Freeze-fracture analysis of structure of plasma membrane of photoreceptor cell outer segments

Abstract

The plasma membrane of the outer segments of photoreceptor cells in the guinea pig retina was analyzed by means of freeze-fracturing. Two fracture faces with greatly different structure were observed. The relationship of these two faces to the edge which was exposed when the fracture passed across the membrane made it possible to establish that they revealed the cytoplasmic and the peripheral surfaces of the plasma membrane. The thickness of the plasma membrane was measured to be about 100 A. The difference in appearance of the cytoplasmic and the peripheral surfaces of the plasma membrane was interpreted to reveal a layered structure with a predominantly lipid layer at the cytoplasmic surface, and a predominantly protein layer at the peripheral surface. A concept describing the molecular structure of the membrane is proposed. The outer segment plasma membrane differs structurally from the outer segment disk membranes. Certain aspects of disk membrane formation as invaginations of the plasma membrane are discussed. The fact that only two fracture faces were observed and no complementary faces was explained by a simple analysis of some effects of the fracturing on the frozen material.