Photoreceptor membrane breakdown in the spider Dinopis: GERL differentiation in the receptors

The dynamic nature of the Golgi complex

The intracellular transport of newly synthesized G protein of vesicular stomatitis virus is blocked at 20 degrees C and this spanning membrane glycoprotein accumulates in the last Golgi compartment, the trans Golgi-network (TGN). Previous morphological evidence suggested that the TGN enlarged significantly under this condition. In the present study we have used stereological procedures to estimate the volume and surface area of the Golgi stack and the TGN of baby hamster kidney cells under different conditions. The results indicate that the increase in the size of the TGN at 20 degrees C is accompanied by a significant decrease in the surface area and volume of the preceding Golgi compartments. A similar effect is also seen in uninfected cells at 20 degrees C, as well as during normal (37 degrees C) infection with Semliki Forest virus. In the latter case, however, the decrease in the size of the Golgi stack and the increase in that of the TGN is not accompanied by inhibition of transport from the Golgi complex to the cell surface. The results indicate that the Golgi stack and the TGN are dynamic and interrelated structures that are capable of rapid alteration in total surface area in response to changes in the rates of membrane transport.

The posterior median eyes of the dinopid spider Menneus

The posterior median (pm) eyes of the dinopid spider Menneus unifasciatus L. Koch are described and compared with the pm eyes of Dinopis, which are highly specialised for night vision. The lenses of Menneus have F-numbers of 0.72 compared to 0.58 in Dinopis, the distance between receptors is ca. 4.0 microns compared to 20-22 microns for Dinopis, and image quality is matched to receptor spacing. The lens of Menneus is simple, while that of Dinopis comprises two components of different refractive indices (Blest and Land 1977). Receptive segments of the pm eyes of Dinopis are hexagonal in transverse section and those of adjacent cells are tightly contiguous, allowing the possibility of both optical and electrical coupling (Blest 1978). Receptive segments of Menneus are separated from each other by glial processes containing little pigment, and each segment possesses two rhabdomeres on opposite faces of the cell. Rhabdomere volumes undergo a daily cycle similar to that described for Dinopis, but of relatively minor extent. It is shown that the pm eye of Dinopis could have evolved from that of Menneus by a simple series of transformations, and that a gain of two logarithmic units of sensitivity can be attributed to changes in optical design alone.

The sources of acid hydrolases for photoreceptor membrane degradation in a grapsid crab

Dawn photoreceptor breakdown in the crab Leptograpsus variegatus is analysed at the ultrastructural level. Coated vesicles derived from microvilli are assembled as mutlivesicular bodies (mvbs), which degrade to multilamellar bodies (mls) and are lysed. Cytochemical markers for hydrolases were a fluoride-inhibited beta-glycerophosphatase and a fluoride-insensitive p-nitrophenyl phosphatse, with indistinguishable distributions when localised at pH 5.0. These enzymes are injected into the secondary lysomes from two sources: (i) immediately after dawn Golgi bodies are highly active, and differentiate a transtubular network, from which tubules and vesicles detach, and can be seen fusing with mvbs and mlbs. (ii) Saccules derived from the rough endoplasmic reticulum (RER) provide a second source and are most often seen in association with late mlbs. Both kinds of primary lysosome rare give AcPh-positive responses when free in the cytosol, but are seen to do so as they make contact with their secondary lysosomal targets. Lipid droplets and lipofuscin bodies are interpreted as the residual products of breakdown. These results are discussed in relation to previous findings on photoreceptor membrane breakdown in a dinopid spider. Attention is drawn to the implied diversity of organisation of lysosomal compartments in receptors which internalise membranes of similar compositions.

Extracellular shedding of photoreceptor membrane in the open rhabdom of a tipulid fly

The compound eyes of the Australia tipulid fly, Ptilogyna, shed the bulk of their rhabdomeral membrane to extracellular space during turnover. The rhabdomeres of the retinulae lie in a common extracellular space (ECS), which is subdivided in the proximal retina. Before dawn, a distal region of the microvilli in each rhabdomere differentiates and becomes less electron-dense after conventional fixation. The differentiated region then dilates and develops an irregular profile. A few hours after dawn, the transformed tips break off and form a detritus in the ECS. The degraded membrane is internalised back into the retinula cells by mass endocytosis. Retinulae develop pseudopodia at sites bordering the ECS and engulf the membrane detritus, which comes to lie first of all in vacuoles within the receptor cells and then forms very large multivesicular bodies. The latter transform to multilamellar and residual bodies and are, presumably, lysed. Surrounding these secondary lysosomes are rough endoplasmic reticulum and smooth tubular systems, tentatively considered on comparative grounds to provide hydrolases. The literature concerning the ultrastructure of compound eyes offers a small number of instances where extracellular shedding can be suspected for morphological reasons. Attention is drawn to analogies with the shedding of photoreceptor membranes in vertebrate retinae.