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 reti na has not been identified in the literature. We have studied this ult racytochemically. After perfusion with glutaraldehyde fixative, the ey es were enucleated, and the retinal tissues, postfixed with OsO4, were embedded in epoxy resin. Some tissues were treated with saliva before postfixation. Ultrathin sections, stained by the periodic acid-thioca rbohydrazide-silver proteinate (PA-TCH-SP) method or with uranyl aceta te and lead citrate, were examined by electron microscopy. On routinel y stained sections, glycogen particles seemed to be absent in the cyto plasmic matrix of the photoreceptor cells because they were indistingu ishable from the numerous ribosomes. This was due to a similarity in s ize and electron density. After PA-TCH-SP staining, fine electron-dens e reaction products appeared on small cytoplasmic particles (but not o n ribosomes) in the inner segments, perikarya and synaptic terminals o f a subpopulation of photoreceptor cells. These particles, 15-25 nm in diameter, were identified as beta-particles of glycogen because of th eir susceptibility to enzyme digestion. The glycogen-rich photorecepto r cells were thought to be cone cells by reason of their morphological features, such as synaptic terminals, nuclei and outer segments. Thes e results suggest that the cone, but not the rod, photoreceptor cells in the rat contain abundant glycogen.