Osmic staining of amphibian and gastropod photoreceptors

Eyes of two species of amphibian tadpoles ( Xenopus laevis and Hyla regilla) and of a pulmonate snail ( Helix aspersa) were impregnated with osmium tetroxide, with or without extraction by chloroform. Amphibian rods, but not cones, stained heavily with aqueous osmium tetroxide. After extraction with chloroform, osmification (OsO 4 in CCl 4) did not produce heavy deposits of osmium black, but 30–45 diameter granules in the outer leaflets of rod disk membranes became visible. The granules may be stained molecules of photopigment. The pigmented epithelium of light-adapted eyes, but not of dark-adapted eyes, exhibited prominent Golgi cisternae and cytoplasmic vesicles filled with reaction product. Photosensory cells of H. aspersa exhibited osmium black in rhabdomeric microvilli and in 800 vesicles, termed photic vesicles. After extraction with chloroform, little reaction product appeared, but 30–50 diameter granules, interpreted as stained molecules of photopigment, were found in the vesicles and microvilli.