LOCALIZATION OF THE SITE OF MAJOR RESISTANCE TO FLUID TRANSPORT IN BRUCHS MEMBRANE

Purpose. To determine tile site of highest resistance to the movement of water across Bruch's membrane in humans. Methods. A hydraulic condu ctivity chamber was designed that enabled us to measure flow across Br uch's membrane while ablating its subepithelial aspect using an excime r laser (193 nm). When resistance was lost, samples were fixed and pro cessed for electron microscopy Results. Changes in the hydraulic condu ctivity of Bruch's membrane in response to the excimer-mediated sequen tial removal of tissue layers was studied in four control eyes of dono rs 26, 46, 61, and 76 years of age and in one eye of an 89-year-old do nor with age-related macular degeneration. Tile number of laser pulses required to abolish the resistance in Bruch's membrane was found to b e dependent oil the age of the donor. The ablation rate was approximat ely 0.11 mu m per pulse. Loss of resistance correlated with removal of layers internal to the label of elastin. Conclusion. This study indic ates that the inner collagenous layer imparts the major resistance to fluid movement between the retinal pigment epithelium and the choroid. Aging changes in the ultrastructure of these compartments could be re sponsible for the decrease in hydraulic conductivity observed in early life in previous studies.