Complementary freeze fracture methods applied to cattle disk membranes.

Abstract

The rod-outer-segment disk membranes were studied in cattle retinas by EM of ultrathin resin sections and complementary freeze replicas. The membrane sections are characterized by discontinuous electron densities along the unit membrane structure in which electron-less-dense globular substructures, forming an intermediate layer, are enhanced. The substructures are estimated as 60 .ANG. in diameter. In freeze replicas, the membranous particules of 77 .ANG. in diameter are distributed quite densely in the P[protoplasmic]-face and scattered in the E[ectoplasmic]-face. Since no significant correlation between similar profiles and the visual pigment molecule was found, the chemical and thermal preparation procedures affecting the dimensions of the final images are questioned. The 2 methods imply the thin-section substructures and freeze-replica particles represent 2 different phases, respectively, of the same molecular structure rather than a complete profile of the visual pigment. Whether 1 membranous particle is associated to 1 molecular pigment or an aggregation of several molecules is undecided. Topographical analyses of the complementary replicas may resolve the problems.