The appearance of the outer segments of isolated retinae fixed with glutaraldehyde or formaldehyde alone, or with these aldehydes and postfixed with osmium tetroxide, is described. Rod outer segments and, to a lesser extent, cone outer segments of retinae fixed only with glutaraldehyde show shrinkage or swelling artifacts which are dependent on the fixative buffer concentration. The rod outer segments are most normal with a fixative phosphate buffer about 50% isotonic with retinal Ringer solution. The disks fixed with glutaraldehyde alone have a granular pentalaminar structure. At the edge of the rod disks, the loops formed by folding over of the disk membranes are seen after glutaraldehyde fixation alone to be filled with stainable material and the edges of adjacent loops to be in contact. The edge of the disk stack thus appears to be a more solid structure than previously thought, and it only partly survives osmium tetroxide treatment, even after glutaraldehyde fixation. Similarly, the arrangement of the disk membranes after glutaraldehyde fixation also appears to be weakened by postfixation with osmium tetroxide. For rod outer segments showing severe shrinkage after glutaraldehyde fixation alone, the interdisk clear spaces are lost and stainable cytoplasmic material, probably protein, is trapped between the disks. The disk membranes then appear as light lines on a dark background, since the central, unstained hydrophobic regions of each disk membrane become prominent.

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