Figure 10.

An electron micrograph of a partial human cone photoreceptor (reprinted with permission from Hogan MJ, Alvarado JA, Weddell JE. Histology of the Human Eye: An Atlas and Textbook. Philadelphia: Saunders; 1971. Copyright Elsevier; annotations modified).⁸⁰ The image shows portions of the cone IS (top) and OS (bottom). The portion of IS shown is a part of the IS ellipsoid (ISe), densely packed with mitochondria (each ~3-μm long), arranged with their long axes parallel to the optical axis of the cone. In the OS, the stacked discs (~50-nm spacing) are clearly visible. Only a small portion of the ISe and OS fall within the micrograph; at this scale, the segments would each span the entirety of this page. Also visible is the distal membrane of the IS, seen here (and in comparable micrographs from other sources, see text) to lie at an angle to the cell's optical axis. The newest 10 to 15 discs, at the proximal edge of the OS, are narrower than mature discs. The slope of the distal IS membrane is similar to the slope of the proximal edge of the OS. In this image, and others like it, a narrow (50–200 nm) gap is visible between the distal IS membrane and proximal discs of the OS (yellow arrow). It is not known whether this gap is an artifact of tissue preparation for electron microscopy or whether it corresponds to a region of interstitial fluid that separates the IS and OS.