The ultrastructure of the developing inner and outer segments of the photoreceptors of chick embryo retina as revealed by the rapid-freezing and deep-etching techniques.

The ultrastructure of retina cell receptors of chick embryos and of one to three week old chicks was examined paying special attention to the membrane structure of receptor discs, mitochondria, cell membrane and other cell organelles, such as the endoplasmic reticulum and the Golgi apparatus. The retinas were rapid-frozen with a liquid-propane jet, deep-etched, and rotary-shadowed replicas produced. The structure of the photolamellae membranes is asymmetrical. The fracture faces showed a smooth (E-face) and a rough (P-face) surface. Both true surfaces ( interdiscal and intradiscal) were also observable by deep-etching. Transverse fractures of the discs showed the globular structure of their membrane. Spherical or polyhedral particles, probably rhodopsin-associated particles, occupying the width of the membrane are 12 nm in diameter and are constituted by 6 subunits of 1.5-2.0 nm arranged around a channel. These particles seem to extend into the membrane of the photolamellae during the last days of incubation and were also found in variable positions in the width of the disc membrane. When observed in transversal fractures of the photolamellae , they were sometimes seen to protrude into the collapsed intradiscal space and into the cytoplasmic surface. Filament-like or particulate structures connect the discs both to each other and to the cell membrane. During development a relationship between the forming discs and the cell membrane was not observed. The mitochondria aggregated in the ellipsoid are connected by filament-like structures that form during development of the inner segment. The structure of the inner cristae membrane of the mitochondria is characterized by the presence of stalked particles as previously described by Fern andez -Morán (1961) using negative staining. An intracristal space is not present. The fracture of the receptor cell membrane shows a particulate cytoplasmic half with particle-free patches. The glycogen granula situated in the cytoplasm between the smooth ER cisternae show a rosette-like composition.