Recognition of hyperbolic periodic cytomembrane morphologies in electron microscopic sections

2 Recognition of hyperbolic periodic cytomembrane morphologies in electron microscopic sections 

The shape and fimction of cytomembranes are intriguing and challenging research areas, inasmuch as their heterogeneous composition and size make them technically very difficult to study. Usually, we have to rely on experimental probes that inevitably perturb the sample - such as electron microscopy (EM) - to study their structures. The preparation techniques used for biological specimens often affect the shape of the membranes. Even if these obstacles can be overcome, the result is a two-dimensional impression of a three-dimensional structure. Serial sections or scanning electron microscopy (SEM), as well as tilting and rotation of the sample, will of course improve the result. 

It is unfortunate that the only approach by which we can currently study cubic membrane structure is from electron microscopy, which often induces artefacts. On the other hand it is more likely that perturbations caused by EM preparation md fixation techniques are destructive to cubic membranes rather than constructive. Therefore it is unlikely that EM preparation techniques would induce the formation of cubic membranes. Indeed similar conclusions have been deduced by several authors dealing with the membrane assemblies that we identify as cubic membranes (see, e.g. [4] and [6]). Besides artefactual problems, our approach is inherently limited, because 

1(a) The diamond (D-) membrane system of the FLB in etiolated leaves. Projection of a section cut approximately normal to the jlOO) plane. The lower mserts show the match between the experimental micrograph and the computer generated constant mean curvature PCS projections for two different distances along the [100] direction. The upper inserts show the Fourier transform (calculated for the regions indicated) for the corresponding experimental and theoretical projections (a and b, and a and b, respectively). 

However, when TEM micrographs of optimal magnification and good resolution are available, the structural evaluation is mostly satisfactory. Thus, excepting the rather rare cases when we are able to identify imique signatures 

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