142 MACROMOLECULAR COMPLEXES 



Fine Structure of Photoreceptors. The lamellar systems in photo- 

 receptors are of particular interest because they represent the pri- 

 mary sites of visual excitation where light is converted into chemical 

 or electrical energy. From earlier polarized-light studies (Schmidt, 

 1937, 1951) it had been deduced that the rod outer segments of 

 the vertebrate retina consist of transversally arranged, thin protein 

 layers alternating with longitudinally oriented layers of lipid mole- 

 cules. Electron microscopy confirmed this conception by demon- 

 strating that the entire rod outer segment is built up of several 

 hundred unit disks about 150 A thick, in regular compact arrange- 

 ment ( Fernandez-Moran, 1954; Sjostrand, 1949, 1953). As first 

 shown by Sjostrand (1949), the isolated unit disks of the guinea-pig 

 rod outer segment are about 140 A in thickness and approximately 

 2 n. in diameter. In thin sections of osmium tetroxide standard prep- 

 arations (DeRobertis, 1956; Sjostrand, 1953), each disk is seen to 

 correspond to two dense layers fused at their ends and enclosing a 

 lighter space, to give a total thickness of approximately 140 A, The 

 light interspaces between the disks are considered to be filled mainly 

 with an aqueous, ionic medium, according to Sjostrand, who has also 

 tentatively assumed that the 30 to 40 A-thick dense osmiophilic 

 layers are essentially of protein nature, while the 70 to 80 A, less 

 dense interspace could accommodate a double layer of lipid mole- 

 cules (Sjostrand, 1959). 



Thin sections of light-adapted guinea-pig retinas which have been 

 subjected to the osmium-cryofixation procedures (Figs. 14, 16) dis- 

 close a more differentiated picture. The rod outer segments now 

 appear exceptionally compact, with few indications of the discon- 

 tinuities frequently observed in standard preparations. The less 

 dense interspace of each unit disk, which in standard preparations 

 appears essentially devoid of fine structure, contains a distinct in- 

 termediate line 15 to 20 A thick (arrows. Figs. 14, 16). This inter- 

 mediate line is associated with a compact granular material which 

 gives each unit disk the appearance of greater density than is ob- 

 served in the corresponding images of standard preparations (Fig. 

 15). The interspaces between the disks have also been reduced to 

 barely visible gaps between the dense lines of each disk. This close 

 apposition of two structurally asymmetric units now bears a close 

 resemblance to the arrangement of the asvmmetric unit membranes 

 as seen in the myelin sheath and in chloroplasts ( Hodge, 1959 ) . The 

 photoreceptor elements, which had apparently constituted an excep- 



