Bruch’s membrane is a thin, specialized layer of tissue within the eye, important for healthy vision. This delicate membrane, typically measuring between 2 to 4 micrometers in thickness, supports the eye’s intricate visual processes. Its integrity is important for the health of the back of the eye and clear sight.
Anatomy of Bruch’s Membrane
Bruch’s membrane is positioned in the back of the eye, situated between the retinal pigment epithelium (RPE) and the choroid. The RPE nourishes the light-sensitive retina, and the choroid provides oxygen and nutrients via its blood vessels. This thin structure is a complex, five-layered extracellular matrix.
The layers of Bruch’s membrane, from the RPE side towards the choroid, include the basal lamina of the RPE, followed by the inner collagenous layer. Next is a central elastic layer, which provides flexibility, bordered by the outer collagenous layer. The outermost layer is the basal lamina of the choriocapillaris, the choroid’s capillary system. These collagenous layers act like structural scaffolding, providing support and organization.
The Function of Bruch’s Membrane
Bruch’s membrane performs two primary roles. It functions as a selective transport system, allowing substances to move between the choroid and retina. Nutrients and oxygen from the choroid pass through the membrane to reach the RPE and photoreceptors, which are highly metabolically active cells.
Waste products generated by the retina are transported across Bruch’s membrane to the choroidal blood supply. The membrane also serves as a physical barrier, providing structural support to the RPE and preventing the uncontrolled growth of blood vessels from the choroid into the retina. This barrier helps maintain the distinct environments of these two regions, which is important for proper retinal function.
Age-Related Changes and Disease
Bruch’s membrane undergoes changes with age, affecting its function and contributing to various eye conditions. The membrane thickens, and lipid-rich deposits accumulate within its layers. These deposits, often referred to as drusen, are a common early sign of age-related macular degeneration (AMD).
The accumulation of drusen and the thickening of Bruch’s membrane can impair its ability to transport nutrients and remove waste products. This disruption can lead to the two main forms of AMD. In dry (atrophic) AMD, which accounts for about 90% of cases, the RPE and photoreceptors slowly break down and thin, leading to gradual vision loss.
The breakdown of the membrane’s barrier function is particularly significant in wet (neovascular) AMD, a less common but more severe form. In this condition, the weakened Bruch’s membrane allows new, fragile blood vessels to grow from the choroid into the retina, a process called choroidal neovascularization. These new vessels are often leaky, causing fluid and blood to seep into the macula, leading to rapid and severe central vision loss. Beyond AMD, other conditions like pseudoxanthoma elasticum (PXE) involve calcification and fragmentation of elastic fibers in Bruch’s membrane, which can result in angioid streaks and choroidal neovascularization.