Application of Tomographic 3D Reconstruction Methods To A Diverse Range of Biological Preparations
Tomography is a completely general three-dimensional (3D) reconstruction approach which can be applied to almost any biological structure which is sufficiently contrasted from its background. We have used tomography to compute 3D reconstructions of a diverse group of biological preparations which include: the cilium and the kinetochore, which were positively stained and imaged in 0.25 μm sections; the dendrite and the Golgi apparatus, which were selectively stained, before embedding, and imaged in 3 μm sections; chromatin fibers, which were negatively stained; and patch-clamped membranes which were critically point dried and imaged as whole mounts in their micropipettes.With the cilium, we found good overall agreement with the known structure but were unable to completely resolve the fine structure for several reasons. Figure 1 illustrates how a different volume rendering technique, which included partial transparency, was able to bring out a dynein repeat that was not detected in our original study. With the kinetochore (figure 2), the structure is too complicated for the eye to readily comprehend and a procedure of selective data reduction is being developed. The dendrite and the Golgi apparatus (figure 3) are favorable objects for tomography because selective staining allows a large depth to be reconstructed (3 μm) and yet the structures are simple enough for surface renditions to provide a clear picture of the 3D ultrastructure. The dendrite reconstructions provided valuable surface area information and the Golgi reconstructions are expected to yield clues as to how this structure is formed. The chromatin fiber afforded the highest resolution reconstruction we obtained (figure 4) and, in conjunction with a 3D peak search, we were able to locate the positions of individual nucleosomes (figure 5). However, fiber flattening and other effects of the preparatory procedures were noted. The cylindrical geometry of the patch-clamped membranes in a micropipette is ideal for tomography because the sample thickness does not change with tilt angle and an unlimited tilt range should be possible. We are currently working on the latter but the availability of the full tilt range and the sides of the micropipette present some problems for the presently implemented alignment schemes.