The distinction by light and electron microscopy of two types of cone containing colorless oil droplets in the retina of the turtle.

In turtle retina the spectral types of cones can be distinguished by the characteristic colors of their oil droplets. Blue cones until recently were thought to be identified by their colorless oil droplets. Now it has been shown that the cones containing colorless oil droplets can be subclassified into two types: one type in which the oil droplet fluoresces under ultraviolet stimulation and the other type in which it does not. We have attempted to recognize these two subclasses of cones with colorless oil droplets by the following anatomical techniques. (1) Light microscopy of fresh retina, (2) light microscopy of isolated cells in tissue culture and (3) light and electron microscopy of photoreceptors stained with horseradish peroxidase in whole-mount. The cone type with the colorless fluorescent oil droplet (FC cone) has an oil droplet size intermediate between those contained in the green single cones and the cone with the clear colorless oil droplet (CC cone); it typically has a short (15 microns long) oblique axon and it forms approximately 13% of the cones in the retina. The CC cone, on the other hand, is recognized by having the smallest oil droplet of all cone types which is nonfluorescent; it has a very long (30 microns long) oblique axon and forms 5% of the cones in the retina. We discuss the similarity of the FC cone morphology and distribution to blue cones in other vertebrate retinas.