Retinal ellipsosomes: morphology, development, identification, and comparison with oil droplets.

We report some unique features of retinal cone ellipsosomes in mountain-stream teleosts. They have also been compared with oil droplets occurring predominantly in many reptilian and avian retinas. Ontogenetically, ellipsosome differentiation from ellipsoidal mitochondria occurs with advanced eye growth (diameter > 1 mm). In juvenile loaches, they arise almost simultaneously in the dorsal and ventral retina, whereas in cyprinids, they appear first dorsally in bottom-dwelling early juveniles (approximate age 3-4 months), and then in the ventral retina in migratory late juveniles (eye diameter > 4 mm, approximate age 2 years). The significance of the pattern of ontogeny of ellipsosomes in these stream fishes is discussed in relation to their utilization of a complex habitat during life. All adult cones possess conspicuous ellipsosomes. Histochemically, they react strongly with phosphotungstic-acid hematoxylin, a dye specific for proteins, whereas oil droplets refuse to do so (studied in turtle and pigeon). This reflects a major chemical difference between the two types of globules. Since ellipsosomes are present in the double cone accessory unit (which in higher vertebrates lacks an oil droplet) and since they appear late ontogenetically during advanced eye growth, they cannot be related to oil droplets, which have an embryonic developmental program.