Direct visualization of microtomy artefacts in sections of twisted fibrous extracellular matrices

Twisted fibrous extracellular matrices observed in section often show alternating clear and dark bands. Three different methods of observation (high voltage electron microscopy, shadowing of thin sections and stereoscopic views) show the presence of ruffling effects and relief at the surface of crab cuticle sections. These effects appear uneven on both sides of the sections. As shown in a series of diagrams, the localization of the microtomy artefact is a function of the orientation of the cuticle laminae relative to the knife direction, and this creates variations in the position and the extent of the microtomy effect over each lamina. Confirmation of this analysis is obtained in a particular geometrical situation which appears in sections of tubercles in the crab cuticle where the twisted plywood stratification is deformed into a dome. By shadowing thin sections, perpendicular to the tubercle axis, nested crescents are visualized on the surface of the samples. All observations demonstrate that the clear and dark lamellae are due to a microtomy artefact which is a three-dimensional process, and not, as usually considered, due to chemical or physical variations in the structure.