Photoreceptors and oil droplet colors in the red area of the pigeon retina.

Six types of photoreceptors in the red area (dorso-temporal quadrant) of the pigeon retina are identified using Golgi impregnation, light microscopy and electron microscopy. Golgi impregnation is used to categorize the receptors into morphological types. Examination of oil droplets in the inner segments of cones in fresh, unfixed tissue shows five different types which can be characterized by color, size and stratification. Therefore, in sections through the length of the receptors examined by electron microscopy, the oil droplets contained in the inner segments of the cones can be identified as to their color by their characteristics (i.e., size and stratification), and the groups of receptors thus classified, further characterized as to the morphology of their terminals. Rods have no oil droplets in their inner segments, and their synaptic terminals are located in the outermost stratum of the outer plexiform layer OPL). Principal members of double cones have yellow oil droplets in their inner segments, while accessory members contain small colorless oil droplets. The synaptic terminals of double cones are located in the same (outermost) stratum of the OPL as rod synaptic terminals. Two types of single, straight cones house either red or orange oil droplets and terminate in the intermediate stratum of the OPL. Oblique single cones with yellow-green oil droplets in their inner segments contribute synaptic terminals to the innermost stratum of the OPL.