Freeze-etch and histochemical evidence for cycling in crayfish photoreceptor membranes

Summary Freeze-etched rhabdoms and adjacent cytoplasmic organelles from crayfish compound eyes have been studied for evidence of photoreceptor membrane cycling. The protoplasmic leaflet face (PF) of split photoreceptor membrane of the microvilli is richly particulate. The particles (92±16 A in diameter in surface fractures; 70±9 A in cross fractures; density about 8000/m²) probably indicate rhodopsin molecule localization. Closely similar particles appear in membranes of pinocytotic vesicles, multivesicular bodies (MVB) and secondary lysosomes. In contrast other retinular cell membranes like plasma membrane remote from the rhabdom are quite distinct (60±23 A particle diameter, density ca 1000/m²). Histochemical tests for acid phosphatase demonstrate its presence in well-developed (but not early stage) MVBs, mixed lamellar vesicular bodies (LVB) and lamellar bodies. Density of PF particles decreases from 8000 in MVB to roughly 4500/m² in LVB indicating a degradative sequence from rhabdom to lamellar bodies. Membrane leaflet orientations show that primary endocytosis from microvilli must be followed by secondary endocytosis of fused coated vesicles to form MVB. Morphological evidence for photoreceptor membrane resynthesis has not been found yet in crayfish but some has been obtained in other crustaceans.This research was supported by grants from the U.S. National Institutes of Health (EY 00405), the National Geographic Society and the Japan Society for the Promotion of ScienceThe authors are grateful to Mr. Washioka of the JEOL Co. Research Laboratory for his essential help in effecting the freeze-etch preparations. They also thank Professor Ryoji Kikuchi of Tokyo Women's Medical College for welcome cooperation and hospitality as well as Dr. Karl Pfenninger of Yale University for his generous assistance in interpreting the freeze-etch data. Technical advice and help were also kindly provided on the acid phosphatase histochemistry by Professor Marilyn Farquhar and others in the Yale Cell Biology Section