Artefacts and morphological changes during chemical fixation

Abstract

The normally ‘condensed’ (darkly stained) chromosomes of dinofiagellates decondense by swelling. This occurs in an increasing number of cells when the concentration of added OsO₄ is decreased. With different fixatives other types of disintegration can be observed, which vary with the concentration. With cryofixation and freeze-substitution the chromosomes are most ‘condensed’. Escherichia colt infected with bacteriophage T4, with or without active lysozyme production, were studied by optical densitometry for partial lysis and by light and electron microscopy for observing swelling. When active lysozyme is present some of the acrolein (2.5%) - glutaraldehyde (2%)-fixed cells swell at 0°C, but do not in the absence of lysozyme nor when fixed at room temperature. If OsO₄ is added at concentrations ≤0.5%, partial lysis occurs when lysozyme is present. The optical density decreases, the cells lose some matter and swell slightly. The corresponding electron micrographs show gap formation by curdling and/or a decreased concentration of the cytoplasm which reveals certain phage-related particles.