The effect of age on the macromolecular permeability of human Bruch's membrane

To determine whether age-related changes in Bruch's membrane affect its permeability to macromolecules. Such changes have been postulated to underlie some pathologic manifestations of age-related macular degeneration.

Bruch's membrane preparations were isolated from the macular region of donated human eyes of differing age and mounted in a modified Ussing chamber. Permeability to macromolecules was assessed by simultaneously placing a physiological concentration of serum proteins adjacent to the choroidal margin of the membrane preparation and a saline solution adjacent to the retinal pigment epithelial basement membrane. After 24 hours, the protein content of the saline solution was measured by standard assay and permeability calculated as the quantity of protein traversing the membrane preparation per unit area. The spectrum of proteins able to cross the membrane was assessed by subjecting the diffusate proteins to electrophoretic separation and the resultant gel to scanning densitometry.

The permeability of Bruch's membrane to serum proteins decreased 10-fold from the first to the ninth decade of life, and on regression analysis this decline exhibited a linear relationship with donor age (P < 0.0005). Membrane preparations from young donors were permeable to proteins with a molecular weight in excess of 200 kDa, but with increasing age, the membrane progressively impeded the passage of high-molecular-weight entities. Even so, elderly membranes were still permeable to macromolecules with molecular weights in excess of 100 kDa. Results from the oldest preparation studied suggest that by the ninth decade, the membrane may selectively impede the flux of specific proteins, based on a criterion other than molecular weight.

The results imply that, with increasing age, the capacity of Bruch's membrane to facilitate macromolecular exchange between the choroidal and the retinal pigment epithelial compartments is reduced.