Defects and Textures in Cholesteric Analogues given by Some Biological Systems

Many biological materials are polymerized analogues of cholesteric liquid crystals. The same types of defect occur both in cholesteric liquids and in these more or less solid biological systems. However, their distribution and therefore the textures are different, owing to certain growth mechanisms in biological analogues. I . Introduction. -In a previous work 111, a comparison was made between certain fibrous and regularly twisted biological materials and cholesteric liquid crystals. The twisted fibrous systems are found in varied cytoplasmic inclusions, in the organic matrix of numerous skeletal structures, in the connective tissue of certain Invertebrates, in the nucleus of many Bacteria and in the Dinoflegallate chromosomes. Very long polymer chains occur in such materials (chitin, collagen, different proteins, D. N. A., etc ...). These systems are more or less solid, but they nevertheless show the optical and geometrical properties of cholesterics. Such biological materials contain defects very similar to those appearing in true cholesteric liquids. The distribution of these defects is often different in cholesteric phases and in their biological analogues. The growth mechanisms of the twisted fibrous arrangements in biological tissues or cells provide interpretations for these differences. 2. Defects in cholesteric liquid crystals and their biological analogues. The cholesteric layering is often altered by many defects and these have been seen very clearly by the first observers ; see, for example, Lehmann 121. The focal conics have been described by Friedel and Grandjean [3] primarily for smectics but these authors also mentioned their occurrence in cholesterics [3, 41. More recently, de Gennes [5], Friedel and Klkrnan [6] have proposed models of edge-dislocations and disclinations in the cholesteric systems. We have proved the existence of several of these arrange(*) Ce travail a btncficie d'unc convention de Recherche D. G . R. S. T. no 73 7 1732. ments [7]. Punctual singularities have been supposed to occur in nematic droplets [8] and they have been shown to exist in certain geometrical situations [6, 91. Many defects are located on different interfaces (as the limit with the isotropic liquid). These singularities are lines or points due to certain angular conditions imposed on the molecular orientation in the vicinity of the limiting surface [lo, 111. The main lines visible in the bulk have been represented in several papers [6, 121. They have been found in synthetic polymerized cholesterics [13]. Figure 1 shows that they exist in different polymerized biological systems. In the latter, we have not studied the possible occurrence of punctual singularities and surface defects. 2.1 TRANSLATION DISLOCATIONS. We have described screw-dislocations in the crab-cuticle [I]. Our model was planar, the cholesteric axis and the dislocation line being vertical. The director trajectories are drawn in horizontal planes and may give either confocal parabolae (f n disclinations) or the complementary arrangement (n disclinations). Electron micrographs are in good agreement with this model. Recently, Smith, Telfer and Neville 1141 have obtained electron micrographs of such patterns in the chorion of a moth : Hyalophora cecropia. The thin sections are exactly horizontal and one sees the filaments running near the core, in the plane of the preparation with a very weak vertical component (Fig. la). Screw-dislocations may occur along the axis of certain Dinoflagellate chromosomes (Fig. Ib). There is no great difference between the aspects given by a screw and an edge-dislocation seen in oblique section. However, the distinction is made easier with serial Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975156