilizing a mouse model of protoporphyria with approximately a 10-fold increase in protoporphyrin IX level and exposure to blue light (14 microW/cm2; bandwidth, 390 to 430 nm), a time- and light-dependent increase in choriocapillary and sub-retinal pigment epithelium basal laminar-like deposits could be demonstrated at 7 months by transmission electron microscopy. Thickening of the choriocapillary endothelial basement membrane with a homogeneous electron-dense material was first noted in protoporphyric mice exposed to blue light for 13 weeks. At 28 weeks the experimental animals exhibited a thick band of homogeneous deposits at the level of the choriocapillary basement membrane and electron-dense fibrillogranular deposits of varying sizes along the inner aspect of Bruch's membrane, with fibrils measuring up to 16 nm, with a periodicity of 13 nm. These deposits contributed to an overall thickening of Bruch's membrane with narrowing of the choriocapillaris. The morphologic appearance and localization of these deposits within Bruch's membrane of this animal model are similar to previously described deposits noted in the aging Bruch's membrane prior to the development of age-related macular degeneration.