Freeze-fracture studies of the structure of rod outer segment membranes: New observations regarding the distribution of particle-free patches and the location of the fracture planes in conventionally prepared retinas

Abstract

In a continuing study by electron microscopy of rod outer segment membranes in replicas from freeze-fractured retinas of various species, new findings have been obtained which bear upon the process of disk membrane assembly. According to current models, disk membranes at the base of the rod outer segment are formed simply by the expansion of either the plasma membrane of the outer segment through an in-folding, or of the plasma membrane of the connecting cilium through paired out-foldings. In both Bufo marinus and northern Rana pipiens we have found particle-free patches in newly formed disks, but not in the outer segment plasma membrane. This structural distinction in the rod outer segments of these species between the densely particulate plasma membrane and forming disks at the base suggests that chemical modification of some elements within the forming disk and plasma membranes may accompany their dynamic interplay during outer segment growth. We also present an experiment supporting the bilayer-splitting hypothesis for the predominant location of the fracture planes in the exceptionally protein-rich membranes of outer segments after conventional fixation and cryoprotection.