Pinocytosis in eyes of a snail, Helix aspersa

Selective illumination of single photoreceptors in the house fly retina: local membrane turnover and uptake of extracellular horseradish peroxidase (HRP) and lucifer yellow

Single photoreceptor cells in the compound eye of the housefly Musca domestica were selectively illuminated and subsequently compared electron-microscopically with the unilluminated photoreceptors in the immediate surroundings. The rhabdomeres of the illuminated cells remain largely unaffected, but the cells show an increase in the number of coated pits, various types of vesicles, and degradative organelles; some of the latter organelles are described for the first time in fly photoreceptors. Coated pits are found not only at the bases of the microvilli, but also in other parts of the plasma membrane. Degradative organelles, endoplasmic reticulum (ER) and mitochondria aggregate in the perinuclear region. The rough ER and smooth ER are more elaborate, the number of Golgi stacks, free ribosomes and polysomes is increased, and the shape and distribution of heterochromatin within the nuclei are altered. Illuminated photoreceptors also interdigitate extensively with their neighbouring secondary pigment cells. These structural changes in illuminated fly photoreceptor cells indicate an increase in membrane turnover and cellular metabolism. When applied to the eye, Lucifer Yellow spreads into the extracellular space and is taken up only by the illuminated photoreceptor cells. These cells show the same structural modifications as above. Horseradish peroxidase applied in the same way is observed in pinocytotic vesicles and degradative organelles of the illuminated cells. Hence, the light-induced uptake of extracellular compounds takes place in vivo at least partially as a result of an increase in pinocytosis.

Rhodopsin and retinochrome in the retina of a marine gastropod, Conomulex luhuanus

Photopigments in the conch retina were examined with special attention given to the photic vesicles characteristic of gastropod photoreceptors. Three different fractions of visual cell fragments were prepared from the retina: the MV-fraction containing the rhabdomal microvilli, and the PVH- and PVL-fractions containing the photic vesicles located in the visual cell body. Rhodopsin was found in the MV-fraction (lambda max = 474 nm), and yielded a photoequilibrium mixture with metarhodopsin (lambda max = 512 nm) on irradiation with blue light. Retinochrome was found in both of the PVH- and PVL-fractions (lambda max = approximately 510 nm), and was bleached into metaretinochrome by exposure to orange light, showing no marked shift of the absorption peak. Unlike the PVH-fraction, the PVL-fraction contains much aporetinochrome in addition to retinochrome, suggesting that the large mass of photic vesicles around the nucleus may serve as storage for retinal in retinochrome and for newly synthesized aporetinochrome. The total amount of retinochrome in the retina was several times higher than that of rhodopsin, distinguishing the gastropod eye from the cephalopod eye.

Acyltransferase and acid hydrolase activities of the abalone photoreceptor cell

In order to study the synthesis and degradation processes of the photoreceptor membranes in the abalone, Nordotis discus, the localization of acyltransferase and acid hydrolase activities, respectively, were determined at the electron-microscopic level. Acyltransferase activity was localized on the cytoplasmic sides of thick (greater than 10 nm) membranes of the following organelles: a few cisternae at the trans (or concave) side of Golgi apparatus, Golgi and probably related vesicles, short tubules, curved pentalaminar disks and limiting membranes of the phagosomal multivesicular bodies; all organelles were scattered in the peri- to supranuclear cytoplasm. The phospholipids, which are major components of the photoreceptor membrane, are considered to be synthesized by these membranes. Acid phosphatase activity was localized in the lumina of Golgi cisternae and vesicles, lysosomes, and smaller multivesicular and related bodies, but not in multilamellar bodies. The matrices of the larger multivesicular bodies and of the pigment granule complexes showed arylsulfatase activity. Vesiculated and autophagocytosed photoreceptor microvilli seemed to be degraded by acid hydrolases, forming multivesicular and related bodies. Supporting cells also showed acyltransferase and acid hydrolase activities.

Autoradiographic and biochemical analysis of photoreceptor membrane renewal in Octopus retina

Using autoradiographic and biochemical methods, we have demonstrated the renewal of light-sensitive membranes and photopigments in Octopus visual cells. After the injection of Octopus with [3H]leucine, electron microscope autoradiography revealed an intracellular pathway similar to that in vertebrates for the synthesis and transport of nascent protein from the inner segments to the rhabdomes. However, migration of labelled protein from synthetic sites to the light-sensitive rhabdomes took longer in Octopus than the equivalent process in vertebrates. Biochemical analysis of [3H]leucine-labelled retinas identified some of the labelled protein observed in autoradiographs of the rhabdomes as the visual pigment, rhodopsin. We have shown that retinochrome, a second photopigment in cephalopod retinas, is also renewed. Biochemical analysis 8 h after injection of [3H]leucine revealed heavy labelling of this photoprotein. Light microscope autoradiography of Octopus retina 8 h after injection of [3H]retinol showed labelling of both the rhabdomes and the myeloid bodies of the inner segments. Biochemical data gathered 8 h after injection of [3H]retinol indicated chromophore addition to both rhodopsin and retinochrome with retinochrome being more heavily labelled than rhodopsin. Thus, silver grains observed over the rhabdomes and inner segments could arise from one or both photopigments. These data suggest that retinal is stored in the myeloid bodies of the photoreceptor inner segments. Retinal could then be transferred, perhaps via retinochrome, to newly synthesized opsin before the visual pigment is assembled into new rhabdomeric membranes. Alternatively, retinochrome may serve to transport retinal from the myeloid bodies to the rhabdomes to regenerate rhodopsin as previously proposed.

Transport of pinocytic vesicles in the eye of a snail, Helix aspersa

The heads of small adult snails, Helix aspersa, were injected with horseradish peroxidase (HRP) for one to five hours before extirpating the eyes and preparing them cytochemically for electron microscopy. There was internalization of tracer by pinocytic vesicles (pinosomes) at the bases of types-I and -II sensory cells, ganglion cells and, in lesser amounts, by pigmented supportive cells. Vesicles and vacuoles filled with HRP were transported in two directions: lensward as far distad as the ends of the cells (retrograde) and brainward down the optic nerve (anterograde). We believe that the numerous reacted vacuoles in the cell somata are formed by fusion of vesicles, tubules and C-shaped organelles filled with tracer; we present evidence that they become secondary lysosomes. Sensory cell type II possesses more HRP-reacted vacuoles distally than the other retinal cells. Other vesicles are also described. There was no uptake of tracer by the distal ends of the retinal cells following injection HRP into the hemolymph. The swelling of the optic nerve, immediately behind the eye, contains more HRP-filled pinosomes and vacuoles than does the nerve below the dilatation. The significance of endocytosis and transport of pinosomes within the eye and down the optic nerve is discussed.