A study of the ultrastructure of visual cell outer segment membranes

Abstract

Aldehyde-fixed chick retina was embedded in a water-containing resin of glutaraldehyde and urea without dehydration. The loss of lipids and other soluble tissue components thereby was minimized and protein denaturation caused by osmium tetroxide postifixation was avoided. Ultrathin sections were stained with (a) uranyl acetate and lead citrate, (b) silicotungstic acid, or (c) osmium vapor prior to electron microscope examination of visual cell outer segment ultra-structure at magnifications up to 800 000 diameters. The two membranes forming an outer segment disc are closely apposed, yielding a total disc thickness of about 230 Å. Interdiscal cytoplasm appears to be retained, and the staining reactions suggest that the membranes are preserved with less alteration in structure than is caused by conventional techniques. A thin layer containing mucopolysaccharide separates the two disc membranes. In cones, this layer is continuous with the intercellular mucopolysaccharide substance.