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Development of rod outer segment (ROS) in the posterior area of the retina in normal (GR) and mutant Campbell rats with inherited retinal dystrophy was studied using transmission electron microscopy and morphometry. In both strains of rats primitive cilia appear first after birth and their number progressively increases up to postnatal day 9. The first ROS membrane disks (MD) appear at postnatal day 5. Originally MD are randomly oriented but later they acquire a regular arrangement. At day 7 MD occupy about 14% area of posterior retina in transverse sections in Campbell rats versus 7% in normal animals. By day 9 MD occupy about 35% area in the same region of the retina in both lines of rats. The retina of 15-day-old rats possesses the definitive number of differentiated ROS. The data obtained show that during the period between birth and eyelid opening ROS morphogenesis in Campbell rats is not slow or disturbed as compared with that in normal rats.

The external photoreceptor segments in the retina of vertebrates have been studied already more than 100 years; the intensity of these studies increased markedly during the recent 20 years due to the introduction of autoradiography and electron microscopy. These methods allowed to study the ultrastructure of external segments of the rods and cones in many vertebrate species, its development in ontogenesis, the renewal of external segments and the dependence of renewal on the diurnal rhythm of illumination. All these data have never been summed up and discussed. This review provides a general picture of studies of different aspects of the ultrastructure of external segments; the models of formation of the external segments' discs proposed by several authors are considered; the necessity of continuation and widening of comparative studies of the ultrastructure of external photoreceptor segments in different vertebrates is founded. The most important conclusions concern the differences between the rods and the cones during renewal of the membranes discs of external segments.

Proteolysis of rhodopsin in disc membranes of right-side out orientation by thermolysin, papain and St. aureus V8 protease allowed to identify two highly exposed regions of polypeptide chain located on the cytoplasmic membrane surface: carboxyl terminal sequence 321-348 and the fragment 236-241. Incubation with chymotrypsin reveals the third site on the cytoplasmic surface, 146-147, accessible to proteolytic enzymes. Frozen-thawed membranes comprise a mixture of vesicles with normal and inverted orientation. Both thermolytic and chymotryptic digests of rhodopsin in these membranes contain the polypeptide which represents the amino terminal sequence lacking the first 30 amino acid residues. Thus at least 30 amino acids from the N-terminus must protrude into the intradiscal space. One additional site was located on the intradiscal surface: papain digests rhodopsin in the inverted membranes at the position 186-187. Localization of the proteolytic cleavage sites allowed to propose a model for rhodopsin topography in disc membrane: the polypeptide chain traverses the bilayer thickness seven times; each of seven transmembrane segments containing approximately 40 amino acid residues includes a sequence of approximately 30 hydrophobic amino acids; which are probably in close contact with hydrocarbon matrix of the membrane. Hydrophobic sequences are terminated with fragments containing clusters of hydrophilic amino acids, possibly interacting with lipid polar head groups and orienting each segment in the bilayer.

Cytoplasmic membranes of rod outer segments from frog retina intact rods in retina were stained with fluorescent dye fluoresceinmonomercur acetate. The dye is covalently bound to proteins of cytoplasmic membrane and doesn't penetrate into the cells. Upon isolation of the purified outer segments with the labeled cytoplasmic membranes the cells were disrupted and fractionated in density sucrose gradient. Cytoplasmic membranes possess floating densities different from those of disk membranes and thus providing a mean for separating them from the latter. The main peptides of cytoplasmic membranes are 56, 53, 45, 30 and 28 kDa proteins.

The influence of cyclic GMP on the structural state of photoreceptor membranes was studied. The spin probe technique was used to obtain the dependences of microviscosity of lipid domains of membranes on cGMP concentration. It was shown that microviscosity of 5-doxylsteraric acid environment in rod outer segment, disc and plasma mambranes changed at concentrations of 2.10(7); 5.10(-6) and 6.10(-6). Non-cyclic GMP was ineffective. The ultrasound treatment of membraneous samples resulted on disappearance of the structural changes.