A Possible Cytological Basis for the ‘R’ Membrane in the Vertebrate Eye

Abstract

THE ‘R’ membrane was the name coined by Brindley¹ for a hypothetical retinal barrier whose penetration by a microelectrode advancing from the vitreous chamber results in a marked drop in electrical resistance and decrease in capacitance. Later, Tomita, Murakami and Hashimoto² found that this barrier could be demonstrated in eyes from which the retina was removed, thus placing the barrier behind the retina either at the level of the pigment epithelium or at Bruch's membrane. This was afterwards confirmed by Brindley³. Since the membrane of Bruch appears to be a substantial structure in light microscopy, it was a likely candidate for the ‘R’ membrane. Electrode marking methods employed by Brown and Tasaki⁴ gave results compatible with this hypothesis and ruling out the retina although in fact not clearly marking Bruch's membrane as distinct from the pigment epithelium. Cohen⁵, using electron microscopy, cast doubt as to the role of Bruch's membrane in this phenomenon by pointing out that Bruch's membrane consisted of the basement membranes of the pigment epithelium and choroid capillaries together with a layer of loosely interwoven connective tissue. Since there is a basement membrane at the retinal-vitreous junction where no electrical change of a comparable type is noted, it seemed unlikely that Bruch's membrane had the requisite structure for significant electrical resistance.