The mitotic spindle


Eukaryotic cells segregate their chromosomes using at least two classes of micortubules. Kinetochore microtubules (shown here in green) attach to the kinetochores (red) and guide the chromosomes (blue) toward opposite poles in Anaphase. Longer microtubules (not shown) can stretch from pole-to-pole and are involved in spindle elongation.

The organism

Picoplankton are unicellular algae (plant cells) found in all of the world's oceans. These aptly named organisms typically less than 3 µm wide. The picoplankton called O. tauri is the smallest known eukaryote: each cell is smaller  than 2 µm and has just 1 chloroplast, 1 mitochondrion, 1 Golgi body, and 1 nucleus. Such a simple ultrastructure makes O. tauri an excellent system to study cell biology. (A) Example immunofluroescence images of O. tauri cells showing the chloroplasts (green), interphase nuclei (blue), and mitotic nuclei (magenta). (B) DIC image of a living O. tauri cell. Scale bars, 2 µm. (C) DIC image of a budding yeast (S. cerevisiae) cell at the same scale. A mammalian cell at the same scale would span this entire webpage!

The technology

Electron tomograms are 3-D "images" reconstructed from a series of 2-D images. (A) First, a "tilt series" of images are recorded as the object (a ribosome shown in gray here) is incrementally tilted, typically over a range of -60 degrees to +60 degrees. Each image is a unique view of the object comprising a "projection" of the all of the object's density. (B) A tomogram is reconstructed by back-projecting (akin to smearing) the aligned 2-D images through a common origin. For more details on tomography of cells, see Gan et al.