Retinal attachment to the pigment epithelium. Linkage through an extracellular sheath surrounding cone photoreceptors.

The interphotoreceptor matrix, which occupies the so-called subretinal space in normal human eyes, was examined with electron microscopy after treatment with Cuprolinic Blue, a dye that selectively stains sulfated polyanions. A cylindrically shaped, extracellular ensheathing domain, composed of Cuprolinic Blue-positive particles, was observed surrounding each cone photoreceptor examined. Equivalent structures were not present around rods. Surrounding peripheral cones, the unit particles which comprised the sheath, were punctate in shape whereas in foveal and foveolar cones the particles were more elongate. A matrix domain completely enveloped each cone photoreceptor, beginning at the outer limiting membrane, extending beyond the tip of the cone outer segment, and ending at the apical surface of the pigment epithelium. Proximal to the pigment epithelium, specialized extensions of apical microvilli were present in the lumen of the matrix sheath and shorter microvilli appeared to be embedded in the sheath wall. When retinas were experimentally detached from the pigment epithelium and washed extensively before fixation and staining, the cone matrix domains remained associated with the cones, and broken fragments of pigment epithelial cell processes were present embedded in the distal tip of the cone matrix sheath wall. These observations indicate that the cone matrix sheath is insoluble and is closely associated with both the cone photoreceptor and the apical surface of the pigment epithelium. The implication of these findings is that the cone matrix sheath is a bridge through which the retina and pigment epithelium are physically attached.