Introduction
Parasitology studies parasites and their interactions with their natural hosts. Parasites are diverse and range from unicellular to multicellular organisms. The size of parasites can vary between micrometres for unicellular parasites up to several millimetres or even centimetres for multicellular organisms. A recent review shows how different imaging modalities (light, X-ray, electron and ion microscopies) can be used to study parasites1. The highest resolution structural studies of parasites are usually performed using electron microscopy. For transmission electron microscopy, parasites are thick samples and cannot be studied as is, they must be thinned down. This is classically performed by conventional EM on dehydrated resin embedded samples (Fig. 1, top-left) which consists of different sample preparation steps:
  1. fixation with paraformaldehyde and glutaraldehyde
  2. staining with osmium tetroxide (also acts as a fixative agent) and heavy metals (e.g. uranyl acetate)
  3. dehydration in ethanol (and acetone)
  4. embedding the sample in a resin which can be solidified using heat or UV-light
This eventually creates a solid block of sample that must be cut into thin sections using a diamond knife in an ultramicrotome (Fig. 1, bottom-left). Then, the sections can be observed at the electron microscope using TEM. Conventional TEM is an extremely useful method to study the ultrastructure of cells and organisms as it allows tracking membrane and organelle contours permitting to identify cell compartments. Resin blocks can be stored for several years and sections deposited on EM grids can be re-imaged several times if required. For decades, the structure of parasites has been explored using conventional TEM2. These studies allowed describing the internal organisation of the cells and helped with the phylogenetic classification of parasite species.