Complementary freeze fracture methods applied to cattle disk membranes.

The rod-outer-segment disk membranes were studied in cattle retinas by electron microscope observation of ultrathin resin sections and complementary freeze replicas. In sections, the membranes are characterized by discontinuous electron densities along the unit membrane structure, in which electron-less-dense globular substructures, forming an intermediate layer, are enhanced. Such substructures are estimated as 60 A in diameter. In freeze replicas, the membranous particles of 77 A in diameter are distributed quite densely in the P-face and scattered in the E-face. Like a number of preceding investigators who have failed to obtain convincing findings to correlate similar profiles with the visual pigment molecules, we should also raise a question about preparation involving chemical and thermal procedures affecting the dimensions of the final images. Our constant results obtained from the two methods, however, will suggest that the thin-section substructures and freeze-replica particles represent two different phases respectively of the same molecular existence rather than a complete profile of the visual pigment. It is also uncertain yet whether one membranous particle should be in fact associated to one molecular pigment or an aggregation of several molecules. The last problem will be resolved to some extent by topographical analyses of the complementary replicas in progress in our laboratory.