Effects of light and dark on photoreceptors in the polychaete annelid Nereis limnicola.

The effects of light and dark on photoreceptors of the brackish-water polychaete annelid Nereis limnicola were studied by electron microscopy. Animals dark-adapted for one or two days exhibited well-formed straight microvilli (rhabdomeres) on the sensory cell processes. Continuous illumination of worms for one or two days caused extensive breakdown of the microvilli into vesicles and debris. Thirty minutes to three h of exposure of dark-adapted animals to light produced increasing severity of degradation of photoreceptoral microvilli. Light-adapted worms placed in darkness for one-half to three h showed progressive restoration of the microvilli to the dark-adapted condition. The products of degradation were internalized by both sensory and pigmented supportive cells by phagocytosis and pinocytosis.

Cytochemical localization of acid phosphatase in light- and dark-adapted eyes of a polychaete worm, Nereis limnicola.

The amount and distribution of the lysosomal enzyme acid phosphatase in light- and dark-adapted eyes of the brackish-water annelid Nereis limnicola were studied by standard cytochemical techniques. Precipitate from the acid phosphatase reaction was observed in Golgi-endoplasmic reticulum-lysosomal complexes, primary lysosomes, and secondary lysosomes, formed by fusion of primary lysosomes with phagocytic and pinocytic vesicles containing products of presumed rhabdomeric degradation. The acid phosphatase reaction occurred in these organelles in both sensory and supportive cells of both light- and dark-adapted ocelli. Secondary lysosomes were more abundant in sensory cells of illuminated ocelli than in those maintained in the dark. Sparse reaction product was found in Golgi cisternae, none in rough endoplasmic reticulum. We suggest that the increase of lysosomal activity in light-adapted eyes is correlated with the breakdown of photosensory microvilli upon exposure to light. A diagram of our interpretation of recycling of photoreceptoral membrane in N. limnicola is presented.

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.

Unique eye of probable evolutionary significance.

The eyes in the brain of a larval flatworm studied by electron microscopy are dissimilar. Light-sensitive structures in the right eye are microvilli; those in the left eye, the unique one, are both microvillar and ciliary. Perhaps this is evidence for the origin of a microvillar line of photoreceptoral evolution from a more primitive ciliary line.