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

Abstract

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.