Theory of microtomy artefacts in arthropod cuticle.

Arthropod cuticles observed in section generally present alternating clear and dark bands. These have often been interpreted in terms of superimposed layers of different structure or composition. It has been shown, however that this material is homogeneous, and is formed by a twisted arrangement of microfibrils. The dense bands correspond actually to a microtomy artefact and they form dark single spirals in certain distorted areas of the cuticle. A model was proposed, involving the interaction between knife motion and microfibrils; it will be referred to as the stepped model, since the proposed mechanism results in the formation of steps at the surface of sections, on both faces. These steps are limited by structures resembling crests or cliffs, whose regular distribution produces alternating thick and thin bands in the section. This explains the observed contrast (Bouligand, (1972)). Two very interesting models were proposed later (Gordon and Winfree, (1978)) and are referred to as the cos F model and the sand model, but steps and crests are absent in these models. However, Giraud-Guille ((1986)), has shown very clearly the existence of these crests, which seem to be quite essential in this microtomy artefact. To clarify the debate, the texts defining the initial and the two new models are reproduced here and the difficulties encountered by each model are discussed. A mathematical formulation of this artefact is presented in an appendix to the present article; this leads to a more complete discussion of the possible models. Other factors are also taken into consideration: microfibril orientation and staining. The main factor of contrasts is undoubtedly the variation in thickness over a single section, as proposed in the stepped model.