LAMELLAR SYSTEMS 



147 



1M 



Fig. 19. Modified mitochondria in pigment epithelium cell of light-adapted 

 guinea-pig retina, showing characteristic condensation of granular material in 

 the layered structure. Numerous intermediate stages are found which show 

 gradual transformation of the layered structure into the highly regular com- 

 ponent of the paracrystalline granules. Osmium-cryofixation preparation. 

 X 55,000. 



crystalline structure of certain types of dense granules within these 

 cells and to study their interrelationship with the lamellar com- 

 ponents of the cytoplasm. The slender, microvilli-like processes of 

 the pigment epithelium cells envelop the retinal rod outer segments 

 and establish intimate contact. In addition to specialized lamellar 

 components (Porter, 1957; Yamada et al., 1958), numerous dense 

 granules approximately 0.3 //, to 1 or 2 /x in diameter are found in 

 these cells. As indicated by Yamada et al. (1958), these "fuscin 

 granules" are extremely dense, and their internal structure is difficult 

 to study because of their mechanical hardness, so that only small 

 particulates or fine tubular networks could be seen. 



In standard osmium-fixed and araldite-embedded preparations of 

 the guinea-pig retinal pigment epithelium, these granules can be 

 sectioned with the diamond knife, and exhibit an extremely com- 

 pact, granular fine-structure (Fig. 18) with only occasional indica- 



