Exp. Eye Res. (1989) 48, 295-301 Discontinuities of the External Limiting Membrane in the fovea centralis of the Primate Retina* INGEBORG P. KREBS AND WOLF KREBS Department of Ophthalmology, College of Physicians and Surgeons of Columbia University, and Departments of Ophthalmology and Pathology, Manhattan Eye, Ear and Throat Hospital, New York, N.Y. 10032, U.S.A. (Received 23 May 1988 and accepted in revised form 24 August 1988) In the center of the fovea centralis of retinas of three cynomolgus monkeys, one baboon and one male human, the external limiting membrane contains circumscribed areas in which zonulae adherentes are lacking between cone inner segments and apices of Miiller cells. These discontinuities of the external limiting membrane begin about 300 #m from the foveolar center, involving only a few cones. Towards the center of the fovea the discontinuities become larger. Key words: retina; fovea; membrana limitaws externa ; zonula adherens; macular edema; cynomolgus monkey; baboon; human. 1. Introduction In the vertebrate retina the apical processes of Mfiller cells are attached to each other and to the inner segments of the photoreceptor cells by continuous junctions of the zonula adherens type. These junctions delineate the inner boundary of the so- called 'interphotoreceptor space' which is derived from the ventricular cavity of the embryonic eye cup. The zonulae, when viewed in cross sections of the retina, appear as a fine line under the light microscope. This line was termed membrana limitans externa by Schultze (1866) over a century ago. It has generally been accepted that this 'membrane' is continuous throughout the entire retina. But, in the fovea centralis of three cynomolgus monkeys, one baboon and one human, we found that the external limiting membrane is interrupted by patches where cones are free of adhering junctions. 2. Materials and Methods Eyes were obtained from three healthy cynomolgus monkeys (Macaca irus, four, eight and twenty years of age), one baboon (Papio anubis, 15 years of age) and one 58 year old male human. The human eye was supplied by the New York Eye Bank. It was taken from an organ donor 45 min after he had died of Hodgkin's disease. The eyes, incised at the limbus corneae, were fixed by immersion in buffered glutaraldehyde/formaldehyde. Pieces of retina containing the fovea were post-fixed with osmium tetroxide and embedded in epoxy resin to allow sectioning parallel to a plane tangential to the surface of the globe. One fovea of the baboon was embedded to allow sectioning normal to the plane of the retina (Fig. 1). Tangential sections were cut through the fovea at the level of the external limiting membrane as shown in Fig. 1. The sections were stained with lead citrate and uranyl acetate. The details of the methods have been published recently (Krebs and Krebs, 1987). * All correspondence should be addressed to I. P. Krebs, Department of Ophthalmology, Columbia lJniversity, 630 W. 168th Street, New York, N.Y. 10032, U.S.A. 0014~4835/89/020295+07 $03.00/0 9 1989 Academic Press Limited