Ultracytochemical demonstration of glycogen in cone, but not in rod, photoreceptor cells in the rat retina

The presence of native glycogen in photoreceptor cells of the rat retina has not been identified in the literature. We have studied this ultracytochemically. After perfusion with glutaraldehyde fixative, the eyes were enucleated, and the retinal tissues, postfixed with OsO 4, were embedded in epoxy resin. Some tissues were treated with saliva before postfixation. Ultrathin sections, stained by the periodic acid-thiocarbohydrazide-silver proteinate (PA-TCH-SP) method or with uranyl acetate and lead citrate, were examined by electron microscopy. On routinely stained sections, glycogen particles seemed to be absent in the cytoplasmic matrix of the photoreceptor cells because they were indistinguishable from the numerous ribosomes. This was due to a similarity in size and electron density. After PA-TCH-SP staining, fine electron-dense reaction products appeared on small cytoplasmic particles (but not on ribosomes) in the inner segments, perikarya and synaptic terminals of a subpopulation of photoreceptor cells. These particles, 15–25 nm in diameter, were identified as β-particles of glycogen because of their susceptibility to enzyme digestion. The glycogen-rich photoreceptor cells were thought to be cone cells by reason of their morphological features, such as synaptic terminals, nuclei and outer segments. These results suggest that the cone, but not the rod, photoreceptor cells in the rat contain abundant glycogen.