Effects of Varying the Vehicle for OsO4 in Tissue Fixation

I t is well known that the quality of OsO4 fixation of tissue cells obtained with any one of the several recommended mixtures (1-3) may vary considerably in the same block or from one tissue to another. In some cells of a sample, the material of the cytoplasmic matrix may appear as an even distribution of partides and finely fibrous elements, in which case it is regarded as well fixed. In other cells in the same sample the matrix components may be condensed into clumps of variable size and density, separated by areas of lesser density devoid of any resolvable structure. Such clumping is usually taken as descriptive of relatively poor fixation, since little or no evidence of it can be detected in equivalent living cells (4). I t has been noted also that in images of what one considers to be well fixed ceils, the contours of the membrane-limited elements of the endoplasmic reticulum are smooth, whereas in another part of the same preparation equivalent structures may appear angular or irregular and vesiculate& In some images the nucleoplasm is homogeneous and relatively uniform in density, whereas in others it is obviously uneven and coagulated. Some of these differences may be referable to structural or compositional variations associated with functional changes, but in most instances the differences seem more properly considered as products of fixation. Especially is this true in frequent